HomeMy WebLinkAbout5ALateDocument1663 3 Observations: Temperature Records 3. Observations: Temperature Records 3.1. Paeloclimatic Data 3.2. Past 1,000 Years 3.3. Urban Heat Island 3.4. Fingerprints 3.5. Satellite Data
3.6. Arctic 3.7. Antarctic Introduction The Intergovernmental Panel on Climate Change (IPCC) claims to have found evidence in paeloclimatic data that higher levels of atmospheric CO2
can cause or amplify an increase in global temperatures (IPCC, 2007-I, Chapter 6). The IPCC further claims to have evidence of an anthropogenic effect on climate in the earth’s temperature
history during the past century (Chapters 3, 9), in the pattern (or “fingerprint”) of more recent warming (Chapter 9, Section 9.4.1.4), in data from land-based temperature stations and
satellites (Chapter 3), and in the temperature records of the Artic region and Antarctica where models predict anthropogenic global warming should be detected first (Chapter 11, Section
8). In this chapter, we critically examine the data used to support each of these claims, startin?g with the relationship between CO2 and temperature in ancient climates. References
IPCC. 2007-I. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon,
S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge, UK. 3.1. Paeloclimatic Data Rothman (2002) derived
a 500-million-year history of the air’s CO2 content based on considerations related to the chemical weathering of rocks, volcanic and metamorphic degassing, and the burial of organic
carbon, along with considerations related to the isotopic content of organic carbon and strontium in marine sedimentary rocks. The results of this analysis suggest that over the majority
of the half-billion-year record, earth’s atmospheric CO2 concentration fluctuated between values that were two to four times greater than those of today at a dominant period on the order
of 100 million years. Over the last 175 million years, however, the data depict a long-term decline in the air’s CO2 content. Rothman reports that the CO2 history “exhibits no systematic
correspondence with the geologic record of climatic variations at tectonic time scales.” A visual examination of Rothman’s plot of CO2 and concomitant major cold and warm periods indicates
the three most striking peaks in the air’s CO2 concentration occur either totally or partially within periods of time when earth’s climate was relatively cool. A more detailed look at
the most recent 50 million years of earth’s thermal and CO2 history was prepared by Pagani et al. (2005). They found about 43 million years ago, the atmosphere’s CO2 concentration was
approximately 1400 ppm and the oxygen isotope ratio (a proxy for temperature) was
Climate Change Reconsidered 64 about 1.0 per mil. Then, over the next ten million years, the air’s CO2 concentration experienced three huge oscillations on the order of 1000 ppm from
peak to valley. In the first two oscillations, temperature did not appear to respond at all to the change in CO2, exhibiting an uninterrupted slow decline. Following the third rise in
CO2, however, temperatures seemed to respond, but in the direction opposite to what the greenhouse theory of global warming predicts, as the rise in CO2 was followed by the sharpest
drop in temperature of the entire record. Following this large drop in temperature between 34 and 33 million years before present (Ma BP), the oxygen isotope ratio hovered around a value
of 2.7 per mil from about 33 to 26 Ma BP, indicating little change in temperature over that period. The corresponding CO2 concentration, on the other hand, experienced about a 500 ppm
increase around 32 Ma BP, after which it dropped 1,000 ppm over the next two million years, only to rise again by a few hundred ppm, refuting – three times – the CO2-induced global warming
hypothesis. Next, around 26 Ma BP, the oxygen isotope ratio dropped to about 1.4 per mil (implying a significant rise in temperature), during which time the air’s CO2 content declined.
From 24 Ma BP to the end of the record at 5 Ma BP, there were relatively small variations in atmospheric CO2 content but relatively large variations in oxygen isotope values, both up
and down. All of these many observations, according to Pagani et al. (2005), “argue for a decoupling between global climate and CO2.” Moving closer to the modern era, Fischer et al.
(1999) examined trends of atmospheric CO2 and air temperature derived from Antarctic ice core data that extended back in time a quarter of a million years. Over this period, the three
most dramatic warming events experienced on earth were the terminations of the last three ice ages; and for each of these climatic transitions, earth’s air temperature always rose well
in advance of the the increase in atmospheric CO2. In fact, the air’s CO2 content did not begin to rise until 400 to 1,000 years after the planet began to warm. Another research team,
Petit et al. (1999), studied the beginnings rather than the ends of glacial ages. They discovered that during all glacial inceptions of the past half million years, temperature always
dropped well before the decline in the air’s CO2 concentration. They said their data indicate that “the CO2 decrease lags the temperature decrease by several thousand years.” Petit et
al. also found the current interglacial is the coolest of the five most recent such periods. In fact, the peak temperatures of the four interglacials that preceded it were, on average,
more than 2°C warmer than that of the one in which we currently live. (See Figure 3.1.) Figure 3.1 tells us three things about the current warm period. First, temperatures of the last
decades of the twentieth century were “unprecedented” or “unusual” only because they were cooler than during past interglacial peaks. Second, the current temperature of the globe cannot
be taken as evidence of an anthropogenic effect since it was warmer during parts of all preceding interglacials for which we have good proxy temperature data. And third, the higher temperatures
of the past four interglacials cannot be attributed to higher CO2 concentrations caused by some non-human influence because atmospheric CO2 concentrations during all four prior interglacials
never rose above approximately 290 ppm, whereas the air’s CO2 concentration today stands at nearly 380 ppm. Likewise, Mudelsee (2001) determined that Figure 3.1. Temperature history
derived by Petit et al. (1999) from an ice core extracted from the Russian Vostok drilling station in East Antarctica.
Observations: Temperature Records 65 variations in atmospheric CO2 concentration lagged behind variations in air temperature by 1,300 to 5,000 years over the past 420,000 years. During
certain climatic transitions characterized by rapid warmings of several degrees Centigrade, which were followed by slower coolings that returned the climate to essentially full glacial
conditions, Staufer et al. (1998) observed the atmospheric CO2 concentration derived from ice core records typically varied by less than 10 ppm. They, too, considered the CO2 perturbations
to have been caused by the changes in climate, rather than vice versa. Other studies have also demonstrated this reverse coupling of atmospheric CO2 and temperature (e.g., Cheddadi et
al., 1998; Gagan et al., 1998; Raymo et al., 1998), where temperature is the independent variable that appears to induce changes in CO2. Steig (1999) noted cases between 7,000 and 5,000
years ago when atmospheric CO2 concentrations increased by just over 10 ppm at a time when temperatures in both hemispheres cooled. Caillon et al. (2003) measured the isotopic composition
of argon – specifically, d40Ar, which they argue “can be taken as a climate proxy, thus providing constraints about the timing of CO2 and climate change” – in air bubbles in the Vostok
ice core over the period that comprises what is called Glacial Termination III, which occurred about 240,000 years ago. The results of their tedious but meticulous analysis led them
to conclude that “the CO2 increase lagged Antarctic deglacial warming by 800 ± 200 years.” This finding, in their words, “confirms that CO2 is not the forcing that initially drives the
climatic system during a deglaciation.” Indermuhle et al. (1999) determined that after the termination of the last great ice age, the CO2 content of the air gradually rose by approximately
25 ppm in almost linear fashion between 8,200 and 1,200 years ago, over a period of time that saw a slow but steady decline in global air temperature. On the other hand, when working
with with a high-resolution temperature and atmospheric CO2 record spanning the period 60 to 20 thousand years ago, Indermuhle et al. (2000) discovered four distinct periods when temperatures
rose by approximately 2°C and CO2 rose by about 20 ppm. However, one of the statistical tests they performed on the data suggested that the shifts in the air’s CO2 content during these
intervals followed the shifts in air temperature by approximately 900 years; while a second statistical test yielded a mean CO2 lag time of 1,200 years. Another pertinent study is that
of Siegenthaler et al. (2005), who analyzed CO2 and proxy temperature (dD, the ratio of deuterium to hydrogen) data derived from an ice core in Antarctica. Results of their analysis
revealed a coupling of Antarctic temperature and CO2 in which they obtained the best correlation between CO2 and temperature “for a lag of CO2 of 1900 years.” Specifically, over the
course of glacial terminations V to VII, they indicate that “the highest correlation of CO2 and deuterium, with use of a 20-ky window for each termination, yields a lag of CO2 to deuterium
of 800, 1600, and 2800 years, respectively.” In addition, they note that “this value is consistent with estimates based on data from the past four glacial cycles,” citing in this regard
the work of Fischer et al. (1999), Monnin et al. (2001) and Caillon et al. (2003). These observations seem to undermine the IPCC’s claims that the CO2 produced by the burning of fossil
fuels will lead to catastrophic global warming. Nevertheless, Siegenthaler et al. stubbornly state that the new findings “do not cast doubt ... on the importance of CO2 as a key amplification
factor of the large observed temperature variations of glacial cycles.” The previously cited Caillon et al. also avoid the seemingly clear implication of their own findings, that CO2
doesn’t cause global warming. We find such disclaimers disingenuous. When temperature is found to lead CO2 by thousands of years, during both glacial terminations and inceptions (Genthon
et et al., 1987; Fischer et al., 1999; Petit et al., 1999; Indermuhle et al., 2000; Monnin et al., 2001; Mudelsee, 2001; Caillon et al., 2003), it is extremely likely that CO2 plays
only a minor role in enhancing temperature changes that are induced by something else. Compared with the mean conditions of the preceding four interglacials, there is currently 90 ppm
more CO2 in the air and yet it is currently more than 2°C colder than it was then. There is no way these real-world observations can be construed to suggest that a significant increase
in atmospheric CO2 would necessarily lead to any global warming, much less the catastrophic type that is predicted by the IPCC. References Caillon, N., Severinghaus, J.P., Jouzel, J.,
Barnola, J.-M., Kang, J. and Lipenkov, V.Y. 2003. Timing of atmospheric CO2 and Antarctic temperature changes across Termination III. Science 299: 1728-1731.
Climate Change Reconsidered 66 Cheddadi, R., Lamb, H.F., Guiot, J. and van der Kaars, S. 1998. Holocene climatic change in Morocco: a quantitative reconstruction from pollen data. Climate
Dynamics 14: 883-890. Fischer, H., Wahlen, M., Smith, J., Mastroianni, D. and Deck, B. 1999. Ice core records of atmospheric CO2 around the last three glacial terminations. Science 283:
1712-1714. Gagan, M.K., Ayliffe, L.K., Hopley, D., Cali, J.A., Mortimer, G.E., Chappell, J., McCulloch, M.T. and Head, M.J. 1998. Temperature and surface-ocean water balance of the mid-Holocene
tropical western Pacific. Science 279: 1014-1017. Genthon, C., Barnola, J.M., Raynaud, D., Lorius, C., Jouzel, J., Barkov, N.I., Korotkevich, Y.S. and Kotlyakov, V.M. 1987. Vostok ice
core: Climatic response to CO2 and orbital forcing changes over the last climatic cycle. Nature 329: 414-418. Indermuhle, A., Monnin, E., Stauffer, B. and Stocker, T.F. 2000. Atmospheric
CO2 concentration from 60 to 20 kyr BP from the Taylor Dome ice core, Antarctica. Geophysical Research Letters 27: 735-738. Indermuhle, A., Stocker, T.F., Joos, F., Fischer, H., Smith,
H.J., Wahllen, M., Deck, B., Mastroianni, D., Tschumi, J., Blunier, T., Meyer, R. and Stauffer, B. 1999. Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica.
Nature 398: 121-126. Monnin, E., Indermühle, A., Dällenbach, A., Flückiger, J, Stauffer, B., Stocker, T.F., Raynaud, D. and Barnola, J.-M. 2001. Atmospheric CO2 concentrations over the
last glacial termination. Nature 291: 112-114. Mudelsee, M. 2001. The phase relations among atmospheric CO2 content, temperature and global ice volume over the past 420 ka. Quaternary
Science Reviews 20: 583-589. Pagani, M., Authur, M.A. and Freeman, K.H. 1999. Miocene evolution of atmospheric carbon dioxide. Paleoceanography 14: 273-292. Pagani, M., Zachos, J.C.,
Freeman, K.H., Tipple, B. and Bohaty, S. 2005. Marked decline in atmospheric carbon dioxide concentrations during the Paleogene. Science 309: 600-603. Pearson, P.N. and Palmer, M.R.
1999. Middle Eocene seawater pH and atmospheric carbon dioxide concentrations. Science 284: 1824-1826. Pearson, P.N. and Palmer, M.R. 2000. Atmospheric carbon dioxide concentrations
over the past 60 million years. Nature 406: 695-699. Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue,
G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E. and Stievenard, M. 1999. Climate and atmospheric history of the past 420,000
years from the Vostok ice core, Antarctica. Nature 399: 429-436. Raymo, M.E., Ganley, K., Carter, S., Oppo, D.W. and McManus, J. 1998. Millennial-scale climate instability during the
early Pleistocene epoch. Nature 392: 699-702. Rothman, D.H. 2002. Atmospheric carbon dioxide levels for the last 500 million years. Proceedings of the National Academy of Sciences USA
99: 4167-4171. Royer, D.L., Wing, S.L., Beerling, D.J., Jolley, D.W., Koch, P.L., Hickey, L.J. and Berner, R.A. 2001. Paleobotanical evidence for near present-day levels of atmospheric
CO2 during part of the Tertiary. Science 292: 2310-2313. Siegenthaler, U., Stocker, T., Monnin, E., Luthi, D., Schwander, J., Stauffer, B., Raynaud, D., Barnola, J.-M., Fischer, H.,
Masson-Delmotte, V. and Jouzel, J. 2005. Stable carbon cycle-climate relationship during the late Pleistocene. Science 310: 1313-1317. Staufer, B., Blunier, T., Dallenbach, A., Indermuhle,
A., Schwander, J., Stocker, T.F., Tschumi, J., Chappellaz, J., Raynaud, D., Hammer, C.U. and Clausen, H.B. 1998. Atmospheric CO2 concentration and millennial-scale climate change during
the last glacial period. Nature 392: 59-62. Steig, E.J. 1999. Mid-Holocene climate change. Science 286: 1485-1487. 3.2. Past 1,000 Years The IPCC claims “average Northern Hemisphere
temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the past 500 years and likely the highest in at least the past
1,300 years [italics in the original]” (IPCC, 2007-I, p. 9). Later in that report, the IPCC says “the warming observed after 1980 is unprecedented compared to the levels measured in
the previous 280 years” (p. 466) and “it is likely that the 20th century was the warmest in at least the past 1.3 kyr. Considering the recent instrumental and longer proxy evidence together,
it is very likely that average NH [Northern Hemisphere] temperatures during the second half of the 20th century were higher than for any other 50-year period in the last 500 years” (p.
474).
Observations: Temperature Records 67 The notions that the warming of the second half of the twentieth century was “unprecedented” and that temperatures during the twentieth century were
“the warmest in at least the past 1.3 kyr” will be questioned and tested again and again in the present report. We start here with an examination of the work of Mann et al. (1998, 1999,
2004) and Mann and Jones (2003), which captured the attention of the world in the early years of the twenty-first century and upon which the IPCC still relies heavily for its conclusions.
We then present a thorough examination of temperature records around the world to test the IPCC’s claim that there was no Medieval Warm Period during which temperatures exceeded those
of the twentieth century, starting with data from Africa and then from Antarctica, the Arctic, Asia, Europe, North America, and finally South America. We return to Antarctica and the
Arctic at the end of this chapter to discuss more recent temperature trends. References IPCC. 2007-I. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I
to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.)
Cambridge University Press, Cambridge, UK. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature
392: 779-787. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research
Letters 26: 759-762. Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814. Mann et al. 2004.
Corrigendum: Global-scale temperature patterns and climate forcing over the past six centuries. Nature 430: 105. 3.2.1. The Hockey Stick One of the most famous pieces of “evidence” for
anthropogenic global warming (AGW) brought forth in recent years was the “hockey stick” diagram of Michael Mann and colleagues (Mann et al., 1998; Mann et al., 1999; Mann and Jones,
2003). (See Figure 3.2.1.) Because the graph played such a big role in mobilizing concern over global warming in the years since it was first released, and since the IPCC continues to
rely upon and defend it in its latest report (see IPCC, 2007-I, pp. 466-471), we devote some space here to explaining its unusual origins and subsequent rejection by much of the scientific
community. The hockey stick graph first appeared in a 1998 study led by Michael Mann, a young Ph.D. from the University of Massachusetts (Mann et al., 1998). Mann and his colleagues
used several temperature proxies (but primarily tree rings) as a basis for assessing past temperature changes from 1000 to 1980. They then grafted the surface temperature record of the
twentieth century onto the pre-1980 proxy record. The effect was visually dramatic. (See Figure 3.2.1.) Gone were were the difficult-to-explain Medieval Warming and the awkward Little
Ice Age. Mann gave us nine hundred years of stable global temperatures—until about 1910. Then the twentieth century’s temperatures seem to rocket upward out of control. The Mann study
gave the Clinton administration the quick answer it wanted to the argument that natural climate variations exceed whatever effect human activity might have had in the twentieth century
by claiming, quite simply, that even the very biggest past historic changes in temperatures simply never happened. The Clinton administration featured it as the first visual in the U.S.
National Assessment of the Potential Consequences of Climate Variability and Change (later published as Climate Change Impacts on the United States: The Potential Consequences of Climate
Variability and Change (National Assessment Synthesis Team, 2001)). Mann was named an IPCC lead author and his graph was prominently displayed in the IPCC’s Third Assessment Report (IPCC-TAR,
2001), and it subsequently appeared in Al Gore’s movie, “An Inconvenient Truth.” Mann was named an editor of The Journal of Climate, a major professional journal, signaling the new order
of things to the rest of his profession.
Climate Change Reconsidered 68 The “hockey stick” graph was severely critiqued by two Canadian nonscientists who were well trained in statistics—metals expert Stephen McIntyre of Toronto
and economist Ross McKitrick from Canada’s University of Guelph (McIntyre and McKitrick, 2003, 2005). McIntyre and McKitrick requested the original study data from Mann. It was provided—haltingly
and incompletely—indicating that no one else had previously requested the data for a peer review in connection with the original publication in Nature. They found the data did not produce
the claimed results “due to collation errors, unjustifiable truncation or extrapolation of source data, obsolete data, geographical location errors, incorrect calculation of principal
components and other quality control defects.” In their exchanges with the Mann research team, McIntyre and McKitrick learned that the Mann studies give by far the heaviest weight to
tree-ring data from 14 sites in California’s Sierra Nevada Mountains. At those sites, ancient, slow-growing, high-elevation bristlecone pine trees (which can live 5,000 years) showed
a strong twentieth century growth spurt. The growth ring data from those trees were collected and presented in a 1993 paper by Donald Graybill and Sherwood Idso. Significantly, that
paper was titled “Detecting the Aerial Fertilization Effect of Atmospheric CO2 Enrichment in Tree Ring Chronologies” (Graybill and Idso, 1993). Graybill and Idso specifically pointed
out in their study that neither local nor regional temperature changes could account for the twentieth century growth spurt in those already-mature trees. But CO2 acts like fertilizer
for trees and plants and also increases their water-use efficiency. All trees with more CO2 in their atmosphere are very likely to grow more rapidly. Trees like the high-altitude bristlecone
pines, on the margins of both moisture and fertility, are likely to exhibit very strong responses to CO2 enrichment—which was the point of the Graybill and Idso study. McIntyre and McKitrick
demonstrated that removing the bristlecone pine tree data eliminates the distinctive rise at the end of the “hockey stick.” Mann and his coauthors could hardly have escaped knowing the
CO2 reality, since it was clearly presented in the title of the study from which they derived their most heavily weighted data sites. Using corrected and updated source data, McIntyre
and McKitrick recalculated the Northern Hemisphere temperature index for the period 1400–1980 using Mann’s own methodology. This was published in Energy & Environment, with the data
refereed by the World Data Center for Paleoclimatology (McIntyre and Figure 3.2.1. The ‘hockey stick’ temperature graph was used by the IPCC to argue that the twentieth century was unusually
warm (IPCC-TAR 2001, p. 3).
Observations: Temperature Records 69 McKitrick, 2003). “The major finding is that the [warming] in the early 15th century exceed[s] any [warming] in the 20th century,” report McIntyre
and McKitrick. In other words, the Mann study was fundamentally wrong. Mann and his team were forced to publish a correction in Science admitting to errors in their published proxy data,
but they still claimed that “none of these errors affect our previously published results” (Mann et al., 2004). That claim, too, was contradicted by later work by McIntyre and McKitrick
(2005), by statistics expert Edward Wegman (Wegman et al., 2006), and by a National Academy of Sciences report (NAS, 2006). The NAS skipped lightly over the errors of the hockey-stick
analysis and concluded it showed only that the twentieth century was the warmest in 400 years, but this conclusion is hardly surprising, since the Little Ice Age was near its nadir 400
years ago, with temperatures at their lowest. It was the claim that temperatures in the second half of the twentieth century were the highest in the last millennium that properly generated
the most attention. Where does the IPCC stand today regarding the “hockey stick”? Surprisingly, it still defends and relies on it. It appears in a series of graphs on page 467. Critiques
by Soon and Baliunas (2003) and McIntyre and McKitrick are reported briefly but both are dismissed, the first because “their qualitative approach precluded any quantitative summary of
the evidence at precise times,” and the latter by citing a defense of Mann by Wahl and Ammann (2006) “who show the impact on the amplitude of the final reconstruction is very small (~0.05ºC)”
(IPCC, 2007-I, p. 466). The Medieval Warm Period appears only in quotes in the index and body of the IPCC 2007-I report. In the glossary (Annex I), it is defined as “an interval between
AD 1000 and 1300 in which some Northern Hemisphere regions were warmer than during the Little Ice Age that followed” (p. 949). In the text it is referred to as “the so-called ‘Medieval
Warm Period.’” In a boxed discussion of “Hemispheric Temperatures in the ‘Medieval Warm Period,’” it says “medieval warmth was heterogeneous in terms of its precise timing and regional
expression” and “the warmest period prior to the 20th century very likely occurred between 950 and 1100, but temperatures were probably between 0.1ºC and 0.2ºC below the 1961 to 1990
mean and significantly below the level shown by instrumental data after 1980” (p. 469). One can disprove the IPCC’s claim by demonstrating that about 1,000 years ago, there was a world-wide
Medieval Warm Period (MWP) when global temperatures were equally as high as or higher than they were over the latter part of the twentieth century, despite there being approximately
25 percent less CO2 in the atmosphere than there is today. This real-world fact conclusively demonstrates there is nothing unnatural about the planet’s current temperature, and that
whatever warming occurred during the twentieth century was likely caused by the recurrence of whatever cyclical phenomena created the equal or even greater warmth of the MWP. The degree
of warming and climatic influence during the MWP varied from region to region and, hence, its consequences were manifested in several ways. But that it occurred and was a global phenomenon
is certain; there are literally hundreds of peer-reviewed scientific articles that bear witness to this truth. The Center for the Study of Carbon Dioxide and Global Change has analyzed
more than 200 peerreviewed research papers produced by more than 660 individual scientists working in 385 separate institutions from 40 different countries that comment on the MWP. Figure
3.2.2 illustrates the spatial distribution of these studies. Squares denote studies where the scientists who conducted the work provided quantitative data that enable one to determine
the degree by which the peak temperature of the MWP differed from the peak temperature of the Current Warm Period (CWP). Circles denote studies where the scientists who conducted the
work provided qualitative data that enable one to determine which of the two periods was warmer, but not by how much. Triangles denote studies where the MWP was evident in the study’s
data, but the data did not provide a means by which the warmth of the MWP could be compared with that of the CWP. The third category includes studies that are based on data related to
parameters other than temperature, such as precipitation. As can be seen from the figure, evidence of the MWP has been uncovered at locations throughout the world, revealing the truly
global nature of this phenomenon. A second question often posed with respect to the MWP is: When did it occur? A histogram of the timeframe (start year to end year) associated with the
MWP of the studies plotted in Figure 3.2.2 is shown in Figure 3.2.3. The peak timeframe of all studies occurs around 1050 AD, within a more generalized 800 to 1300 AD warm era.
Climate Change Reconsidered 70 With respect to how warm it was during this period, we have plotted the frequency distribution of all MWP-CWP temperature differentials from all quantitative
studies (squares) shown in Figure 3.2.2 to create Figure 3.2.4. This figure reveals there are a few studies in
which the MWP was determined to have been cooler than the CWP, but the vast majority of the temperature differentials are positive, indicating the MWP was warmer than the CWP. The average
of all such differentials is 1.01°C, while the median is 0.90°C. We can further generalize the superior warmth of the MWP by analyzing the qualitative studies in Figure 3.2.2, which
we have done in Figure 3.2.5. Here we have plotted the number of studies in Figure 3.2.2 in which the MWP was warmer than, cooler than, or about the same as, the CWP, based upon data
presented by the authors of the original works. The vast majority of studies indicates the MWP was warmer than the CWP. It is often claimed that temperatures over the latter part of
the twentieth century were higher than those experienced at any other time over the past one to two millennia. Based upon the synthesis of realworld data presented here (and hereafter),
however, that claim is seen to be false. Figure 3.2.2. Plot of the locations of proxy climate studies for which (a) quantitative determinations of the temperature difference between
the MWP and CWP can be made (squares), (b) qualitative determinations of the temperature difference between the MWP and CWP can be made (circles), and (c) neither quantitative nor qualitative
determinations can be made, with the studies simply indicating that the Medieval Warm Period did indeed occur in the studied region (triangles). Figure 3.2.3. Histogram showing the timeframe
associated with all MWP studies plotted in Figure 3.2.2. Quantitative MWP -CWP Temperature Differences 0123456789 10 11 12 13 14-4.25 -3.25 -2.25 -1.25 -0.25 0.75 1.75 2.75 3.75 Temperature
Difference: MWP-CWP (°C) Figure 3.2.4. The distribution, in 0.5°C increments, of studies that allow one to identify the degree by which peak Medieval Warm Period temperatures either
exceeded or fell short of peak Current Warm Period temperatures.
Observations: Temperature Records 71 In the rest of this section, we highlight the results of studies from regions across the globe that show the existence of a Medieval Warm Period.
Additional information on this topic, including reviews on the Medieval Warm Period not discussed here, can be found at http://www.co2science.org/subject/m/subject_m.php under the heading
Medieval Warm Period. References Graybill, D.A. and Idso, S.B. 1993. Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree ring chronologies. Global Biogeochemical
Cycles 7:81–95. IPCC. 2007-I. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
Change. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge, UK. IPCC-TAR 2001. Climate Change
2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. McIntyre, S.
and McKitrick, R. 2003. Corrections to Mann et al. (1998) proxy data base and northern hemisphere average temperature series. Energy & Environment 14: 751-777. McIntyre, S. and McKitrick,
R. 2005. Hockey sticks, principal components and spurious significance. Geophysical Research Letters 32 L03710. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature
patterns and climate forcing over the past six centuries. Nature 392: 779-787. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium:
Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762. Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical
Research Letters 30: 10.1029/2003GL017814. Mann et al. 2004. Corrigendum: Global-scale temperature patterns and climate forcing over the past six centuries. Nature 430: 105. NAS 2006.
Surface Temperature Reconstructions for the Last 2,000 Years. National Academy Press, Washington, DC. National Assessment Synthesis Team. 2001. Climate Change Impacts on the United States:
The Potential Consequences of Climate Variability and Change, Report for the U.S. Global Change Research Program. Cambridge University Press, Cambridge UK. Soon, W. and Baliunas, S.
2003. Proxy climatic and environmental changes of the past 1000 years. Climate Research 23 (2): 89-110. Wahl, E.R. and Ammann, C.M. 2007. Robustness of the Mann, Bradley, Hughes reconstruction
of Northern Hemisphere surface temperatures: Examination of criticisms based on the nature and processing of proxy climate evidence. Climate Change 85: 33-69. Wegman, E., Scott, D.W.
and Said, Y. 2006. Ad Hoc Committee Report to Chairman of the House Committee on Energy & Commerce and to the Chairman of the House sub-committee on Oversight & Investigations on the
Hockey-stick Global Climate Reconstructions. US House of Representatives, Washington, DC. Available at http://energycommerce.house.gov/108/home/07142006 Wegman Report.pdf. 3.2.2. Africa
Based on the temperature and water needs of the crops that were cultivated by the first agropastoralists of southern Africa, Huffman (1996) constructed a climate history of the region
based on archaeological evidence acquired from various Iron Age settlements. In the course of completing this project, dated relic evidence of the presence of cultivated sorghum and
millets was considered by Huffman to be so strong as to essentially prove that the climate of the subcontinent-wide region must have been warmer and Qualitative MWP -CWP Temperature
Differences 0 10 20 30 40 50 60 MWP < CWP MWP ˜ CWP MWP > CWP Figure 3.2.5. The distribution of studies that allow one to determine whether peak Medieval Warm Period temperatures were
warmer than, equivalent to, or cooler than peak Current Warm Period temperatures.
Climate Change Reconsidered 72 wetter than it is today from approximately AD 900-1300, for these crops cannot be grown in this part of southern Africa under current climatic conditions,
which are much too cool and dry. Other evidence for this conclusion comes from Tyson et al. (2000), who obtained a quasi-decadal record of oxygen and carbon-stable isotope data from
a well-dated stalagmite of Cold Air Cave in the Makapansgat Valley (30 km southwest of Pietersburg, South Africa), which they augmented with five-yearresolution temperature data that
they reconstructed from color variations in banded growth-layer laminations of the stalagmite that were derived from a relationship calibrated against actual air temperatures obtained
from a surrounding 49-station climatological network over the period 1981-1995, which had a correlation of +0.78 that was significant at the 99 percent confidence level. This record
revealed the existence of a significantly warmer-than-present period that began prior to AD 1000 and lasted to about AD 1300. Tyson et al. report that the “maximum warming at Makapansgat
at around 1250 produced conditions up to 3-4°C hotter than those of the present.” In a similar study, Holmgren et al. (2001) derived a 3,000-year temperature record for South Africa
that revealed several multi-century warm and cold periods. They found a dramatic warming at approximately AD 900, when temperatures reached a level that was 2.5°C higher than that prevailing
at the time of their analysis of the data. Lamb et al. (2003) provided strong evidence for the hydrologic fingerprint of the Medieval Warm Period in Central Kenya in a study of pollen
data obtained from a sediment core taken from Crescent Island Crater, which is a sub-basin of Lake Naivasha. Of particular interest in this regard is the strong similarity between their
results and those of Verschuren et al. (2000). The most striking of these correspondences occurred over the period AD 980 to 1200, when lake-level was at an 1,100-year low and woody
taxa were significantly underrepresented in the pollen assemblage. Holmgren et al. (2003) developed a 25,000-year temperature history from a stalagmite retrieved from Makapansgat Valley’s
Cold Air Cave based on d18O and d13C measurements dated by 14C and highprecision thermal ionization mass spectrometry using the 230Th/234U method. This work revealed, in the words of
the nine researchers (together with our interspersed notes), that “cooling is evident from ~6 to 2.5ka [thousand years before present, during the long interval of coolness that preceded
the Roman Warm Period], followed by warming between 1.5 and 2.5 ka [the Roman Warm Period] and briefly at ~AD 1200 [the Medieval Warm Period, which followed the Dark Ages Cold Period],”
after which “maximum Holocene cooling occurred at AD 1700 [the depth of the Little Ice Age].” They also note that “the Little Ice Age covered the four centuries between AD 1500 and 1800
and at its maximum at AD 1700 represents the most pronounced negative d18O deviation in the entire record.” This new temperature record from far below the equator (24°S) reveals the
existence of all of the major millennial-scale oscillations of climate that are evident in data collected from regions surrounding the North Atlantic Ocean. Two years later, Kondrashov
et al. (2005) applied advanced spectral methods to fill data gaps and locate interannual and interdecadal periodicities in historical records of annual low-and high-water levels on the
Nile River over the 1,300-year period AD 622-1922. In doing so, several statistically significant periodicities were noted, including cycles at 256, 64, 19, 12, 7, 4.2 and 2.2 years.
With respect to the causes of these cycles, the three researchers say that the 4.2-and 2.2-year oscillations are likely due to El Niño-Southern Oscillation variations, that the 7-year
cycle may be related to North Atlantic influences, and that the longer-period oscillations could be due to astronomical forcings. They also note that the annualscale resolution of their
results provides a “sharper and more reliable determination of climatic-regime transitions” in tropical east Africa, including the documentation of fairly abrupt shifts in river flow
at the beginning and end of the Medieval Warm Period. Ngomanda et al. (2007) derived high-resolution (<40 years) paleoenvironmental reconstructions for the past 1,500 years based on
pollen and carbon isotope data obtained from sediment cores retrieved from Lakes Kamalete and Nguene in the lowland rainforest of Gabon. The nine researchers state that after a sharp
rise at ~1200 cal yr BP, “A/H [aquatic/hygrophytic] pollen ratios showed intermediate values and varied strongly from 1150 to 870 cal yr BP, suggesting decadal-scale fluctuations in
the water balance during the ‘Medieval Warm Period’.” Thereafter, lower A/H pollen ratios “characterized the interval from ~500 to 300 cal yr BP, indicating lower water levels during
the ‘Little Ice Age’.” In addition, they report that “all inferred lake-level low stands, notably between 500 and 300 cal yr BP, are associated with decreases in the score of the TRFO
[Tropical Rainforest] biome.”
Observations: Temperature Records 73 In discussing their findings, Ngomanda et al. state that “the positive co-variation between lake level and rainforest cover changes may indicate
a direct vegetational response to regional precipitation variability,” noting that “evergreen rainforest expansion occurs during wet intervals, with contraction during periods of drought.”
It appears that in this part of Western Equatorial Africa, the Little Ice Age was a time of low precipitation, low lake levels, and low evergreen rainforest presence, while much the
opposite was the case during the Medieval Warm Period, when fluctuating wet-dry conditions led to fluctuating lake levels and a greater evergreen rainforest presence. Placing these findings
within a broader temporal context, Ngomanda et al. additionally note that “rainforest environments during the late Holocene in western equatorial Africa are characterized by successive
millennial-scale changes according to pollen (Elenga et al., 1994, 1996; Reynaud-Farrera et al., 1996; Maley and Brenac, 1998; Vincens et al., 1998), diatom (Nguetsop et al., 2004),
geochemical (Delegue et al., 2001; Giresse et al., 1994), and sedimentological data (Giresse et al., 2005; Wirrmann et al., 2001),” and that “these changes were essentially driven by
natural climatic variability (Vincens et al., 1999; Elenga et al., 2004).” Esper et al. (2007) used Cedrus atlantica ringwidth data “to reconstruct long-term changes in the Palmer Drought
Severity Index (PDSI) over the past 953 years in Morocco, Northwest Africa.” They report “the long-term PDSI reconstruction indicates generally drier conditions before ~1350, a transition
period until ~1450, and generally wetter conditions until the 1970s,” after which there were “dry conditions since the 1980s.” In addition, they determined that “the driest 20-year period
reconstructed is 1237-1256 (PDSI = -4.2),” adding that “1981-2000 conditions are in line with this historical extreme (-3.9).” Also of significance, the six researchers note that “millennium-long
temperature reconstructions from Europe (Buntgen et al., 2006) and the Northern Hemisphere (Esper et al., 2002) indicate that Moroccan drought changes are broadly coherent with well-documented
temperature fluctuations including warmth during medieval times, cold in the Little Ice Age, and recent anthropogenic warming,” which latter coherency would tend to suggest that the
peak warmth of the Medieval Warm Period was at least as great as that of the last two decades of the twentieth century throughout the entire Northern Hemisphere; and, if the coherency
is strictly interpreted, it suggests that the warmth of the MWP was likely even greater than that of the late twentieth century. In light of these research findings, it appears that
(1) the Medieval Warm Period did occur over wide reaches of Africa, and (2) the Medieval Warm Period was probably more extreme in Africa than has been the Current Warm Period to this
point in time. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/africamwp.php.
References Buntgen, U., Frank, D.C., Nievergelt, D. and Esper, J. 2006. Summer temperature variations in the European Alps, A.D. 755-2004. Journal of Climate 19: 5606-5623. Delegue,
A.M., Fuhr, M., Schwartz, D., Mariotti, A. and Nasi, R. 2001. Recent origin of large part of the forest cover in the Gabon coastal area based on stable carbon isotope data. Oecologia
129: 106-113. Elenga, H., Maley, J., Vincens, A. and Farrera, I. 2004. Palaeoenvironments, palaeoclimates and landscape development in Central Equatorial Africa: A review of major terrestrial
key sites covering the last 25 kyrs. In: Battarbee, R.W., Gasse, F. and Stickley, C.E. (Eds.) Past Climate Variability through Europe and Africa. Springer, pp. 181-196. Elenga, H., Schwartz,
D. and Vincens, A. 1994. Pollen evidence of Late Quaternary vegetation and inferred climate changes in Congo. Palaeogeography, Palaeoclimatology, Palaeoecology 109: 345-356. Elenga,
H., Schwartz, D., Vincens, A., Bertraux, J., de Namur, C., Martin, L., Wirrmann, D. and Servant, M. 1996. Diagramme pollinique holocene du Lac Kitina (Congo): mise en evidence de changements
paleobotaniques et paleoclimatiques dans le massif forestier du Mayombe. Compte-Rendu de l’Academie des Sciences, Paris, serie 2a: 345-356. Esper, J., Cook, E.R. and Schweingruber, F.H.
2002. Lowfrequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253. Esper, J., Frank, D., Buntgen, U., Verstege, A., Luterbacher,
J. and Xoplaki, E. 2007. Long-term drought severity variations in Morocco. Geophysical Research Letters 34: 10.1029/2007GL030844. Giresse, P., Maley, J. and Brenac, P. 1994. Late Quaternary
palaeoenvironments in Lake Barombi Mbo (West
Climate Change Reconsidered 74 Cameroon) deduced from pollen and carbon isotopes of organic matter. Palaeogeography, Palaeoclimatology, Palaeoecology 107: 65-78. Giresse, P., Maley,
J. and Kossoni, A. 2005. Sedimentary environmental changes and millennial climatic variability in a tropical shallow lake (Lake Ossa, Cameroon) during the Holocene. Palaeogeography,
Palaeoclimatology, Palaeoecology 218: 257-285. Holmgren, K., Lee-Thorp, J.A., Cooper, G.R.J., Lundblad, K., Partridge, T.C., Scott, L., Sithaldeen, R., Talma, A.S. and Tyson, P.D. 2003.
Persistent millennial-scale climatic variability over the past 25,000 years in Southern Africa. Quaternary Science Reviews 22: 2311-2326. Holmgren, K., Tyson, P.D., Moberg, A. and Svanered,
O. 2001. A preliminary 3000-year regional temperature reconstruction for South Africa. South African Journal of Science 97: 49-51. Huffman, T.N. 1996. Archaeological evidence for climatic
change during the last 2000 years in southern Africa. Quaternary International 33: 55-60. Kondrashov, D., Feliks, Y. and Ghil, M. 2005. Oscillatory modes of extended Nile River records
(A.D. 622-1922). Geophysical Research Letters 32: doi:10.1029/2004 GL022156. Lamb, H., Darbyshire, I. and Verschuren, D. 2003. Vegetation response to rainfall variation and human impact
in central Kenya during the past 1100 years. The Holocene 13: 285-292. Maley, J. and Brenac, P. 1998. Vegetation dynamics, paleoenvironments and climatic changes in the forests of western
Cameroon during the last 28,000 years B.P. Review of Palaeobotany and Palynology 99: 157-187. Ngomanda, A., Jolly, D., Bentaleb, I., Chepstow-Lusty, A., Makaya, M., Maley, J., Fontugne,
M., Oslisly, R. and Rabenkogo, N. 2007. Lowland rainforest response to hydrological changes during the last 1500 years in Gabon, Western Equatorial Africa. Quaternary Research 67: 411-425.
Nguetsop, V.F., Servant-Vildary, S. and Servant, M. 2004. Late Holocene climatic changes in west Africa, a high resolution diatom record from equatorial Cameroon. Quaternary Science
Reviews 23: 591-609. Reynaud-Farrera, I., Maley, J. and Wirrmann, D. 1996. Vegetation et climat dans les forets du Sud-Ouest Cameroun depuis 4770 ans B.P.: analyse pollinique des sediments
du Lac Ossa. Compte-Rendu de l’Academie des Sciences, Paris, serie 2a 322: 749-755. Tyson, P.D., Karlén, W., Holmgren, K. and Heiss, G.A. 2000. The Little Ice Age and medieval warming
in South Africa. South African Journal of Science 96: 121-126. Verschuren, D., Laird, K.R. and Cumming, B.F. 2000. Rainfall and drought in equatorial east Africa during the past 1,100
years. Nature 403: 410-414. Vincens, A., Schwartz, D., Bertaux, J., Elenga, H. and de Namur, C. 1998. Late Holocene climatic changes in Western Equatorial Africa inferred from pollen
from Lake Sinnda, Southern Congo. Quaternary Research 50: 34-45. Vincens, A., Schwartz, D., Elenga, H., Reynaud-Farrera, I., Alexandre, A., Bertauz, J., Mariotti, A., Martin, L., Meunier,
J.-D., Nguetsop, F., Servant, M., Servant-Vildary, S. and Wirrmann, D. 1999. Forest response to climate changes in Atlantic Equatorial Africa during the last 4000 years BP and inheritance
on the modern landscapes. Journal of Biogeography 26: 879-885. Wirrmann, D., Bertaux, J. and Kossoni, A. 2001. Late Holocene paleoclimatic changes in Western Central Africa inferred
from mineral abundance in dated sediments from Lake Ossa (Southwest Cameroon). Quaternary Research 56: 275-287. 3.2.3. Antarctica Hemer and Harris (2003) extracted a sediment core from
beneath the Amery Ice Shelf, East Antarctica, at a point that is currently about 80 km landward of the location of its present edge. In analyzing the core’s characteristics over the
past 5,700 14C years, the two scientists observed a peak in absolute diatom abundance in general, and the abundance of Fragilariopsis curta in particular—which parameters, in their words,
“are associated with increased proximity to an area of primary production, such as the sea-ice zone”—at about 750 14C yr B.P., which puts the time of maximum Ice Shelf retreat in close
proximity proximity to the historical time frame of the Medieval Warm Period. Khim et al. (2002) likewise analyzed a sediment core removed from the eastern Bransfield Basin just off
the northern tip of the Antarctic Peninsula, including grain size, total organic carbon content, magnetic susceptibility, biogenic silica content, 210Pb geochronology, and radiocarbon
(14C) age, all of which data clearly depicted, in their words, the presence of the “Little Ice Age and Medieval Warm period, together with preceding climatic events of similar intensity
and duration.” Hall and Denton (2002) mapped the distribution and elevation of surficial deposits along the southern Scott Coast of Antarctica in the vicinity of the Wilson Piedmont
Glacier, which runs parallel to the coast of the western Ross Sea from McMurdo Sound north to
Observations: Temperature Records 75 Granite Harbor. The chronology of the raised beaches they studied was determined from more than 60 14C dates of incorporated organic materials they
had previously collected from hand-dug excavations (Hall and Denton, 1999); the record the dates helped define demonstrated that near the end of the Medieval Warm Period, “as late as
890 14C yr BP,” as Hall and Denton describe it, “the Wilson Piedmont Glacier was still less extensive than it is now,” demonstrating that the climate of that period was in all likelihood
considerably warmer than it is currently. Noon et al. (2003) used oxygen isotopes preserved in authigenic carbonate retrieved from freshwater sediments of Sombre Lake on Signy Island
(60°43’S, 45°38’W) in the Southern Ocean to construct a 7,000-year history of that region’s climate. This work revealed that the general trend of temperature at the study site has been
downward. Of most interest to us, however, is the millennial-scale oscillation of climate that is apparent in much of the record. This climate cycle is such that approximately 2,000
years ago, after a thousand-year gap in the data, Signy Island experienced the relative warmth of the last vestiges of the Roman Warm Period, as delineated by McDermott et al. (2001)
on the basis of a high-resolution speleothem d18O record from southwest Ireland. Then comes the Dark Ages Cold period, which is also contemporaneous with what McDermott et al. observe
in the Northern Hemisphere, after which the Medieval Warm Period appears at the same point in time and persists for the same length of time that it does in the vicinity of Ireland, whereupon
the Little Ice Age sets in just as it does in the Northern Hemisphere. Finally, there is an indication of late twentieth century warming, but with still a long way to go before conditions
comparable to those of the Medieval Warm Period are achieved. Two years later, Castellano et al. (2005) derived a detailed history of Holocene volcanism from the sulfate record of the
first 360 meters of the Dome Concordia ice core that covered the period 0-11.5 kyr BP, after which they compared their results for the past millennium with similar results obtained from
eight other Antarctic ice cores. Before doing so, however, they normalized the results at each site by dividing its several volcanic-induced sulfate deposition values by the value produced
at that site by the AD 1816 Tambora eruption, in order to reduce deposition differences among sites that might have been induced by differences in local site characteristics. This work
revealed that most volcanic events in the early last millennium (AD 1000-1500) exhibited greater among-site variability in normalized sulphate deposition than was observed thereafter.
Citing Budner and Cole-Dai (2003) in noting that “the Antarctic polar vortex is involved in the distribution of stratospheric volcanic aerosols over the continent,” Castellano et al.
say that assuming the intensity and persistence of the polar vortex in both the troposphere and stratosphere “affect the penetration of air masses to inland Antarctica, isolating the
continental area during cold periods and facilitating the advection of peripheral air masses during warm periods (Krinner and Genthon, 1998), we support the hypothesis that the pattern
of volcanic deposition intensity and geographical variability [higher values at coastal sites] could reflect a warmer climate of Antarctica in the early last millennium,” and that “the
re-establishment of colder conditions, starting in about AD 1500, reduced the variability of volcanic depositions.” Describing this phenomenon in terms of what it implies, Castellano
et al. say “this warm/cold step could be like a Medieval Climate Optimum-like to Little Ice Age-like transition.” They additionally cite Goosse et al. (2004) as reporting evidence from
Antarctic ice-core dD and d18O data “in support of a Medieval Warming-like period in the Southern Hemisphere, delayed by about 150 years with respect to Northern Hemisphere Medieval
Warming.” The researchers conclude by postulating that “changes in the extent and intra-Antarctic variability of volcanic depositional fluxes may have been consequences of the establishment
of a Medieval Warming-like period that lasted until about AD 1500.” A year later, Hall et al. (2006) collected skin and hair (and even some whole-body mummified remains) from Holocene
raised-beach excavations at various locations along Antarctica’s Victoria Land Coast, which they identified by both visual inspection and DNA analysis as coming from southern elephant
seals, and which they analyzed for age by radiocarbon dating. By these means they obtained data from 14 different locations within their study region—which they describe as being “well
south” of the seals’ current “core sub-Antarctic breeding and molting grounds”—that indicate that the period of time they denominate the Seal Optimum began about 600 BC and ended about
AD1400, the latter of which dates they describe as being “broadly contemporaneous with the onset of Little Ice Age climatic conditions in the Northern Hemisphere and with glacier advance
near [Victoria Land’s] Terra Nova Bay.”
Climate Change Reconsidered 76 In describing the significance of their findings, the US, British, and Italian researchers say they are indicative of “warmer-than-present climate conditions”
at the times and locations of the identified presence of the southern elephant seal, and that “if, as proposed in the literature, the [Ross] ice shelf survived this period, it would
have been exposed to environments substantially warmer than present,” which would have included both the Roman Warm Period and Medieval Warm Period. More recently, Williams et al. (2007)
presented methyl chloride (CH3Cl) measurements of air extracted from a 300-m ice core that was obtained at the South Pole, Antarctica, covering the time period 160 BC to AD 1860. In
describing what they found, the researchers say “CH3Cl levels were elevated from 900-1300 AD by about 50 ppt relative to the previous 1000 years, coincident with the warm Medieval Climate
Anomaly (MCA),” and that they “decreased to a minimum during the Little Ice Age cooling (1650-1650-1800 AD), before rising again to the modern atmospheric level of 550 ppt.” Noting that
“today, more than 90% of the CH3Cl sources and the majority of CH3Cl sinks lie between 30°N and 30°S (Khalil and Rasmussen, 1999; Yoshida et al., 2004),” they say “it is likely that
climate-controlled variability in CH3Cl reflects changes in tropical and subtropical conditions.” They go on to say that “ice core CH3Cl variability over the last two millennia suggests
a positive relationship between atmospheric CH3Cl and global [our italics] mean temperature.” As best we can determine from the graphical representation of their data, the peak CH3Cl
concentration measured by Williams et al. during the MCA is approximately 533 ppt, which is within 3 percent of its current mean value of 550 ppt and well within the range of 520 to
580 ppt that characterizes methyl chloride’s current variability. Hence, we may validly conclude that the mean peak temperature of the MCA (which we refer to as the Medieval Warm Period)
over the latitude range 30°N to 30°S—and possibly over the entire globe—may not have been materially different from the mean peak temperature so far attained during the Current Warm
Period. This conclusion, along with the findings of the other studies we have reviewed of the climate of Antarctica, suggests there is nothing unusual, unnatural, or unprecedented about
the current level of earth’s warmth. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/antarc
icmwp.php.
References Budner, D. and Cole-Dai, J. 2003. The number and magnitude of large explosive volcanic eruptions between 904 and 1865 A.D.: Quantitative evidence from a new South Pole ice
core. In: Robock, A. and Oppenheimer, C. (Eds.) Volcanism and the Earth’s Atmosphere, Geophysics Monograph Series 139: 165-176. Castellano, E., Becagli, S., Hansson, M., Hutterli, M.,
Petit, J.R., Rampino, M.R., Severi, M., Steffensen, J.P., Traversi, R. and Udisti, R. 2005. Holocene volcanic history as recorded in the sulfate stratigraphy of the European Project
for Ice Coring in Antarctica Dome C (EDC96) ice core. Journal of Geophysical Research 110: 10.1029/JD005259. Goosse, H., Masson-Delmotte, V., Renssen, H., Delmotte, M., Fichefet, T.,
Morgan, V., van Ommen, T., Khim, B.K. and Stenni, B. 2004. A late medieval warm period in the Southern Ocean as a delayed response to external forcing. Geophysical Research Letters 31:
10.1029/2003GL019140. Hall, B.L. and Denton, G.H. 1999. New relative sea-level curves for the southern Scott Coast, Antarctica: evidence for Holocene deglaciation of the western Ross
Sea. Journal of Quaternary Science 14: 641-650. Hall, B.L. and Denton, G.H. 2002. Holocene history of the Wilson Piedmont Glacier along the southern Scott Coast, Antarctica. The Holocene
12: 619-627. Hall, B.L., Hoelzel, A.R., Baroni, C., Denton, G.H., Le Boeuf, B.J., Overturf, B. and Topf, A.L. 2006. Holocene elephant seal distribution implies warmer-than-present climate
in in the Ross Sea. Proceedings of the National Academy of Sciences USA 103: 10,213-10,217. Hemer, M.A. and Harris, P.T. 2003. Sediment core from beneath the Amery Ice Shelf, East Antarctica,
suggests mid-Holocene ice-shelf retreat. Geology 31: 127-130. Khalil, M.A.K. and Rasmussen, R.A. 1999. Atmospheric methyl chloride. Atmospheric Environment 33: 1305-1321. Khim, B-K.,
Yoon, H.I., Kang, C.Y. and Bahk, J.J. 2002. Unstable climate oscillations during the Late Holocene in the Eastern Bransfield Basin, Antarctic Peninsula. Quaternary Research 58: 234-245.
Krinner, G. and Genthon, C. 1998. GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica. Climate Dynamics 14: 741-758. McDermott, F., Mattey, D.P. and
Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a
Observations: Temperature Records 77 high-resolution speleothem ð18O record from SW Ireland. Science 294: 1328-1331. Noon, P.E., Leng, M.J. and Jones, V.J. 2003. Oxygenisotope (ð18O)
evidence of Holocene hydrological changes at Signy Island, maritime Antarctica. The Holocene 13: 251-263. Williams, M.B., Aydin, M., Tatum, C. and Saltzman, E.S. 2007. A 2000 year atmospheric
history of methyl chloride from a South Pole ice core: Evidence for climate-controlled variability. Geophysical Research Letters 34: 10.1029/2006GL029142. Yoshida, Y., Wang, Y.H., Zeng,
T. and Yantosea, R. 2004. A three-dimensional global model study of atmospheric methyl chloride budget and distributions. Journal of Geophysical Research 109: 10.1029/2004JD004951. 3.2.4.
Arctic Dahl-Jensen et al. (1998) used temperature measurements from two Greenland Ice Sheet boreholes to reconstruct the temperature history of this portion of the earth over the past
50,000 years. Their data indicate that after the termination of the glacial period, temperatures steadily rose to a maximum of 2.5°C warmer than at present during the Holocene Climatic
Optimum (4,000 to 7,000 years ago). The Medieval Warm Period and Little Ice Age were also documented in the record, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present,
respectively. After the Little Ice Age, they report that temperatures once again rose, but that they “have decreased during the last decades.” These results thus clearly indicate that
the Medieval Warm Period in this part of the Arctic was significantly warmer than current temperatures. Wagner and Melles (2001) also worked on Greenland, where they extracted a 3.5-m-long
sediment core from a lake (Raffels So) on an island (Raffles O) located just off Liverpool Land on the east coast of Greenland, which they analyzed for a number of properties related
to the past presence of seabirds there, obtaining a 10,000-year record that tells us much about the region’s climatic history. Key to the study were biogeochemical data that, in the
words of the researchers, reflect “variations in seabird breeding colonies in the catchment which influence nutrient and cadmium supply to the lake.” Wagner and Melles’ data reveal sharp
increases in the values of the parameters they measured between about 1100 and 700 years before present (BP), indicative of the summer presence of significant numbers of seabirds during
that “medieval warm period,” as they describe it, which had been preceded by a several-hundred-year period (Dark Ages Cold Period) of little to no bird presence. Thereafter, their data
suggest another absence of birds during what they call “a subsequent Little Ice Age,” which they note was “the coldest period since the early Holocene in East Greenland.” The Raffels
So data also show signs of a “resettlement of seabirds during the last 100 years, indicated by an increase of organic matter in the lake sediment and confirmed by bird observations.”
However, values of the most recent measurements are not as great as those obtained from the earlier Medieval Warm Period, which indicates that higher temperatures prevailed during the
period from 1,100 to 700 years BP than what has been observed over the most recent hundred years. A third relevant Greenland study was conducted by Kaplan et al. (2002), who derived
a climatic history of the Holocene by analyzing the physicalchemical properties of sediments obtained from a small lake in southern Greenland. They determined that the interval from
6,000 to 3,000 years BP was marked by warmth and stability, but that the climate cooled thereafter until its culmination in the Little Ice Age. From 1,300-900 years BP, however, there
was a partial amelioration during the Medieval Warm Period, which was associated with an approximate 1.5°C rise in temperature. In a non-Greenland Arctic study, Jiang et al. (2002) analyzed
diatom assemblages from a highresolution core extracted from the seabed of the north Icelandic shelf to reconstruct a 4,600-year history of mean summer sea surface temperature at that
location. Starting from a maximum value of about 8.1°C at 4,400 years BP, the climate was found to have cooled fitfully for about 1,700 years and then more consistently over the final
2,700 years of the record. The most dramatic departure from this longterm decline was centered on about 850 years BP, during the Medieval Warm Period, when the temperature rose by more
than 1°C above the line describing the long-term downward trend to effect an almost complete recovery from the colder temperatures of the Dark Ages Cold Period, after which temperatures
continued their descent into the Little Ice Age, ending with a final most recent value of approximately 6.3°C. These data also clearly indicate that the Medieval Warm Period in this
part of the Arctic was significantly warmer than it is there now.
Climate Change Reconsidered 78 Moore et al. (2001) analyzed sediment cores from Donard Lake, Baffin Island, Canada, producing a 1,240-year record of average summer temperatures for this
Arctic region. Over the entire period from AD 750-1990, temperatures averaged 2.9°C. However, anomalously warm decades with summer temperatures as high as 4°C occurred around AD 1000
and 1100, while at the beginning of the thirteenth century, Donard Lake witnessed “one of the largest climatic transitions in over a millennium,” as “average summer temperatures rose
rapidly by nearly 2°C from 1195-1220 AD, ending in the warmest decade in the record” with temperatures near 4.5°C. This rapid warming of the thirteenth century was followed by a period
of extended warmth that lasted until an abrupt cooling event occurred around 1375 and made the following decade one of the coldest in the record. This event signaled the onset of the
Little Ice Age, which lasted for 400 years, until a gradual warming trend began about 1800, which was followed by a dramatic cooling event in 1900 that brought temperatures back to levels
similar to those of the Little Ice Age. This cold regime lasted until about 1950, whereupon temperatures warmed for about two decades but then tended downwards again all the way to the
end of the record in 1990. Hence, in this part of the Arctic the Medieval Warm Period was also warmer than it is there currently. Grudd et al. (2002) assembled tree-ring widths from
880 living, dead, and subfossil northern Swedish pines into a continuous and precisely dated chronology covering the period 5407 BC to AD 1997. The strong association between these data
and summer (June-August) mean temperatures of the last 129 years of the period then enabled them to produce a 7,400-year history of summer mean temperature for northern Swedish Lapland.
The most dependable portion of this record, based upon the number of trees that were sampled, consisted of the last two millennia, which the authors say “display features of century-timescale
climatic variation known from other proxy and historical sources, including a warm ‘Roman’ period in the first centuries AD and a generally cold ‘Dark Ages’ climate from about AD 500
to about AD 900.” They also note that “the warm period around AD 1000 may correspond to a so-called ‘Mediaeval Warm Period,’ known from a variety of historical sources and other proxy
records.” Lastly, they say “the climatic deterioration in the twelfth century can be regarded as the starting point of a prolonged cold period that continued to the first decade of the
twentieth century,” which “Little Ice Age,” in their words, is also “known from instrumental, historical and proxy records.” Going back further in time, the tree-ring record displays
several more of these relatively warmer and colder periods. They report that “the relatively warm conditions of the late twentieth century do not exceed those reconstructed for several
earlier time intervals.” Seppa and Birks (2002) used a recently developed pollen-climate reconstruction model model and a new pollen stratigraphy from Toskaljarvi—a tree-line lake in
the continental sector of northern Fenoscandia (located just above 69°N latitude)—to derive quantitative estimates of annual precipitation and July mean temperature. As they describe
it, their reconstructions “agree with the traditional concept of a ‘Medieval Warm Period’ (MWP) and ‘Little Ice Age’ in the North Atlantic region (Dansgaard et al., 1975) and in northern
Fennoscandia (Korhola et al., 2000).” In addition, they report there is “a clear correlation between [their] MWP reconstruction and several records from Greenland ice cores,” and that
“comparisons of a smoothed July temperature record from Toskaljavri with measured borehole temperatures of the GRIP and Dye 3 ice cores (Dahl-Jensen et al., 1998) and the d18O record
from the Crete ice core (Dansgaard et al., 1975) show the strong similarity in timing of the MWP between the records.” Finally, they note that “July temperature values during the Medieval
Warm Period (ca. 1400-1000 cal yr B.P.) were ca. 0.8°C higher than at present,” where present means the last six decades of the twentieth century. Noting that temperature changes in
high latitudes are (1) sensitive indicators of global temperature changes, and that they can (2) serve as a basis for verifying climate model calculations, Naurzbaev et al. (2002) developed
a 2,427-year proxy temperature history for the part of the Taimyr Peninsula of northern Russia that lies between 70°30’ and 72°28’ North latitude, based on a study of ring-widths of
living and preserved larch trees, noting further that “it has been established that the main driver of tree-ring variability at the polar timber-line [where they worked] is temperature
(Vaganov et al., 1996; Briffa et al., 1998; Schweingruber and Briffa, 1996).” In doing so, they found that “the warmest periods over the last two millennia in this region were clearly
in the third [Roman Warm Period], tenth to twelfth [Medieval Warm Period] and during the twentieth [Current Warm Period] cent
uries.” Observations: Temperature Records 79 With respect to the second of these periods, they emphasize that “the warmth of the two centuries AD 1058-1157 and 950-1049 attests to the
reality of relative mediaeval warmth in this region.” Their data also reveal three other important pieces of information: (1) the Roman and Medieval Warm Periods were both warmer than
the Current Warm Period has been to date, (2) the “beginning of the end” of the Little Ice Age was somewhere in the vicinity of 1830, and (3) the Current Warm Period peaked somewhere
in the vicinity of 1940. All of these observations are at odds with what is portrayed in the thousand-year Northern Hemispheric “hockey stick” temperature history of Mann et al. (1998,
1999) and its thousand-year global extension developed by Mann and Jones (2003). Knudsen et al. (2004) documented climatic changes over the past 1,200 years by means of highresolution
multi-proxy studies of benthic and planktonic foraminiferal assemblages, stable isotopes, and ice-rafted debris found in three sediment cores retrieved from the North Icelandic shelf.
This work revealed that “the time period between 1200 and around 7,800 cal. years BP, including the Medieval Warm Period, was characterized by relatively high bottom and surface water
temperatures,” after which “a general temperature decrease in the area marks the transition to ... the Little Ice Age.” They also note that “minimum sea-surface temperatures were reached
at around 350 cal. BP, when very cold conditions were indicated by several proxies.” Thereafter, they say “a modern warming of surface waters ... is not [our italics] registered in the
proxy data,” and that “there is no clear indication of warming of water masses in the area during the last decades,” even in sea surface temperatures measured over the period 1948-2002.
Grinsted et al. (2006) developed “a model of chemical fractionation in ice based on differing elution rates for pairs of ions ... as a proxy for summer melt (1130-1990),” based on data
obtained from a 121-meter-long ice core they extracted from the highest ice field in Svalbard (Lomonosovfonna: 78°51’53”N, 17°25’30”E), which was “validated against twentieth-century
instrumental records and longer historical climate proxies.” This history indicated that “in the oldest part of the core (1130-1200), the washout indices [were] more than 4 times as
high as those seen during the last century, indicating a high degree of runoff.” In addition, they report they have performed regular snow pit studies near the ice core site since 1997
(Virkkunen, 2004) and that “the very warm 2001 summer resulted in similar loss of ions and washout ratios as the earliest part of the core.” They then state that “this suggests that
the Medieval Warm Period in Svalbard summer conditions [was] as warm (or warmer) as present-day, consistent with the Northern Hemisphere temperature reconstruction of Moberg et al. (2005).”
In addition, they conclude that “the degree of summer melt was significantly larger during the period 1130-1300 than in the 1990s,” which likewise suggests that a large portion of the
Medieval Warm Period was significantly warmer than the peak warmth (1990s) of the Current Warm Period. Besonen et al. (2008) derived thousand-year histories of varve thickness and sedimentation
accumulation rate for Canada’s Lower Murray Lake (81°20’N, 69°30’W), which is typically covered for about 11 months of each year by ice that reaches a thickness of 1.5 to 2 meters at
the end of each winter. With respect to these parameters, they say—citing seven other studies—that “field-work on other High Arctic lakes clearly indicates that sediment transport and
varve thickness are related to temperatures during the short summer season that prevails in this region, and we have no reason to think that this is not the case for Lower Murray Lake.”
They found “the twelfth and thirteenth centuries were relatively warm,” with their data indicating that Lower Murray Lake and its environs were often much warmer during this time period
(AD 1080-1320) than they were at any point in the twentieth century, which has also been shown to be the case for Donard Lake (66.25°N, 62°W) by Moore et al. (2001). The studies reviewed
above indicate that the Arctic—which climate models suggest should be sensitive to greenhouse-gas-induced warming—is still not as warm as it was many centuries ago during portions of
the Medieval Warm Period, when there was much less CO2 and methane in the air than there is today. This further suggests the planet’s more modest current warmth need not be the result
of historical increases in these two greenhouse gases. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.or
/subject/a/arcticmwp.php. References Besonen, M.R., Patridge, W., Bradley, R.S., Francus, P., Stoner, J.S. and Abbott, M.B. 2008. A record of climate over the last millennium based on
varved lake sediments from the Canadian High Arctic. The Holocene 18: 169-180.
Climate Change Reconsidered 80 Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., Shiyatov, S.G. and Vaganov, E.A. 1998. Reduced sensitivity of recent tree-growth to temperature
at high northern latitudes. Nature 391: 678-682. Dahl-Jensen, D., Mosegaard, K., Gundestrup, N., Clow, G.D., Johnsen, S.J., Hansen, A.W. and Balling, N. 1998. Past temperatures directly
from the Greenland Ice Sheet. Science 282: 268-271. Dansgaard, W., Johnsen, S.J., Gundestrup, N., Clausen, H.B. and Hammer, C.U. 1975. Climatic changes, Norsemen and modern man. Nature
255: 24-28. Grinsted, A., Moore, J.C., Pohjola, V., Martma, T. and Isaksson, E. 2006. Svalbard summer melting, continentality, and sea ice extent from the Lomonosovfonna ice core. Journal
of Geophysical Research 111: 10.1029/2005JD006494. Grudd, H., Briffa, K.R., Karlén, W., Bartholin, T.S., Jones, P.D. and Kromer, B. 2002. A 7400-year tree-ring chronology in northern
Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. timescales. The Holocene 12: 657-665. Jiang, H., Seidenkrantz, M-S., Knudsen, K.L. and Eiriksson,
J. 2002. Late-Holocene summer sea-surface temperatures based on a diatom record from the north Icelandic shelf. The Holocene 12: 137-147. Kaplan, M.R., Wolfe, A.P. and Miller, G.H. 2002.
Holocene environmental variability in southern Greenland inferred from lake sediments. Quaternary Research 58: 149-159. Knudsen, K.L., Eiriksson, J., Jansen, E., Jiang, H., Rytter, F.
and Gudmundsdottir, E.R. 2004. Palaeoceanographic changes off North Iceland through the last 1200 years: foraminifera, stable isotopes, diatoms and ice rafted debris. Quaternary Science
Reviews 23: 2231-2246. Korhola, A., Weckstrom, J., Holmstrom, L. and Erasto, P. 2000. A quantitative Holocene climatic record from diatoms in northern Fennoscandia. Quaternary Research
54: 284-294. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787. Mann, M.E., Bradley,
R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762. Mann, M.E.
and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814. Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko,
N.M. and Karlén, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low-and high-resolution proxy data. Nature 433: 613-617. Moore, J.J., Hughen, K.A., Miller,
G.H. and Overpeck, J.T. 2001. Little Ice Age recorded in summer temperature reconstruction from varved sediments of Donard Lake, Baffin Island, Canada. Journal of Paleolimnology 25:
503-517. Naurzbaev, M.M., Vaganov, E.A., Sidorova, O.V. and Schweingruber, F.H. 2002. Summer temperatures in eastern Taimyr inferred from a 2427-year late-Holocene tree-ring chronology
and earlier floating series. The Holocene 12: 727-736. Schweingruber, F.H. and Briffa, K.R. 1996. Tree-ring density network and climate reconstruction. In: Jones, P.D., Bradley, R.S.
and Jouzel, J. (Eds.), Climatic Variations and Forcing Mechanisms of the Last 2000 Years, NATO ASI Series 141. Springer-Verlag, Berlin, Germany, pp. 43-66. Seppa, H. and Birks, H.J.B.
2002. Holocene climate reconstructions from the Fennoscandian tree-line area based on pollen data from Toskaljavri. Quaternary Research 57: 191-199. Vaganov, E.A., Shiyatov, S.G. and
Mazepa, V.S. 1996. Dendroclimatic Study in Ural-Siberian Subarctic. Nauka, Novosibirsk, Russia. Virkkunen, K. 2004. Snowpit Studies in 2001-2002 in Lomonosovfonna, Svalbard. M.S. Thesis,
University of Oulu, Oulu, Finland. Wagner, B. and Melles, M. 2001. A Holocene seabird record from Raffles So sediments, East Greenland, in response to climatic and oceanic changes. Boreas
30: 228-239. 3.2.5. Asia 3.2.5.1. China Using a variety of climate records derived from peat, lake sediment, ice core, tree-ring and other proxy sources, Yang et al. (2002) identified
a period of exceptional warmth throughout China between AD 800 and 1100. Yafeng et al. (1999) also observed a warm period between AD 970 and 1510 in d18O data obtained from the Guliya
ice cap of the Qinghai-Tibet Plateau. Similarly, Hong et al. (2000) developed a 6,000-year d18O record from plant cellulose deposited in a peat bog in the Jilin Province (42° 20’ N,
126° 22’ E), within which they found evidence of “an obvious warm period represented by the high d18O
Observations: Temperature Records 81 from around AD 1100 to 1200 which may correspond to the Medieval Warm Epoch of Europe.” Shortly thereafter, Xu et al. (2002) determined from a study
of plant cellulose d18O variations in cores retrieved from peat deposits at the northeastern edge of the Qinghai-Tibet Plateau that from AD 1100-1300 “the d18O of Hongyuan peat cellulose
increased, consistent with that of Jinchuan peat cellulose and corresponding to the ‘Medieval Warm Period’.” In addition, Qian and Zhu (2002) analyzed the thickness of laminae in a stalagmite
found in Shihua Cave, Beijing, from whence they inferred the existence of a relatively wet period running from approximately AD 940 to 1200. Hong et al. (2000) also report that at the
time of the MWP “the northern boundary of the cultivation of citrus tree (Citrus reticulata Blanco) and Boehmeria nivea (a perennial herb), both subtropical and thermophilous plants,
moved gradually into the northern part of China, and it has been estimated that the annual annual mean temperature was 0.9-1.0°C higher than at present.” Considering the climatic conditions
required to successfully grow these plants, they further note that annual mean temperatures in that part of the country during the Medieval Warm Period must have been about 1.0°C higher
than at present, with extreme January minimum temperatures fully 3.5°C warmer than they are today, citing De’er (1994). Chu et al. (2002) studied the geochemistry of 1,400 years of dated
sediments recovered from seven cores taken from three locations in Lake Huguangyan (21°9’N, 110°17’E) on the low-lying Leizhou Peninsula in the tropical region of South China, together
with information about the presence of snow, sleet, frost, and frozen rivers over the past 1,000 years obtained from historical documents. They report that “recent publications based
on the phenological phenomena, distribution patterns of subtropical plants and cold events (Wang and Gong, 2000; Man, 1998; Wu and Dang, 1998; Zhang, 1994) argue for a warm period from
the beginning of the tenth century AD to the late thirteenth century AD,” as their own data also suggest. In addition, they note there was a major dry period from AD 880-1260, and that
“local historical chronicles support these data, suggesting that the climate of tropical South China was dry during the ‘Mediaeval Warm Period’.” Paulsen et al. (2003) used high-resolution
d13C and d18O data derived from a stalagmite found in Buddha Cave [33°40’N, 109°05’E] to infer changes in climate in central China for the past 1,270 years. Among the climatic episodes
evident in their data were “those corresponding to the Medieval Warm Period, Little Ice Age and 20th-century warming, lending support to the global extent of these events.” In terms
of timing, the dry-then-wet-then-dry-again MWP began about AD 965 and continued to approximately AD 1475. Also working with a stalagmite, this one from Jingdong Cave about 90 km northeast
of Beijing, Ma et al. (2003) assessed the climatic history of the past 3,000 years at 100-year intervals on the basis of d18O data, the Mg/Sr ratio, and the solid-liquid distribution
coefficient of Mg. They found that between 200 and 500 years ago, “air temperature was about 1.2°C lower than that of the present,” but that between 1,000 and 1,300 years ago, there
was an equally aberrant but warm period that “corresponded to the Medieval Warm Period in Europe.” Based on 200 sets of phenological and meteorological records extracted from a number
of historical sources, many of which are described by Gong and Chen (1980), Man (1990, 2004), Sheng (1990), and Wen and Wen (1996), Ge et al. (2003) produced a 2,000-year history of
winter half-year temperature (October to April, when CO2-induced global warming is projected to be most evident) for the region of China bounded by latitudes 27° and 40°N and longitudes
107° and 120°E. Their work revealed a significant warm epoch that lasted from the AD 570s to the 1310s, the peak warmth of which was “about 0.3-0.6°C higher than present for 30-year
periods, but over 0.9°C warmer on a 10-year basis.” Bao et al. (2003) utilized proxy climate records (ice-core d18O, peat-cellulose d18O, tree-ring widths, tree-ring stable carbon isotopes,
total organic carbon, lake water temperatures, glacier fluctuations, ice-core CH4, magnetic parameters, pollen assemblages, and sedimentary pigments) obtained from 20 prior studies to
derive a 2,000-year temperature history of the northeastern, southern and western sections of the Tibetan Plateau. In each case, there was more than one prior 50-year period of time
when the mean temperature of each region was warmer than it was over the most recent 50-year period. In the case of the northeastern sector of the plateau, all of the maximum-warmth
intervals occurred during the Medieval Warm Period; in the case of the western sector, they occurred near the end of the Roman Warm Period, and in the case of the southern sector they
occurred during both warm periods. From these several studies, it is evident that for a considerable amount of time during the Medieval
Climate Change Reconsidered 82 Warm Period, many parts of China exhibited warmer conditions than those of modern times. Since those earlier high temperatures were caused by something
other than high atmospheric CO2 concentrations, whatever was responsible for them could be responsible for the warmth of today. Additional information on this topic, including reviews
of newer publications as they become available, can be found at http://www.co2science.org/subject/m/mwpchina.php. References Bao, Y., Brauning, A. and Yafeng, S. 2003. Late Holocene
temperature fluctuations on the Tibetan Plateau. Quaternary Science Reviews 22: 2335-2344. Chu, G., Liu, J., Sun, Q., Lu, H., Gu, Z., Wang, W. and Liu, T. 2002. The ‘Mediaeval Warm Period’
drought recorded in Lake Huguangyan, tropical South China. The Holocene 12: 511-516. De’er, Z. 1994. Evidence for the existence of the medieval warm period in China. Climatic Change
26: 289-297. Ge, Q., Zheng, J., Fang, X., Man, Z., Zhang, X., Zhang, P. and Wang, W.-C. 2003. Winter half-year temperature reconstruction for the middle and lower reaches of the Yellow
River and Yangtze River, China, during the past 2000 years. The Holocene 13: 933-940. Gong, G. and Chen, E. 1980. On the variation of the growing season and agriculture. Scientia Atmospherica
Sinica 4: 24-29. Hong, Y.T., Jiang, H.B., Liu, T.S., Zhou, L.P., Beer, J., Li, H.D., Leng, X.T., Hong, B. and Qin, X.G. 2000. Response of climate to solar forcing recorded in a 6000-year
d18O time-series of Chinese peat cellulose. The Holocene 10: 1-7. Ma, Z., Li, H., Xia, M., Ku, T., Peng, Z., Chen, Y. and Zhang, Z. 2003. Paleotemperature changes over the past 3000
years
in eastern Beijing, China: A reconstruction based on Mg/Sr records in a stalagmite. Chinese Science Bulletin 48: 395-400. Man, M.Z. 1998. Climate in Tang Dynasty of China: discussion
for its evidence. Quaternary Sciences 1: 20-30. Man, Z. 1990. Study on the cold/warm stages of Tang Dynasty and the characteristics of each cold/warm stage. Historical Geography 8: 1-15.
Man, Z. 2004. Climate Change in Historical Period of China. Shandong Education Press, Ji’nan, China. Paulsen, D.E., Li, H.-C. and Ku, T.-L. 2003. Climate variability in central China
over the last 1270 years revealed by high-resolution stalagmite records. Quaternary Science Reviews 22: 691-701. Qian, W. and Zhu, Y. 2002. Little Ice Age climate near Beijing, China,
inferred from historical and stalagmite records. Quaternary Research 57: 109-119. Sheng, F. 1990. A preliminary exploration of the warmth and coldness in Henan Province in the historical
period. Historical Geography 7: 160-170. Wang, S.W. and Gong, D.Y. 2000. The temperature of several typical periods during the Holocene in China. The Advance in Nature Science 10: 325-332.
Wen, H. and Wen, H. 1996. Winter-Half-Year Cold/Warm Change in Historical Period of China. Science Press, Beijing, China. Wu, H.Q. and Dang, A.R. 1998. Fluctuation and characteristics
of climate change in temperature of Sui-Tang times in China. Quaternary Sciences 1: 31-38. Xu, H., Hong, Y., Lin, Q., Hong, B., Jiang, H. and Zhu, Y. 2002. Temperature variations in
the past 6000 years inferred from d18O of peat cellulose from Hongyuan, China. Chinese Science Bulletin 47: 1578-1584. Yafeng, S., Tandong, Y. and Bao, Y. 1999. Decadal climatic variations
recorded in Guliya ice core and comparison with the historical documentary data from East China during the last 2000 years. Science in China Series D-Earth Sciences 42 Supp.: 91-100.
Yang, B., Braeuning, A., Johnson, K.R. and Yafeng, S. 2002. General characteristics of temperature variation in China during the last two millennia. Geophysical Research Letters 29:
10.1029/2001GL014485. Zhang, D.E. 1994. Evidence for the existence of the Medieval Warm Period in China. Climatic Change 26: 287-297. 3.2.5.2. Russia Demezhko and Shchapov (2001) studied
a borehole extending to more than 5 km depth, reconstructing an 80,000-year history of ground surface temperature in the Middle Urals within the western rim of the Tagil subsidence (58°24’
N, 59°44’E). The reconstructed temperature history revealed the existence of a number of climatic excursions, including, in their words, the “Medieval Warm Period with a culmination
about 1000 years ago.” Further north, Hiller et al. (2001) analyzed subfossil wood samples from the Khibiny mountains
Observations: Temperature Records 83 on the Kola Peninsula of Russia (67-68°N, 33-34°E) in an effort to reconstruct the region’s climate history over the past 1,500 years. They determined
that between AD 1000 and 1300 the tree-line was located at least 100-140 m above its current elevation. This observation, in their words, suggests that mean summer temperatures during
this “Medieval climatic optimum” were “at least 0.8°C higher than today,” and that “the Medieval optimum was the most pronounced warm climate phase on the Kola Peninsula during the last
1500 years.” Additional evidence for the Medieval Warm Period in Russia comes from Naurzbaev and Vaganov (2000), who developed a 2,200-year proxy temperature record (212 BC to 1996 AD)
using treering data obtained from 118 trees near the upper timberline in Siberia. Based on their results, they concluded that the warming experienced in the twentieth century was “not
extraordinary,” and that “the warming at the border of the first and second millennia was longer in time and similar in amplitude.” Krenke and Chernavskaya (2002) present an impressive
overview of what is known about the MWP within Russia, as well as throughout the world, based on historical evidence, glaciological evidence, hydrologic evidence, dendrological data,
archaeological data, and palynological data. Concentrating on data wholly from within Russia, they report large differences in a number of variables between the Little Ice Age (LIA)
and MWP. With respect to the annual mean temperature of northern Eurasia, they report an MWP to LIA drop on the order of 1.5°C. They also say that “the frequency of severe winters reported
was increased from once in 33 years in the early period of time, which corresponds to the MWP, to once in 20 years in the LIA,” additionally noting that “the abnormally severe winters
[of the LIA] were associated with the spread of Arctic air masses over the entire Russian Plain.” Finally, they note that the data they used to draw these conclusions were “not used
in the reconstructions performed by Mann et al.,” which perhaps explains why the Mann et al. temperature history of the past millennium does not depict the coolness of the LIA or the
warmth of the MWP nearly as well as the more appropriately derived temperature history of Esper et al. (2002). In discussing their approach to the subject of global warming detection
and attribution, the Russians state that “an analysis of climate variations over 1000 years should help … reveal natural multicentennial variations possible at present but not detectable
in available 100-200-year series of instrumental records.” In this endeavor, they were highly successful, stating unequivocally that “the Medieval Warm Period and the Little Ice Age
existed globally.” Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/m/mwprussia.php.
References Demezhko, D.Yu. and Shchapov, V.A. 2001. 80,000 years ground surface temperature history inferred from the temperature-depth log measured in the superdeep hole SG-4 (the Urals,
Russia). Global and Planetary Change 29: 167-178. Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Lowfrequency signals in long tree-ring chronologies for reconstructing past temperature
variability. Science 295: 2250-2253. Hiller, A., Boettger, T. and Kremenetski, C. 2001. Medieval climatic warming recorded by radiocarbon dated alpine tree-line shift on the Kola Peninsula,
Russia. The Holocene 11: 491-497. Krenke, A.N. and Chernavskaya, M.M. 2002. Climate changes in the preinstrumental period of the last millennium and their manifestations over the Russian
Plain. Isvestiya, Atmospheric and Oceanic Physics 38: S59-S79. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six
centuries. Nature 392: 779-787. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.
Geophysical Research Letters 26: 759-762. Naurzbaev, M.M. and Vaganov, E.A. 2000. Variation of early summer and annual temperature in east Taymir and Putoran (Siberia) over the last
two millennia inferred from tree rings. Journal of Geophysical Research 105: 7317-7326. 3.2.5.3. Other Asia Locations In addition to China and Russia, the Medieval Warm Period (MWP)
has been identified in several other parts of Asia.
Climate Change Reconsidered 84 Schilman et al. (2001) analyzed foraminiferal oxygen and carbon isotopes, together with the physical and geochemical properties of sediments, contained
in two cores extracted from the bed of the southeastern Mediterranean Sea off the coast of Israel, where they found evidence for the MWP centered on AD 1200. In discussing their findings,
they note there is an abundance of other evidence for the existence of the MWP in the Eastern Mediterranean as well, including, in their words, “high Saharan lake levels (Schoell, 1978;
Nicholson, 1980), high Dead Sea levels (Issar et al., 1989, 1991; Issar, 1990, 1998; Issar and Makover-Levin, 1996), and high levels of the Sea of Galilee (Frumkin et al., 1991; Issar
and Makover-Levin, 1996),” in addition to “a precipitation maximum at the Nile headwaters (Bell and Menzel, 1972; Hassan, 1981; Ambrose and DeNiro, 1989) and in the northeastern Arabian
Sea (von Rad et al., 1999).” Further to the east, Kar et al. (2002) explored the nature of climate change preserved in the sediment profile of an outwash plain two to three km from the
snout of the Gangotri Glacier in the Uttarkashi district of Uttranchal, Western Himalaya. Between 2,000 and 1,700 years ago, their data reveal the existence of a relatively cool climate.
Then, from 1,700 to 850 years ago, there was what they call an “amelioration of climate,” during the transition from the depth of the Dark Ages Cold Period to the midst of the Medieval
Warm Period. Subsequent to that time, Kar et al.’s data indicate the climate “became much cooler,” indicative of its transition to Little Ice Age conditions, while during the last 200
years there has been a rather steady warming, as shown by Esper et al. (2002a) to have been characteristic of the entire Northern Hemisphere. At a pair of other Asian locations, Esper
et al. (2002b) used more than 200,000 ring-width measurements obtained from 384 trees at 20 individual sites ranging from the lower to upper timberline in the Northwest Karakorum of
Pakistan (35-37°N, 74-76°E) and the Southern Tien Shan of Kirghizia (40°10’N, 72°35’E) to reconstruct regional patterns of climatic variations in Western Central Asia since AD 618. According
to their analysis, the Medieval Warm Period was already firmly established and growing even warmer by the early seventh century; and between AD 900 and 1000, tree growth was exceptionally
rapid, at rates they say “cannot be observed during any other period of the last millennium.” Between AD 1000 and 1200, however, growing conditions deteriorated; and at about 1500, minimum
tree ring-widths were reached that persisted well into the seventeenth century. Towards the end of the twentieth century, ring-widths increased once again; but Esper et al. (2002b) report
that “the twentiethcentury trend does not approach the AD 1000 maximum.” In fact, there is almost no comparison between the two periods, with the Medieval Warm Period being much more
conducive to good tree growth than the Current Warm Period. As the authors describe the situation, “growing conditions in the twentieth century exceed the long-term average, but the
amplitude of this trend is not comparable to the conditions around AD 1000.” The latest contribution to Asian temperature reconstruction is the study of Esper et al. (2003), who processed
several extremely long juniper ring-width chronologies for the Alai Range of the western Tien Shan in Kirghizia in such a way as to preserve multicentennial growth trends that are typically
“lost during the processes of tree ring data standardization and chronology building (Cook and Kairiukstis, 1990; Fritts, 1976).” In doing so, they used two techniques that maintain
low frequency signals: long-term mean standardization (LTM) and regional curve standardization (RCS), as well as the more conventional spline standardization (SPL) technique that obscures
(actually removes) long-term trends. Carried back in time a full thousand years, the SPL chronologies depict significant inter-decadal variations but no longer-term trends. The LTM and
RCS RCS chronologies, on the other hand, show long-term decreasing trends from the start of the record until about AD 1600, broad minima from 1600 to 1800, and long-term increasing trends
from about 1800 to the present. As a result, in the words of Esper et al. (2003), “the main feature of the LTM and RCS Alai Range chronologies is a multi-centennial wave with high values
towards both ends.” This grand result has essentially the same form as the Northern Hemisphere extratropic temperature history of Esper et al. (2002a), which is vastly different from
the hockey stick temperature history of Mann et al. (1998, 1999) and Mann and Jones (2003), in that it depicts the existence of both the Little Ice Age and preceding Medieval Warm Period,
which are nowhere to be found in the Mann reconstructions. In addition, the new result—especially the LTM chronology, which has a much smaller variance than the RCS chronology—depicts
several periods in the first half of the last millennium that were warmer than
Observations: Temperature Records 85 any part of the last century. These periods include much of the latter half of the Medieval Warm Period and a good part of the first half of the
fifteenth century, which has also been found to have been warmer than it is currently by McIntyre and McKitrick (2003) and by Loehle (2004). In commenting on their important findings,
Esper et al. (2003) remark that “if the tree ring reconstruction had been developed using ‘standard’ detrending procedures only, it would have been limited to inter-decadal scale variation
and would have missed some of the common low frequency signal.” We would also remark, with respect to the upward trend of their data since 1800, that a good portion of that trend may
have been due to the aerial fertilization effect of the concomitantly increasing atmospheric CO2 content, which is known to greatly stimulate the growth of trees. Properly accounting
for this very real effect would make the warmer-thanpresent temperatures of the first half of the past millennium even warmer, relative to those of the past century, than they appear
to be in Esper et al.’s LTM and RCS reconstructions. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/
ubject/a/asiamwp.php. References Ambrose, S.H. and DeNiro, M.J. 1989. Climate and habitat reconstruction using stable carbon and nitrogen isotope ratios of collagen in prehistoric herbivore
teeth from Kenya. Quaternary Research 31: 407-422. Bell, B. and Menzel, D.H. 1972. Toward the observation and interpretation of solar phenomena. AFCRL F19628-69-C-0077 and AFCRL-TR-74-0357,
Air Force Cambridge Research Laboratories, Bedford, MA, pp. 8-12. Cook, E.R. and Kairiukstis, L.A. 1990. Methods of Dendrochronology: Applications in the Environmental Sciences. Kluwer,
Dordrecht, The Netherlands. Esper, J., Cook, E.R. and Schweingruber, F.H. 2002a. Low-frequency signals in long tree-ring chronologies and the reconstruction of past temperature variability.
Science 295: 2250-2253. Esper, J., Schweingruber, F.H. and Winiger, M. 2002b. 1300 years of climatic history for Western Central Asia inferred from tree-rings. The Holocene 12: 267-277.
Esper, J., Shiyatov, S.G., Mazepa, V.S., Wilson, R.J.S., Graybill, D.A. and Funkhouser, G. 2003. Temperaturesensitive Tien Shan tree ring chronologies show multicentennial growth trends.
Climate Dynamics 21: 699-706. Fritts, H.C. 1976. Tree Rings and Climate. Academic Press, London, UK. Frumkin, A., Magaritz, M., Carmi, I. and Zak, I. 1991. The Holocene climatic record
of the salt caves of Mount Sedom, Israel. Holocene 1: 191-200. Hassan, F.A. 1981. Historical Nile floods and their implications for climatic change. Science 212: 1142-1145. Issar, A.S.
1990. Water Shall Flow from the Rock. Springer, Heidelberg, Germany. Issar, A.S. 1998. Climate change and history during the Holocene in the eastern Mediterranean region. In: Issar,
A.S. and Brown, N. (Eds.) Water, Environment and Society in Times of Climate Change, Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 113-128. Issar, A.S. and Makover-Levin,
D. 1996. Climate changes during the Holocene in the Mediterranean region. In: Angelakis, A.A. and Issar, A.S. (Eds.) Diachronic Climatic Impacts on Water Resources with Emphasis on the
Mediterranean Region, NATO ASI Series, Vol. I, 36, Springer, Heidelberg, Germany, pp. 55-75. Issar, A.S., Tsoar, H. and Levin, D. 1989. Climatic changes in Israel during historical times
and their impact on hydrological, pedological and socio-economic systems. In: Leinen, M. and Sarnthein, M. (Eds.), Paleoclimatology and Paleometeorology: Modern and Past Patterns of
Global Atmospheric Transport, Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 535-541. Issar, A.S., Govrin, Y., Geyh, M.A., Wakshal, E. and Wolf, M. 1991. Climate changes
during the Upper Holocene in Israel. Israel Journal of Earth-Science 40: 219-223. Kar, R., Ranhotra, P.S., Bhattacharyya, A. and Sekar B. 2002. Vegetation vis-à-vis climate and glacial
glacial fluctuations of the Gangotri Glacier since the last 2000 years. Current Science 82: 347-351. Loehle, C. 2004. Climate change: detection and attribution of trends from long-term
geologic data. Ecological Modelling 171: 433-450. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries.
Nature 392: 779-787. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical
Research Letters 26: 759-762. Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.
Climate Change Reconsidered 86 McIntyre, S. and McKitrick, R. 2003. Corrections to the Mann et al. (1998) proxy data base and Northern Hemispheric average temperature series. Energy
and Environment 14: 751-771. Nicholson, S.E. 1980. Saharan climates in historic times. In: Williams, M.A.J. and Faure, H. (Eds.) The Sahara and the Nile, Balkema, Rotterdam, The Netherlands,
pp. 173-200. Schilman, B., Bar-Matthews, M., Almogi-Labin, A. and Luz, B. 2001. Global climate instability reflected by Eastern Mediterranean marine records during the late Holocene.
Palaeogeography, Palaeoclimatology, Palaeoecology 176: 157-176. Schoell, M. 1978. Oxygen isotope analysis on authigenic carbonates from Lake Van sediments and their possible bearing
on the climate of the past 10,000 years. In: Degens, E.T. (Ed.) The Geology of Lake Van, Kurtman. The Mineral Research and Exploration Institute of Turkey, Ankara, Turkey, pp. 92-97.
von Rad, U., Schulz, H., Riech, V., den Dulk, M., Berner, U. and Sirocko, F. 1999. Multiple monsoon-controlled breakdown of oxygen-minimum conditions during the past 30,000 years documented
in laminated sediments off Pakistan. Palaeogeography, Palaeoclimatology, Palaeoecology 152: 129-161. 3.2.6. Europe Based on analyses of subfossil wood samples from the Khibiny mountains
on the Kola Peninsula of Russia, Hiller et al. (2001) were able to reconstruct a 1,500-year history of alpine tree-line elevation. This record indicates that between AD 1000 and 1300,
the tree-line there was located at least 100 to 140 meters above its current location. The researchers state that this fact implies a mean summer temperature that was “at least 0.8°C
higher than today.” Moving from land to water, in a study of a welldated sediment core from the Bornholm Basin in the southwestern Baltic Sea, Andren et al. (2000) found evidence for
a period of high primary production at approximately AD 1050. Many of the diatoms of that period were warm water species that the scientists say “cannot be found in the present Baltic
Sea.” This balmy period, they report, “corresponds to the time when the Vikings succeeded in colonizing Iceland and Greenland.” The warmth ended rather abruptly, however, at about AD
1200, when they note there was “a major decrease in warm water taxa in the diatom assemblage and an increase in cold water taxa,” which latter diatoms are characteristic of what they
call the Recent Baltic Sea Stage that prevails to this day. In another marine study, Voronina et al. (2001) analyzed dinoflagellate cyst assemblages in two sediment cores retrieved from
the southeastern Barents Sea, one spanning a period of 8,300 years and one spanning a period of 4,400 years. The longer of the two cores indicated a warm interval from about 8,000 to
3,000 years before present, followed by cooling pulses coincident with lowered salinity and extended ice cover in the vicinity of 5,000, 3,500, and 2,500 years ago. The shorter core
additionally revealed cooling pulses at tentative dates of 1,400, 300, and 100 years before present. For the bulk of the the past 4,400 years, however, ice cover lasted only two to three
months per year, as opposed to the modern mean of 4.3 months per year. In addition, August temperatures ranged between 6° and 8°C, significantly warmer than the present mean of 4.6°C.
Moving back towards land, Mikalsen et al. (2001) made detailed measurements of a number of properties of sedimentary material extracted from the bottom of a fjord on the west coast of
Norway, deriving a relative temperature history of the region that spanned the last five millennia. This record revealed the existence of a period stretching from A.D. 1330 to 1600 that,
in their words, “had the highest bottom-water temperatures in Sulafjorden during the last 5000 years.” In eastern Norway, Nesje et al. (2001) analyzed a sediment core obtained from Lake
Atnsjoen, deriving a 4,500-year record of river flooding. They observed “a period of little flood activity around the Medieval period (AD 1000-1400),” which was followed by “a period
of the most extensive flood activity in the Atnsjoen catchment.” This flooding, in their words, resulted from the “post-Medieval climate deterioration characterized by lower air temperature,
thicker and more long-lasting snow cover, and more frequent storms associated with the ‘Little Ice Age’.” Working in both Norway and Scotland, Brooks and Birks (2001) studied midges,
the larval-stage head capsules of which are well preserved in lake sediments and are, in their words, “widely recognized as powerful biological proxies for inferring past climate change.”
Applying this technique to sediments derived from a lake in the Cairngorms region of the Scottish Highlands, they determined that temperatures there peaked at about 11°C during what
they refer to as the “Little Climatic Optimum”— which we typically call the Medieval Warm Period—
Observations: Temperature Records 87 “before cooling by about 1.5°C which may coincide with the ‘Little Ice Age’.” These results, according to Brooks and Birks, “are in good agreement
with a chironomid stratigraphy from Finse, western Norway (Velle, 1998),” where summer temperatures were “about 0.4°C warmer than the present day” during the Medieval Warm Period. This
latter observation also appears to hold for the Scottish site, since the upper sample of the lake sediment core from that region, which was collected in 1993, “reconstructs the modern
temperature at about 10.5°C,” which is 0.5°C less than the 11°C value the authors found for the Medieval Warm Period. Moving to Switzerland, Filippi et al. (1999) analyzed a sediment
core extracted from Lake Neuchatel in the western Swiss Lowlands. During this same transition from the Medieval Warm Period (MWP) to the Little Ice Age (LIA), they detected a drop of
approximately 1.5°C in mean annual air temperature. To give some context to this finding, they say that that “the warming during the 20th century does not seem to have fully compensated
the cooling at the MWP-LIA transition.” And to make the message even more clear, they add that during the Medieval Warm Period, the mean annual air temperature was “on average higher
than at present.” In Ireland, in a cave in the southwestern part of the country, McDermott et al. (2001) derived a ð18O record from a stalagmite that provided evidence for climatic variations
that are “broadly consistent with a Medieval Warm Period at ~1000 ± 200 years ago and a two-stage Little Ice Age.” Also evident in the data were the ð18O signatures of the earlier Roman
Warm Period and Dark Ages Cold Period that comprised the preceding millennial-scale cycle of climate in that region. In another study of three stalagmites found in a cave in northwest
Germany, Niggemann et al. (2003) discovered that the climate records they contained “resemble records from an Irish stalagmite (McDermott et al., 1999),” specifically noting that their
own records provide evidence for the existence of the Little Ice Age, the Medieval Warm Period and the Roman Warm Period, which evidence also implies the existence of what McDermott
et al. (2001) call the Dark Ages Cold Period that separated the Medieval and Roman Warm Periods, as well as the existence of the unnamed cold period that preceded the Roman Warm Period.
Bodri and Cermak (1999) derived individual ground surface temperature histories from the temperature-depth logs of 98 separate boreholes drilled in the Czech Republic. From these data
they detected “the existence of a medieval warm epoch lasting from A.D. 1100-1300,” which they describe as “one of the warmest postglacial times. Noting that this spectacular warm period
was followed by the Little Ice Age, they went on to suggest that “the observed recent warming may thus be easily a natural return of climate from the previous colder conditions back
to a ‘normal’.” Filippi et al. (1999) share similar views, as is demonstrated by their citing of Keigwin (1996) to the effect that “sea surface temperature (SST) reconstructions show
that SST was ca. 1°C cooler than today about 400 years ago and ca. 1°C warmer than today during the MWP.” Citing Bond et al. (1997), they further note that the MWP and LIA are merely
the most recent manifestations of “a pervasive millennial-scale coupled atmosphere-ocean climate oscillation,” which, we might add, has absolutely nothing to do with variations in the
air’s CO2 content. Lastly, we report the findings of Berglund (2003), who identified several periods of expansion and decline of human cultures in northwest Europe and compared them
with a history of reconstructed climate “based on insolation, glacier activity, lake and sea levels, bog growth, tree line, and tree growth.” In doing so, he determined there was a positive
correlation between human impact/land-use and climate change. Specifically, in the latter part of the record, where both cultural and climate changes were best defined, there was, in
his words, a great “retreat of of agriculture” centered on about AD 500, which led to “reforestation in large areas of central Europe and Scandinavia.” He additionally notes that “this
period was one of rapid cooling indicated from tree-ring data (Eronen et al., 1999) as well as sea surface temperatures based on diatom stratigraphy in [the] Norwegian Sea (Jansen and
Koc, 2000), which can be correlated with Bond’s event 1 in the North Atlantic sediments (Bond et al., 1997).” Next came what Berglund calls a “boom period” that covered “several centuries
from AD 700 to 1100.” This interval of time proved to be “a favourable period for agriculture in marginal areas of Northwest Europe, leading into the so-called Medieval Warm Epoch,”
when “the climate was warm and dry, with high treelines, glacier retreat, and reduced lake catchment erosion.” This period “lasted until around AD 1200, when there was a gradual change
to cool/moist climate, the beginning of the
Climate Change Reconsidered 88 Little Ice Age ... with severe consequences for the agrarian society.” The story from Europe seems quite clear. There was a several-hundred-year period
in the first part of the last millennium that was significantly warmer than it is currently. In addition, there is reason to believe the planet may be on a natural climate trajectory
that is taking it back to a state reminiscent of the Medieval Warm Period. There is nothing we can do about this natural cycle except, as is implied by the study of Berglund (2003),
reap the benefits. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/e/europemwp.php.
References Andren, E., Andren, T. and Sohlenius, G. 2000. The Holocene history of the southwestern Baltic Sea as reflected in a sediment core from the Bornholm Basin. Boreas 29: 233-250.
Berglund, B.E. 2003. Human impact and climate changes—synchronous events and a causal link? Quaternary International 105: 7-12. Bodri, L. and Cermak, V. 1999. Climate change of the last
millennium inferred from borehole temperatures: Regional patterns of climatic changes in the Czech Republic—Part III. Global and Planetary Change 21: 225-235. Bond, G., Showers, W.,
Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priori, P., Cullen, H., Hajdes, I. and Bonani, G. 1997. A pervasive millennial-scale climate cycle in the North Atlantic: The Holocene
and late glacial record. Science 278: 1257-1266. Brooks, S.J. and Birks, H.J.B. 2001. Chironomid-inferred air temperatures from Lateglacial and Holocene sites in north-west Europe: progress
and
problems. Quaternary Science Reviews 20: 1723-1741. Eronen, M., Hyvarinen, H. and Zetterberg, P. 1999. Holocene humidity changes in northern Finnish Lapland inferred from lake sediments
and submerged Scots pines dated by tree-rings. The Holocene 9: 569-580. Filippi, M.L., Lambert, P., Hunziker, J., Kubler, B. and Bernasconi, S. 1999. Climatic and anthropogenic influence
influence on the stable isotope record from bulk carbonates and ostracodes in Lake Neuchatel, Switzerland, during the last two millennia. Journal of Paleolimnology 21: 19-34. Hiller,
A., Boettger, T. and Kremenetski, C. 2001. Medieval climatic warming recorded by radiocarbon dated alpine tree-line shift on the Kola Peninsula, Russia. The Holocene 11: 491-497. Jansen,
E. and Koc, N. 2000. Century to decadal scale records of Norwegian sea surface temperature variations of the past 2 millennia. PAGES Newsletter 8(1): 13-14. Keigwin, L.D. 1996. The Little
Ice Age and Medieval Warm Period in the Sargasso Sea. Science 174: 1504-1508. McDermott, F., Frisia, S., Huang, Y., Longinelli, A., Spiro, S., Heaton, T.H.E., Hawkesworth, C., Borsato,
A., Keppens, E., Fairchild, I., van Borgh, C., Verheyden, S. and Selmo, E. 1999. Holocene climate variability in Europe: evidence from delta18O, textural and extensionrate variations
in speleothems. Quaternary Science Reviews 18: 1021-1038. McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution
speleothem ð18O record from SW Ireland. Science 294: 1328-1331. Mikalsen, G., Sejrup, H.P. and Aarseth, I. 2001. Late-Holocene changes in ocean circulation and climate: foraminiferal
and isotopic evidence from Sulafjord, western Norway. The Holocene 11: 437-446. Nesje, A., Dahl, S.O., Matthews, J.A. and Berrisford, M.S. 2001. A ~ 4500-yr record of river floods obtained
from a sediment core in Lake Atnsjoen, eastern Norway. Journal of Paleolimnology 25: 329-342. Niggemann, S., Mangini, A., Richter, D.K. and Wurth, G. 2003. A paleoclimate record of the
last 17,600 years in stalagmites from the B7 cave, Sauerland, Germany. Quaternary Science Reviews 22: 555-567. Velle, G. 1998. A paleoecological study of chironomids (Insecta: Diptera)
with special reference to climate. M.Sc. Thesis, University of Bergen. Voronina, E., Polyak, L., De Vernal, A. and Peyron, O. 2001. Holocene variations of sea-surface conditions in the
southeastern Barents Sea, reconstructed from dinoflagellate cyst assemblages. Journal of Quaternary Science 16: 717-726. 3.2.7. North America Arseneault and Payette (1997) analyzed tree-ring
and growth-form sequences obtained from more than 300 spruce remains buried in a presently treeless peatland in northern Quebec to produce a proxy record of climate for this region of
the continent between 690 and 1591 AD. Perhaps the most outstanding feature of this history was the warm period it revealed between 860 and 1000 AD. Based on the fact that the northernmost
twentieth century location of the forest
Observations: Temperature Records 89 tree-line is presently 130 km south of their study site, the scientists concluded that the “Medieval Warm Period was approximately 1°C warmer than
the 20th century.” Shifting to the other side of the continent, Calkin et al. (2001) carefully reviewed what they termed “the most current and comprehensive research of Holocene glaciation”
along the northernmost Gulf of Alaska between the Kenai Peninsula and Yakutat Bay, where they too detected a Medieval Warm Period that lasted for “at least a few centuries prior to A.D.
1200.” Also identifying the Medieval Warm Period, as well as other major warm and cold periods of the millennial-scale climatic oscillation that is responsible for them, was Campbell
(2002), who analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada (52°N, 113.5°W) to provide a non-vegetation-based high-resolution record of climate variability
for this part of North America over the past 4,000 years. Periods of both increasing and decreasing grain size (related to moisture availability) were noted throughout the 4,000-year
record at decadal, centennial, and millennial time scales. The most predominant departures were several-centuries-long epochs that corresponded to the Little Ice Age (about AD 1500-1900),
the Medieval Warm Period (about AD 700-1300), the Dark Ages Cold Period (about BC 100 to AD 700), and the Roman Warm Period (about BC 900-100). Laird et al. (2003) studied diatom assemblages
in sediment cores taken from three Canadian and three United States lakes situated within the northern prairies of North America, finding that “shifts in drought conditions on decadal
through multicentennial scales have prevailed in this region for at least the last two millennia.” In Canada, major shifts occurred near the beginning of the Medieval Warm Period, while
in the United States they occurred near its end. In giving some context to these findings, the authors state that “distinct patterns of abrupt change in the Northern Hemisphere are common
at or near the termination of the Medieval Warm Period (ca. A.D. 800-1300) and the onset of the Little Ice Age (ca. A.D. 1300-1850).” They also note that “millennial-scale shifts over
at least the past 5,500 years, between sustained periods of wetter and drier conditions, occurring approximately every 1,220 years, have been reported from western Canada (Cumming et
al., 2002),” and that “the striking correspondence of these shifts to large changes in fire frequencies, inferred from two sites several hundreds of kilometers to the southwest in the
mountain hemlock zone of southern British Columbia (Hallett et al., 2003), suggests that these millennial-scale dynamics are linked and operate over wide spatial scales.” In an effort
to determine whether these climatedriven millennial-scale cycles are present in the terrestrial pollen record of North America, Viau et al. (2002) analyzed a set of 3,076 14C dates from
the North American Pollen Database used to date sequences in more than 700 pollen diagrams across North America. Results of their statistical analyses indicated there were nine millennial-scale
oscillations during the past 14,000 years in which continent-wide synchronous vegetation changes with a periodicity of roughly 1,650 years were recorded in the pollen records. The most
recent of the vegetation transitions was centered at approximately 600 years BP (before present). This event, in the words of the authors, “culminat[ed] in the Little Ice Age, with maximum
cooling 300 years ago.” Prior to that event, a major transition that began approximately 1,600 years BP represents the climatic amelioration that “culminat[ed] in the maximum warming
of the Medieval Warm Period 1000 years ago.” And so it goes, on back through the Holocene and into the preceding late glacial period, with the times of all major pollen transitions being
“consistent,” in the words of the authors of the study, “with ice and marine records.” Viau et al. additionally note that “the large-scale nature of these transitions and the fact that
they are found in different proxies confirms the hypothesis that Holocene and late glacial climate variations of millennial-scale were abrupt transitions between climatic regimes as
the atmosphere-ocean system reorganized in response to some forcing.” They go on to say that “although several mechanisms for such natural [our italics] forcing have been advanced, recent
evidence points to a potential solar forcing (Bond et al., 2001) associated with oceanatmosphere feedbacks acting as global teleconnections agents.” Furthermore, they note that “these
transitions are identifiable across North America and presumably the world.” Additional evidence for the solar forcing of these millennial-scale climate changes is provided by Shindell
et al. (2001), who used a version of the Goddard Institute for Space Studies GCM to estimate climatic differences between the period of the Maunder Minimum in solar irradiance (mid-1600s
to early 1700s) and a century later, when solar output was relatively high for several decades. Their results
Climate Change Reconsidered 90 compared so well with historical and proxy climate data that they concluded, in their words, that “colder winter temperatures over the Northern Hemispheric
continents during portions of the 15th through the 17th centuries (sometimes called the Little Ice Age) and warmer temperatures during the 12th through 14th centuries (the putative Medieval
Warm Period) may have been influenced by long-term solar variations.” Rounding out our mini-review of the Medieval Warm Period in North America are two papers dealing with the climatic
history of the Chesapeake Bay region of the United States. The first, by Brush (2001), consists of an analysis of sediment cores obtained from the Bay’s tributaries, marshes and main
stem that covers the past millennium, in which it is reported that “the Medieval Climatic Anomaly and the Little Ice Age are recorded in Chesapeake sediments by terrestrial indicators
of dry conditions for 200 years, beginning about 1000 years ago, followed by increases in wet wet indicators from about 800 to 400 years ago.” Willard et al. (2003) studied the same
region for the period 2,300 years BP to the present, via an investigation of fossil dinoflagellate cysts and pollen from sediment cores. Their efforts revealed that “several dry periods
ranging from decades to centuries in duration are evident in Chesapeake Bay records.” The first of these periods of lower-thanaverage precipitation, which spanned the period 200 BC-AD
300, occurred during the latter part of the Roman Warm Period, as delineated by McDermott et al. (2001) on the basis of a high-resolution speleothem ð18O record from southwest Ireland.
The next such dry period (~AD 800-1200), in the words of the authors, “corresponds to the ‘Medieval Warm Period’, which has been documented as drier than average by tree-ring (Stahle
and Cleaveland, 1994) and pollen (Willard et al., 2001) records from the southeastern USA.” Willard et al. go on to say that “mid-Atlantic dry periods generally correspond to central
and southwestern USA ‘megadroughts’, described by Woodhouse and Overpeck (1998) as major droughts of decadal or more duration that probably exceeded twentieth-century droughts in severity.”
They further indicate that “droughts in the late sixteenth century that lasted several decades, and those in the ‘Medieval Warm Period’ and between ~AD 50 and AD 350 spanning a century
or more have been indicated by Great Plains tree-ring (Stahle et al., 1985; Stahle and Cleaveland, 1994), lacustrine diatom and ostracode (Fritz et al., 2000; Laird et al., 1996a, 1996b)
and detrital clastic records (Dean, 1997).” It is evident that the Medieval Warm Period has left its mark throughout North America in the form of either warm temperature anomalies or
periods of relative dryness. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/northamericamw
.php. References Arseneault, D. and Payette, S. 1997. Reconstruction of millennial forest dynamics from tree remains in a subarctic tree line peatland. Ecology 78: 1873-1883. Bond, G.,
Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during
the Holocene. Science 294: 2130-2136. Brush, G.S. 2001. Natural and anthropogenic changes in Chesapeake Bay during the last 1000 years. Human and Ecological Risk Assessment 7: 1283-1296.
Calkin, P.E., Wiles, G.C. and Barclay, D.J. 2001. Holocene coastal glaciation of Alaska. Quaternary Science Reviews 20: 449-461. Campbell, C. 2002. Late Holocene lake sedimentology and
climate change in southern Alberta, Canada. Quaternary Research 49: 96-101. Cumming, B.F., Laird, K.R., Bennett, J.R., Smol, J.P. and Salomon, A.K. 2002. Persistent millennial-scale
shifts in moisture regimes in western Canada during the past six millennia. Proceedings of the National Academy of Sciences USA 99: 16,117-16,121. Dean, W.E. 1997. Rates, timing, and
cyclicity of Holocene eolian activity in north-central United States: evidence from varved lake sediments. Geology 25: 331-334. Fritz, S.C., Ito, E., Yu, Z., Laird, K.R. and Engstrom,
D.R. 2000. Hydrologic variation in the northern Great Plains during the last two millennia. Quaternary Research 53: 175-184. Hallett, D.J., Lepofsky, D.S., Mathewes, R.W. and Lertzman,
K.P. 2003. 11,000 years of fire history and climate in the mountain hemlock rain forests of southwestern British Columbia based on sedimentary charcoal. Canadian Journal of Forest Research
33: 292-312.
Observations: Temperature Records 91 Laird, K.R., Fritz, S.C., Grimm, E.C. and Mueller, P.G. 1996a. Century-scale paleoclimatic reconstruction from Moon Lake, a closed-basin lake in
the northern Great Plains. Limnology and Oceanography 41: 890-902. Laird, K.R., Fritz, S.C., Maasch, K.A. and Cumming, B.F. 1996b. Greater drought intensity and frequency before AD 1200
in the Northern Great Plains, USA. Nature 384: 552-554. Laird, K.R., Cumming, B.F., Wunsam, S., Rusak, J.A., Oglesby, R.J., Fritz, S.C. and Leavitt, P.R. 2003. Lake sediments record
large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia. Proceedings of the National Academy of Sciences USA 100: 2483-2488.
McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem ð18O record from SW Ireland. Science 294:
1328-1331. Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D. and Waple, A. 2001. Solar forcing of regional climate change during the Maunder Minimum. Science 294: 2149-2152. Stahle,
D.W. and Cleaveland, M.K. 1994. Tree-ring reconstructed rainfall over the southeastern U.S.A. during the Medieval Warm Period and Little Ice Age. Climatic Change 26: 199-212. Stahle,
D.W., Cleaveland, M.K. and Hehr, J.G. 1985. A 450-year drought reconstruction for Arkansas, United States. Nature 316: 530-532. Viau, A.E., Gajewski, K., Fines, P., Atkinson, D.E. and
Sawada, M.C. 2002. Widespread evidence of 1500 yr climate variability in North America during the past 14,000 yr. Geology 30: 455-458. Willard, D.A., Cronin, T.M. and Verardo, S. 2003.
Late-Holocene climate and ecosystem history from Chesapeake Bay sediment cores, USA. The Holocene 13: 201-214. Willard, D.A., Weimer, L.M. and Holmes, C.W. 2001. The Florida Everglades
ecosystem, climatic and anthropogenic impacts over the last two millennia. Bulletins of American Paleontology 361: 41-55. Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought
variability in the Central United States. Bulletin of the American Meteorological Society 79: 2693-2714. 3.2.8. South America In Argentina, Cioccale (1999) assembled what was known at
the time about the climatic history of the central region of that country over the past 1,400 years, highlighting a climatic “improvement” that began some 400 years before the start
of the last millennium, which ultimately came to be characterized by “a marked increase of environmental suitability, under a relatively homogeneous climate.” As a result of this climatic
amelioration that marked the transition of the region from the Dark Ages Cold Period to the Medieval Warm Period, Cioccale says “the population located in the lower valleys ascended
to higher areas in the Andes,” where they remained until around AD 1320, when the transition to the stressful and extreme climate of the Little Ice Age began. Down at the southern tip
of the country in Tierra del Fuego, Mauquoy et al. (2004) inferred similar changes in temperature and/or precipitation from plant macrofossils, pollen, fungal spores, testate amebae,
and humification associated with peat monoliths collected from the Valle de Andorra. These new chronologies were compared with other chronologies of pertinent data from both the Southern
and Northern Hemispheres in an analysis that indicated there was evidence for a period of warminginduced drier conditions from AD 960-1020, which, in their words, “seems to correspond
to the Medieval Warm Period (MWP, as derived in the Northern Hemisphere).” They note that “this interval compares well to the date range of AD 950-1045 based on Northern Hemisphere extratropical
tree-ring data (Esper et al., 2002),” and they conclude that this correspondence “shows that the MWP was possibly synchronous in both hemispheres, as suggested by Villalba (1994).” In
Chile, Jenny et al. (2002) studied geochemical, sedimentological, and diatom-assemblage data derived from sediment cores extracted from one of the largest natural lakes (Laguna Aculeo)
in the central part of the country. From 200 BC, when the record began, until AD 200, conditions there were primarily dry, during the latter stages of the Roman Warm Period. Subsequently,
from AD 200-700, with a slight respite in the central hundred years of that period, there was a high frequency of flood events, during the Dark Ages Cold Period. Then came a several-hundred-year
period of less flooding that was coeval with the Medieval Warm Period. This more benign period was then followed by another period of frequent flooding from 1300-1700 that was coincident
with the Little Ice Age, after which flooding picked up again after 1850. In Peru, Chepstow-Lusty et al. (1998) derived a 4,000-year climate history from a study of pollen in sediment
cores obtained from a recently in-filled lake
Climate Change Reconsidered 92 in the Patacancha Valley near Marcacocha. Their data indicated a several-century decline in pollen content after AD 100, as the Roman Warm Period gave
way to the Dark Ages Cold Period. However, a “more optimum climate,” as they describe it, with warmer temperatures and drier conditions, came into being and prevailed for several centuries
after about AD 900, which was, of course, the Medieval Warm Period, which was followed by the Little Ice Age, all of which climatic periods are in nearly perfect temporal agreement with
the climatic history derived by McDermott et al. (2001) from a study of a stalagmite recovered from a cave nearly half the world away in Ireland. Subsequent work in this area was conducted
by Chepstow-Lusty and Winfield (2000) and Chepstow-Lusty et al. (2003). Centered on approximately 1,000 years ago, Chepstow-Lusty and Winfield researchers identified what they describe
as “the warm global climatic interval frequently referred to as the Medieval Warm Epoch.” This extremely arid interval in this part of South America, in their opinion, may have played
a significant role in the collapse of the Tiwanaku civilization further south, where a contemporaneous prolonged drought occurred in and around the area of Lake Titicaca (Binford et
al., 1997; Abbott et al., 1997). Near the start of this extended dry period, which had gradually established itself between about AD 700 and 1000, Chepstow-Lusty and Winfield report
that “temperatures were beginning to increase after a sustained cold period that had precluded agricultural activity at these altitudes.” This earlier colder and wetter interval was
coeval with the Dark Ages Cold Period of the North Atlantic region, which in the Peruvian Andes had held sway for a good portion of the millennium preceding AD 1000, as revealed by a
series of climatic records developed from sediment cores extracted from yet other lakes in the Central Peruvian Andes (Hansen et al., 1994) and by proxy evidence of concomitant Peruvian
glacial expansion (Wright, 1984; Seltzer and Hastorf, 1990). Preceding the Dark Ages Cold Period in both parts of the world was what in the North Atlantic region is called the Roman
Warm Period. This welldefined climatic epoch is also strikingly evident in the pollen records of Chepstow-Lusty et al. (2003), straddling the BC/AD calendar break with one to two hundred
years of relative warmth and significant aridity on both sides of it. Returning to the Medieval Warm Period and proceeding towards the present, the data of Chepstow-Lusty et al. (2003)
reveal the occurrence of the Little Ice Age, which in the Central Peruvian Andes was characterized by relative coolness and wetness. These characteristics of that climatic interval are
also evident in ice cores retrieved from the Quelccaya ice cap in southern Peru, the summit of which extends 5,670 meters above mean sea level (Thompson et al., 1986, 1988). Finally,
both the Quelccaya ice core data and the Marcacocha pollen data reveal the transition to the drier Current Warm Period that occurred over the past 100-plus years. In harmony with these
several findings are the related observations of Rein et al. (2004), who derived a high-resolution flood record of the entire Holocene from an analysis of the sediments in a 20-meter
core retrieved from a sheltered basin situated on the edge of the Peruvian shelf about 80 km west of Lima. These investigators found a major Holocene anomaly in the flux of lithic components
from the continent onto the Peruvian shelf during the Medieval period. Specifically, they report that “lithic concentrations were very low for about 450 years during the Medieval climatic
anomaly from A.D. 800 to 1250.” In fact, they state that “all known terrestrial deposits of El Niño mega-floods (Magillian and Goldstein, 2001; Wells, 1990) precede or follow the medieval
anomaly in our marine records and none of the El Niño mega-floods known from the continent date within the marine anomaly.” In addition, they report that “this precipitation anomaly
also occurred in other other high-resolution records throughout the ENSO domain,” citing 11 other references in support of this statement. Consequently, because heavy winter rainfalls
along and off coastal Peru occur only during times of maximum El Niño strength, and because El Niños are typically more prevalent and stronger during cooler as opposed to warmer periods
(see El Niño (Relationship to Global Warming) in Chapter 5), the lack of strong El Niños from A.D. 800 to 1250 suggests that this period was truly a Medieval Warm Period; and the significance
of this observation was not lost on Rein et al. In the introduction to their paper, they note that “discrepancies exist between the Mann curve and alternative time series for the Medieval
period.” Most notably, to use their words, “the global Mann curve has no temperature optimum, whereas the Esper et al. (2002) reconstruction shows northern hemisphere temperatures almost
as high as those of the 20th century” during the Medieval period. As a result, in the final sentence of their paper they suggest that “the occurrence of a Medieval climatic anomaly (A.D.
Observations: Temperature Records 93 800-1250) with persistently weak El Niños may therefore assist the interpretation of some of the regional discrepancies in thermal reconstructions
of Medieval times,” which is a polite way of suggesting that the Mann et al. (1998, 1999) hockey stick temperature history is deficient in not depicting the presence of a true Medieval
Warm Period. In Venezuala, Haug et al. (2001) found a temperature/precipitation relationship that was different from that of the rest of the continent. In examining the titanium and
iron concentrations of an ocean sediment core taken from the Cariaco Basin on the country’s northern shelf, they determined that the concentrations of these elements were lower during
the Younger Dryas cold period between 12.6 and 11.5 thousand years ago, corresponding to a weakened hydrologic cycle with less precipitation and runoff, while during the warmth of the
Holocene Optimum of 10.5 to 5.4 thousand years ago, titanium and iron concentrations remained at or near their highest values, suggesting wet conditions and an enhanced hydrologic cycle.
Closer to the present, higher precipitation was also noted during the Medieval Warm Period from 1.05 to 0.7 thousand years ago, followed by drier conditions associated with the Little
Ice Age between 550 and 200 years ago. In an update of this study, Haug et al. (2003) developed a hydrologic history of pertinent portions of the record that yielded “roughly bi-monthly
resolution and clear resolution of the annual signal.” This record revealed that “before about 150 A.D.,” which according to the climate history of McDermott et al. corresponds to the
latter portion of the Roman Warm Period (RWP), Mayan civilization had flourished. However, during the transition to the Dark Ages Cold Period (DACP), which was accompanied by a slow
but long decline in precipitation, Haug et al. report that “the first documented historical crisis hit the lowlands, which led to the ‘Pre-Classic abandonment’ (Webster, 2002) of major
cities.” This crisis occurred during the first intense multiyear drought of the RWP-to-DACP transition, which was centered on about the year 250 A.D. Although the drought was devastating
to the Maya, Haug et al. report that when it was over, “populations recovered, cities were reoccupied, and Maya culture blossomed in the following centuries during the so-called Classic
period.” Ultimately, however, there came a time of reckoning, between about 750 and 950 A.D., during what Haug et al. determined was the driest interval of the entire Dark Ages Cold
Period, when they report that “the Maya experienced a demographic disaster as profound as any other in human history,” in response to a number of other intense multi-year droughts. During
this Terminal Classic Collapse, as it is called, Haug et al. say that “many of the densely populated urban centers were abandoned permanently, and Classic Maya civilization came to an
end.” In assessing the significance of these several observations near the end of their paper, Haug et al. conclude that the latter droughts “were the most severe to affect this region
in the first millennium A.D.” Although some of these spectacular droughts were “brief,” lasting “only” between three and nine years, Haug et al. report “they occurred during an extended
period of reduced overall precipitation that may have already pushed the Maya system to the verge of collapse.” Although the Mayan civilization thus faded away, Haug et al.’s data soon
thereafter depict the development of the Medieval Warm Period, when the Vikings established their historic settlement on Greenland. Then comes the Little Ice Age, which just as quickly
led to the Vikings’ demise in that part of the world. This distinctive cold interval of the planet’s millennial-scale climatic oscillation also must have led to hard times for the people
of Mesoamerica and northern tropical South America; according to the data of Haug et al., the Little Ice Age produced the lowest precipitation regime (of several hundred years’ duration)
of the last two millennia in that part of the world. In conclusion, it is difficult to believe that the strong synchronicity of the century-long Northern Hemispheric and South American
warm and cold periods described above was coincidental. It is much more realistic to believe it was the result of a millennial-scale oscillation of climate that is global in scope and
driven by some regularly varying forcing factor. Although one can argue about the identity of that forcing factor and the means by which it exerts its influence, one thing should be
clear: It is not the atmosphere’s CO2 concentration, which has exhibited a significant in-phase variation with global temperature change only over the Little Ice Age to Current Warm
Period transition. This being the case, it should be clear that the climatic amelioration of the past century or more has had little or nothing to do with the concomitant rise in the
air’s CO2 content but everything to do with the influential forcing factor that has governed the millennial-scale oscillation of earth’s climate as far back in time as we have been able
to detect it.
Climate Change Reconsidered 94 Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/s/southamerica
wp.php. References Abbott, M.B., Binford, M.W., Brenner, M. and Kelts, K.R. 1997. A 3500 14C yr high resolution record of water-level changes in Lake Titicaca. Quaternary Research 47:
169-180. Binford, M.W., Kolata, A.L, Brenner, M., Janusek, J.W., Seddon, M.T., Abbott, M. and Curtis. J.H. 1997. Climate variation and the rise and fall of an Andean civilization. Quaternary
Research 47: 235-248. Chepstow-Lusty, A.J., Bennett, K.D., Fjeldsa, J., Kendall, A., Galiano, W. and
Herrera, A.T. 1998. Tracing 4,000 years of environmental history in the Cuzco Area, Peru, from the pollen record. Mountain Research and Development 18: 159-172. Chepstow-Lusty, A.,
Frogley, M.R., Bauer, B.S., Bush, M.B. and Herrera, A.T. 2003. A late Holocene record of arid events from the Cuzco region, Peru. Journal of Quaternary Science 18: 491-502. Chepstow-Lusty,
A. and Winfield, M. 2000. Inca agroforestry: Lessons from the past. Ambio 29: 322-328. Cioccale, M.A. 1999. Climatic fluctuations in the Central Region of Argentina in the last 1000
years. Quaternary International 62: 35-47. Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Lowfrequency signals in long tree-ring chronologies for reconstructing past temperature
variability. Science 295: 2250-2253. Hansen, B.C.S., Seltzer, G.O. and Wright Jr., H.E. 1994. Late Quaternary vegetational change in the central Peruvian Andes. Palaeogeography, Palaeoclimatology,
Palaeoecology 109: 263-285. Haug, G.H., Gunther, D., Peterson, L.C., Sigman, D.M., Hughen, K.A. and Aeschlimann, B. 2003. Climate and the collapse of Maya civilization. Science 299:
1731-1735. Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C. and Rohl, U. 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293: 1304-1308.
Jenny, B., Valero-Garces, B.L., Urrutia, R., Kelts, K., Veit, H., Appleby, P.G. and Geyh, M. 2002. Moisture changes and fluctuations of the Westerlies in Mediterranean Central Chile
during the last 2000 years: The Laguna Aculeo record (33°50’S). Quaternary International 87: 3-18. Magillian, F.J. and Goldstein, P.S. 2001. El Niño floods and culture change: A late
Holocene flood history for the Rio Moquegua, southern Peru. Geology 29: 431-434. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing
over the past six centuries. Nature 392: 779-787. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties,
and limitations. Geophysical Research Letters 26: 759-762. Mauquoy, D., Blaauw, M., van Geel, B., Borromei, A., Quattrocchio, M., Chambers, F.M. and Possnert, G. 2004. Late Holocene
climatic changes in Tierra del Fuego based on multiproxy analyses of peat deposits. Quaternary Research 61: 148-158. McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale
Holocene climate variability revealed by a high-resolution speleothem d18O record from SW Ireland. Science 294: 1328-1331. Rein, B., Luckge, A. and Sirocko, F. 2004. A major Holocene
ENSO anomaly during the Medieval period. Geophysical Research Letters 31: 10.1029/2004GL020161. Seltzer, G. and Hastorf, C. 1990. Climatic change and its effect on Prehispanic agriculture
in the central Peruvian Andes. Journal of Field Archaeology 17: 397-414. Thompson, L.G., Mosley-Thompson, E., Dansgaard, W. and Grootes, P.M. 1986. The Little Ice Age as recorded in
the stratigraphy of the tropical Quelccaya ice cap. Science 234: 361-364. Thompson, L.G., Davis, M.E., Mosley-Thompson, E. and Liu, K.-B. 1988. Pre-Incan agricultural activity recorded
in dust layers in two tropical ice cores. Nature 307: 763-765. Villalba, R. 1994. Tree-ring and glacial evidence for the Medieval Warm Epoch and the ‘Little Ice Age’ in southern South
America. Climatic Change 26: 183-197. Webster, D. 2002. The Fall of the Ancient Maya. Thames and Hudson, London, UK. Wells, L.E. 1990. Holocene history of the El Niño phenomenon as recorded
in flood sediments of northern coastal Peru. Geology 18: 1134-1137. Wright Jr., H.E. 1984. Late glacial and Late Holocene moraines in the Cerros Cuchpanga, central Peru. Quaternary Research
21: 275-285.
Observations: Temperature Records 95 3.3. Urban Heat Islands How accurate are the surface temperature records cited by the IPCC as showing unprecedented millennial warmth over the past
couple decades? The IPCC considers them very accurate and nearly free of any contaminating influence, yielding a 1905-2005 increase of 0.74°C ± 0.18°C (IPCC, 2007-I, p. 237). Warming
in many growing cities, on the other hand, may have been a full order of magnitude greater. Since nearly all near-surface air temperature records of this period have been obtained from
sensors located in population centers that have experienced significant growth, it is essential that urban heat island (UHI) effects be removed from all original temperature records
when attempting to accurately assess what has truly happened in the natural non-urban environment. The IPCC dismisses this concern, saying the UHI is “an order of magnitude smaller than
decadal and longer time-scale trends” (p. 244) and “UHI effects are real but local, and have a negligible influence (less than 0.006° C per decade over land and zero over the oceans)
on these [observed temperature] values” (p. 5). On this extremely important matter, the IPCC is simply wrong, as the rest of this section demonstrates. References IPCC. 2007-I. Climate
Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning,
Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge, UK. 3.3.1. Global Hegerl et al. (2001) describe UHI-induced temperature perversions
as one of three types of systematic error in the surface air temperature record whose magnitude “cannot be assessed at present.” Nevertheless, they go on to do just that, claiming “it
has been estimated that temperature trends over rural stations only are very similar to trends using all station data, suggesting that the effect of urbanization on estimates of global-scale
signals should be small.” This statement is patently false. De Laat and Maurellis (2004) used a global dataset developed by Van Aardenne et al. (2001), which reveals the spatial distribution
of various levels of industrial activity over the planet as quantified by the intensity of anthropogenic CO2 emissions to divide the surface of the earth into non-industrial and industrial
sectors of various intensity levels, after which they plotted the 1979-2001 temperature trends (°C/decade) of the different sectors using data from both the surface and the lower and
middle troposphere. The two scientists report that “measurements of surface and lower tropospheric temperature change give a very different picture from climate model predictions and
show strong observational evidence that the degree of industrialization is correlated with surface temperature increases as well as lower tropospheric temperature changes.” Specifically,
they find that the surface and lower tropospheric warming trends of all industrial regions are greater than the mean warming trend of the earth’s non-industrial regions, and that the
difference in warming rate between the two types of land use grows ever larger as the degree of industrialization increases. In discussing the implications of their findings, De Laat
and Maurellis say “areas with larger temperature trends (corresponding to higher CO2 emissions) cover a considerable part of the globe,” which implies that “the ‘real’ global mean surface
temperature trend is very likely to be considerably smaller than the temperature trend in the CRU [Hadley Center/Climate Research Unit] data,” since the temperature measurements that
comprise that data base “are often conducted in the vicinity of human (industrial) activity.” These observations, in their words, “suggest a hitherto-overlooked driver of local surface
temperature increases, which is linked to the degree of industrialization” and “lends strong support to other indications that surface processes (possibly changes in land-use or the
urban heat effect) are crucial players in observed surface temperature changes (Kalnay and Cai, 2003; Gallo et al., 1996, 1999).” They conclude that “the observed surface temperature
changes might be a result of local surface heating processes and not related to radiative greenhouse gas forcing.” A similar study was conducted by McKitrick and Michaels (2004), who
calculated 1979-2000 linear trends of monthly mean near-surface air temperature for 218 stations in 93 countries, based upon data they obtained from the Goddard Institute of Space Studies
(GISS), after which they regressed the results against
Climate Change Reconsidered 96 indicators of local economic activity—such as income, gross domestic product growth rates, and coal use—to see if there was any evidence of these socioeconomic
factors affecting the supposedly “pristine as possible” temperature data. Then, they repeated the process using the gridded surface air temperature data of the IPCC. The two scientists
report that the spatial pattern of trends they derived from the GISS data was “significantly correlated with non-climatic factors, including economic activity and sociopolitical characteristics.”
Likewise, with respect to the IPCC data, they say “very similar correlations appear, despite previous attempts to remove non-climatic effects.” These “socioeconomic effects,” in the
words of McKitrick and Michaels, “add up to a net warming bias,” although they say “precise estimation of its magnitude will require further work.” We can get a good feel for the magnitude
of the “socioeconomic effect” in some past work, such as that of Oke (1973), who measured the urban heat island strength of 10 settlements in the St. Lawrence Lowlands of Canada that
had populations ranging from approximately 1,000 to 2,000,000 people, after which he compared his results with those obtained for a number of other cities in North America, as well as
some in Europe. Over the population range studied, Oke found that the magnitude of the urban heat island was linearly correlated with the logarithm of population; this relationship indicated
that at the lowest population value encountered, i.e., 1,000 inhabitants, there was an urban heat island effect of 2° to 2.5°C, which warming is more than twice as great as the increase
in mean global air temperature believed to have occurred since the end of the Little Ice Age. It should be abundantly clear there is ample opportunity for large errors to occur in thermometerderived
surface air temperature histories of the twentieth century, and that error is probably best described as a large and growing warming bias. That this urban heat heat island-induced error
has indeed corrupted data bases that are claimed to be immune from it is suggested by the work of Hegerl and Wallace (2000), who attempted to determine if trends in recognizable atmospheric
modes of variability could account for all or part of the observed trend in surface-troposphere temperature differential, i.e., lapse rate, which has been driven by the upward-inclined
trend in surface-derived temperatures and the nearly level trend in satellitederived tropospheric temperatures over the last two decades of the twentieth century. After doing everything
they could conceive of doing, they had to conclude that “modes of variability that affect surface temperature cannot explain trends in the observed lapse rate,” and that “no mechanism
with clear spatial or time structure can be found that accounts for that trend.” In addition, they had to acknowledge that “all attempts to explain all or a significant part of the observed
lapse rate trend by modes of climate variability with structured patterns from observations have failed,” and that “an approach applying model data to isolate such a pattern has also
failed.” Nor could they find any evidence “that interdecadal variations in radiative forcing, such as might be caused by volcanic eruptions, variations in solar output, or stratospheric
ozone depletion alone, offer a compelling explanation.” Hence, the two scientists ultimately concluded that “there remains a gap in our fundamental understanding of the processes that
cause the lapse rate to vary on interdecadal timescales.” On the other hand, the reason why no meteorological or climatic explanation could be found for the ever-increasing difference
between the surface-and satellite-derived temperature trends of the past 20-plus years may be that one of the temperature records is incorrect. Faced with this possibility, one would
logically want to determine which of the records is likely to be erroneous and then assess the consequences of that determination. Although this task may seem daunting, it is really
not that difficult. One reason why is the good correspondence Hegerl and Wallace found to exist between the satellite and radiosonde temperature trends, which leaves little reason for
doubting the veracity of the satellite results, since this comparison essentially amounts to an in situ validation of the satellite record. A second important reason comes from the realization
that it would be extremely easy for a spurious warming of 0.12°C per decade to be introduced into the surface air temperature trend as a consequence of the worldwide intensification
of the urban heat island effect that was likely driven by the world population increase that occurred in most of the places where surface air temperature measurements were made over
the last two decades of the twentieth century. It appears almost certain that surface-based temperature histories of the globe contain a significant warming bias introduced by insufficient
corrections for the non-greenhouse-gas-induced urban heat island effect. Furthermore, it may well be next to impossible to make proper corrections for this
Observations: Temperature Records 97 deficiency, as the urban heat island of even small towns dwarfs any concomitant augmented greenhouse effect that may be present. References De Laat,
A.T.J. and Maurellis, A.N. 2004. Industrial CO2 emissions as a proxy for anthropogenic influence on lower tropospheric temperature trends. Geophysical Research Letters 31: 10.1029/2003GL019024.
Gallo, K.P., Easterling, D.R. and Peterson, T.C. 1996. The influence of land use/land cover on climatological values of the diurnal temperature range. Journal of Climate 9: 2941-2944.
Gallo, K.P., Owen, T.W., Easterling, D.R. and Jameson, P.F. 1999. Temperature trends of the historical climatologic network based on satellite-designated land use/land cover. Journal
of Climate 12: 1344-1348. Hegerl, G.C., Jones, P.D. and Barnett, T.P. 2001. Effect of observational sampling error on the detection of anthropogenic climate change. Journal of Climate
14: 198-207. Hegerl, G.C. and Wallace, J.M. 2002. Influence of patterns of climate variability on the difference between satellite and surface temperature trends. Journal of Climate
15: 2412-2428. IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
Change. Cambridge University Press. Kalnay, E. and Cai, M. 2003. Impact of urbanization and land use change on climate. Nature 423: 528-531. McKitrick, R. and Michaels, P.J. 2004. A
test of corrections for extraneous signals in gridded surface temperature data. Climate Research 26: 159-173. Oke, T.R. 1973. City size and the urban heat island. Atmospheric Environment
7: 769-779. Van Aardenne, J.A., Dentener, F.J., Olivier, J.G.J., Klein Goldewijk, C.G.M. and Lelieveld, J. 2001. A 1° x 1° resolution dataset of historical anthropogenic trace gas emissions
for the period 1890-1990. Global Biogeochemical Cycles 15: 909-928. 3.3.2. North America In studying the urban heat island (UHI) of Houston, Texas, Streutker (2003) analyzed 82 sets
of nighttime radiation data obtained from the split-window infrared channels of the Advanced Very High Resolution Radiometer on board the NOAA-9 satellite during March 1985 through February
1987 and from 125 sets of similar data obtained from the NOAA-14 satellite during July 1999 through June 2001. Between these two periods, it was found that the mean nighttime surface
temperature of Houston rose by 0.82 ± 0.10 °C. In addition, Streutker notes that the growth of the Houston UHI, both in magnitude and spatial extent, “scales roughly with the increase
in population,” and that the mean rural temperature measured during the second interval was “virtually identical to the earlier interval.” This informative study demonstrates that the
UHI phenomenon can sometimes be very powerful, for in just 12 years the UHI of Houston grew by more than the IPCC contends the mean surface air temperature of the planet rose over the
entire past century, during which period earth’s population rose by approximately 280 percent, or nearly an an order of magnitude more than the 30 percent population growth experienced
by Houston over the 12 years of Streutker’s study. A very different type of study was conducted by Maul and Davis (2001), who analyzed air and seawater temperature data obtained over
the past century at the sites of several primary tide gauges maintained by the U.S. Coast and Geodetic Survey. Noting that each of these sites “experienced significant population growth
in the last 100 years,” and that “with the increase in maritime traffic and discharge of wastewater one would expect water temperatures to rise” (due to a maritime analogue of the urban
heat island effect), they calculated trends for the 14 longest records and derived a mean century-long seawater warming of 0.74°C, with Boston registering a 100-year warming of 3.6°C.
In addition, they report that air temperature trends at the tide gauge sites, which represent the standard urban heat island effect, were “much larger” than the seawater temperature
trends. In another different type of study, Dow and DeWalle (2000) analyzed trends in annual evaporation and Bowen ratio measurements on 51 eastern U.S. watersheds that had experienced
various degrees of urbanization between 1920 and 1990. In doing so, they determined that as residential development progressively occurred on what originally were rural watersheds, watershed
evaporation decreased and sensible heating of the atmosphere increased. And from relationships derived from the suite of watersheds investigated, they
Climate Change Reconsidered 98 calculated that complete transformation from 100 percent rural to 100 percent urban characteristics resulted in a 31 percent decrease in watershed evaporation
and a 13 W/m2 increase in sensible heating of the atmosphere. Climate modeling exercises suggest that a doubling of the air’s CO2 concentration will result in a nominal 4 W/m2 increase
in the radiative forcing of earth’s surface-troposphere system, which has often been predicted to produce an approximate 4°C increase in the mean near-surface air temperature of the
globe, indicative of an order-of-magnitude climate sensitivity of 1°C per W/m2 change in radiative forcing. Thus, to a first approximation, the 13 W/m2 increase in the sensible heating
of the near-surface atmosphere produced by the total urbanization of a pristine rural watershed in the eastern United States could be expected to produce an increase of about 13°C in
near-surface air temperature over the central portion of the watershed, which is consistent with maximum urban heat island effects observed in large and densely populated cities. Hence,
a 10 percent rural-to-urban transformation could well produce a warming on the order of 1.3°C, and a mere 2 percent transformation could increase the near-surface air temperature by
as much as a quarter of a degree Centigrade. This powerful anthropogenic but non-greenhousegas-induced effect of urbanization on the energy balance of watersheds and the temperature
of the boundary-layer air above them begins to express itself with the very first hint of urbanization and, hence, may be readily overlooked in studies seeking to identify a greenhouse-gas-induced
global warming signal. In fact, the fledgling urban heat island effect may already be present in many temperature records that have routinely been considered “rural enough” to be devoid
of all human influence. A case in point is provided by the study of Changnon (1999), who used a series of measurements of soil temperatures obtained in a totally rural setting in central
Illinois between 1889 and 1952 and a contemporary set of air temperature measurements made in an adjacent growing community (as well as similar data obtained from other nearby small
towns), to evaluate the magnitude of unsuspected heat island effects that may be present in small towns and cities that are typically assumed to be free of urban-induced warming. This
work revealed that soil temperature in the totally rural setting experienced an increase from the decade of 1901-1910 to the decade of 1941-1950 that amounted to 0.4°C. This warming
is 0.2°C less than the 0.6°C warming determined for the same time period from the entire dataset of the U.S. Historical Climatology Network, which is supposedly corrected for urban heating
effects. It is also 0.2°C less than the 0.6°C warming determined for this time period by 11 benchmark stations in Illinois with the highest quality long-term temperature data, all of
which are located in communities that had populations of less than 6,000 people as of 1990. And it is 0.17°C less than the 0.57°C warming derived from data obtained at the three benchmark
stations closest to the site of the soil temperature measurements and with populations of less than 2,000 people. Changnon says his findings suggest that “both sets of surface air temperature
data for Illinois believed to have the best data quality with little or no urban effects may contain urban influences causing increases of 0.2°C from 1901 to 1950.” He further notes—in
a grand understatement—that “this could be significant because the IPCC (1995) indicated that the global mean temperature increased 0.3°C from 1890 to 1950.” DeGaetano and Allen (2002b)
used data from the U.S. Historical Climatology Network to calculate trends in the occurrence of maximum and minimum temperatures greater than the 90th, 95th, and 99th percentile across
the United States over the period 1960-1996. In the case of daily warm minimum temperatures, the slope of the regression line fit to the data of a plot of the annual number of 95th percentile
exceedences vs. year was found to be 0.09 exceedences per year for rural stations, 0.16 for suburban stations, and 0.26 for urban stations, making the rate of increase in extreme warm
minimum temperatures at urban stations nearly three times greater than the rate of increase at rural stations less affected by growing urban heat islands. Likewise, the rate of increase
in the annual number of daily maximum temperature 95th percentile exceedences per year over the same time period was found to be 50 percent greater at urban stations than it was at rural
stations. Working on the Arctic Coastal Plain near the Chuckchi Sea at Barrow, Alaska—which is described by Hinkel et al. (2003) as “the northernmost settlement in the USA and the largest
native community in the Arctic,” the population of which “has grown from about 300 residents in 1900 to more than 4600 in 2000”—the four researchers installed 54 temperature-recording
instruments in mid-June of 2001, half of them within the urban area and the other
Observations: Temperature Records 99 half distributed across approximately 150 km2 of surrounding land, all of which provided air temperature data at hourly intervals approximately two
meters above the surface of the ground. In this paper, they describe the results they obtained for the following winter. Based on urban-rural spatial averages for the entire winter period
(December 2001-March 2002), they determined the urban area to be 2.2°C warmer than the rural area. During this period, the mean daily urban-rural temperature difference increased with
decreasing temperature, “reaching a peak value of around 6°C in January-February.” It was also determined that the daily urban-rural temperature difference increased with decreasing
wind speed, such that under calm conditions (< 2 m s-1) the daily urban-rural temperature difference was 3.2°C in the winter. Last of all, under simultaneous calm and cold conditions,
the urban-rural temperature difference was observed to achieve hourly magnitudes exceeding 9°C. Four years later, Hinkel and Nelson (2007) reported that for the period 1 December to
31 March of four consecutive winters, the spatially averaged temperature of the urban area of Barrow was about 2°C warmer than that of the rural area, and that it was not uncommon for
the daily magnitude of the urban heat island to exceed 4°C. In fact, they say that on some days the magnitude of the urban heat island exceeded 6°C, and that values in excess of 8°C
were sometimes recorded, while noting that the warmest individual site temperatures were “consistently observed in the urban core area.” These results indicate just how difficult it
is to measure a background global temperature increase that is believed to have been less than 1°C over the past century (representing a warming of less than 0.1°C per decade), when
the presence of a mere 4,500 people can create a winter heat island that may be two orders of magnitude greater than the signal being sought. Clearly, there is no way that temperature
measurements made within the range of influence of even a small village can be adjusted to the degree of accuracy that is required to reveal the true magnitude of the pristine rural
temperature change. Moving south, we find Ziska et al. (2004) working within and around Baltimore, Maryland, where they characterized the gradual changes that occur in a number of environmental
variables as one moves from a rural location (a farm approximately 50 km from the city center) to a suburban location (a park approximately 10 km from the city center) to an urban location
(the Baltimore Science Center approximately 0.5 km from the city center). At each of these locations, four 2 x 2 m plots were excavated to a depth of about 1.1 m, after which they were
filled with identical soils, the top layers of which contained seeds of naturally occurring plants of the area. These seeds sprouted in the spring of the year, and the plants they produced
were allowed to grow until they senesced in the fall, after which all of them were cut at ground level, removed, dried and weighed. Ziska et al. report that along the rural-tosuburban-to-urban
transect, the only consistent differences in the environmental variables they measured were a rural-to-urban increase of 21 percent in average daytime atmospheric CO2 concentration and
increases of 1.6 and 3.3°C in maximum (daytime) and minimum (nighttime) daily temperatures, respectively, which changes, in their words, are “consistent with most short-term (~50 year)
global change scenarios regarding CO2 concentration and air temperature.” In addition, they determined that “productivity, determined as final above-ground biomass, and maximum plant
height were positively affected by daytime and soil temperatures as well as enhanced CO2, increasing 60 and 115% for the suburban and urban sites, respectively, relative to the rural
site.” The three researchers say their results suggest that “urban environments may act as a reasonable surrogate for investigating future climatic change in vegetative communities,”
and those results indicate that rising air temperatures and CO2 concentrations tend to produce dramatic increases in the productivity of the natural ecosystems typical of the greater
Baltimore area and, by inference, probably those of many other areas as well. Three years later, George et al. (2007) reported on five years of work at the same three transect locations,
stating that “atmospheric CO2 was consistently and significantly increased on average by 66 ppm from the rural to the urban site over the five years of the study,” and that “air temperature
was also consistently and significantly higher at the urban site (14.8°C) compared to the suburban (13.6°C) and rural (12.7°C) sites.” And they again noted that the increases in atmospheric
CO2 and air temperature they observed “are similar to changes predicted in the short term with global climate change, therefore providing an environment suitable for studying future
effects of climate change on terrestrial ecosystems,” specifically noting that “urban areas are currently experiencing elevated atmospheric CO2 and
Climate Change Reconsidered 100 temperature levels that can significantly affect plant growth compared to rural areas.” Working further south still, LaDochy et al. (2007) report that
“when speculating on how global warming would impact the state [of California], climate
change models and assessments often assume that the influence would be uniform (Hansen et al., 1998; Hayhoe et al., 2004; Leung et al., 2004).” Feeling a need to assess the validity
of this assumption, they calculated temperature trends over the 50-year period 1950-2000 to explore the extent of warming in various sub-regions of the state, after which they attempted
to evaluate the influence of human-induced changes to the landscape on the observed temperature trends and determine their significance compared to those caused by changes in atmospheric
composition, such as the air’s CO2 concentration. In pursuing this protocol, the three researchers found that “most regions showed a stronger increase in minimum temperatures than with
mean and maximum temperatures,” and that “areas of intensive urbanization showed the largest positive trends, while rural, non-agricultural regions showed the least warming.” In fact,
they report that the Northeast Interior Basins of the state actually experienced cooling. Large urban sites, on the other hand, exhibited rates of warming “over twice those for the state,
for the mean maximum temperature, and over five times the state’s mean rate for the minimum temperature.” Consequently, they concluded that “if we assume that global warming affects
all regions of the state, then the small increases seen in rural stations can be an estimate of this general warming pattern over land,” which implies that “larger increases,” such as
those they observed in areas of intensive urbanization, “must then be due to local or regional surface changes.” Noting that “breezy cities on small tropical islands ... may not be exempt
from the same local climate change effects and urban heat island effects seen in large continental cities,” Gonzalez et al. (2005) describe the results of their research into this topic,
which they conducted in and about San Juan, Puerto Rico. In this particular study, a NASA Learjet—carrying the Airborne Thermal and Land Applications Sensor (ATLAS) that operates in
visual and infrared wavebands—flew several flight lines, both day and night, over the San Juan metropolitan area, the El Yunque National Forest east of San Juan, plus other nearby areas,
obtaining surface temperatures, while strategically placed ground instruments recorded local air temperatures. This work revealed that surface temperature differences between urbanized
areas and limited vegetated areas were higher than 3°C during daytime, creating an urban heat island with “the peak of the high temperature dome exactly over the commercial area of downtown,”
where noontime air temperatures were as much as 3°C greater than those of surrounding rural areas. In addition, the eleven researchers report that “a recent climatological analysis of
the surface [air] temperature of the city has revealed that the local temperature has been increasing over the neighboring vegetated areas at a rate of 0.06°C per year for the past 30
years.” In discussing their findings, Gonzalez et al. state that “the urban heat island dominates the sea breeze effects in downtown areas,” and they say that “trends similar to those
reported in [their] article may be expected in the future as coastal cities become more populated.” Indeed, it is probable that this phenomenon has long been operative in coastal cities
around the world, helping to erroneously inflate the surface air temperature record of the planet and contributing to the infamous “hockey stick” representation of this parameter that
has been so highly touted by the Intergovernmental Panel on Climate Change. One year later, Velazquez-Lozada et al. (2006) evaluated the thermal impacts of historical land cover and
land use (LCLU) changes in San Juan, Puerto Rico over the last four decades of the twentieth century via an analysis of air temperatures measured at a height of approximately two meters
above ground level within four different LCLU types (urbancoastal, rural-inland, rural-coastal and urban-inland), after which they estimated what the strength of the urban heat island
might be in the year 2050, based on anticipated LCLU changes and a model predicated upon their data of the past 40 years. In doing so, their work revealed “the existence of an urban
heat island in the tropical coastal city of San Juan, Puerto Rico that has been increasing at a rate of 0.06°C per year for the last 40 years.” In addition, they report that predicted
LCLU changes between now and 2050 will lead to an urban heat island effect “as high as 8°C for the year 2050.” Noting that a mass population migration from rural Mexico into medium-and
large-sized cities took place throughout the second half of the twentieth century, Jáuregui (2005) examined the effect of this rapid urbanization on city air temperatures, analyzing
the 1950-1990 minimum air temperature temperature series of seven large cities with populations in excess of a
Observations: Temperature Records 101 million people and seven medium-sized cities with populations ranging from 125,000 to 700,000 people. This work indicated that temperature trends
were positive at all locations, ranging from 0.02°C per decade to 0.74°C per decade. Grouped by population, the average trend for the seven large cities was 0.57°C per decade, while
the average trend for the seven mid-sized cities was 0.37°C per decade. In discussing these results, Jáuregui says they “suggest that the accelerated urbanization process in recent decades
may have substantially contributed to the warming of the urban air observed in large cities in Mexico.” One additional question that may arise in relation to this topic is the direct
heating of near-surface air in towns and cities by the urban CO2 dome that occurs above them. Does it contribute significantly to the urban heat island? In a study designed to answer
this question, Balling et al. (2002) obtained vertical profiles of atmospheric CO2 concentration, temperature, and humidity over Phoenix, Arizona from measurements made in association
with once-daily aircraft flights conducted over a 14-day period in January 2000 that extended through, and far above, the top of the city’s urban CO2 dome during the times of its maximum
manifestation. They then employed a onedimensional infrared radiation simulation model to determine the thermal impact of the urban CO2 dome on the near-surface temperature of the city.
These exercises revealed that the CO2 concentration of the air over Phoenix dropped off rapidly with altitude, returning from a central-city surface value on the order of 600 ppm to
a normal non-urban background value of approximately 378 ppm at an air pressure of 800 hPa, creating a calculated surface warming of only 0.12°C at the time of maximum CO2-induced warming
potential, which is about an order of magnitude less than the urban heat island effect of cities the size of Phoenix. In fact, the authors concluded that the warming induced by the urban
CO2 dome of Phoenix is possibly two orders of magnitude smaller than that produced by other sources of the city’s urban heat island. Although the doings of man are indeed responsible
for high urban air temperatures (which can sometimes rise 10°C or more above those of surrounding rural areas), these high values are not the result of a local CO2-enhanced greenhouse
effect. Meteorologist Anthony Watts (2009), in research too new to have appeared yet in a peer-reviewed journal, discovered compelling evidence that the temperature stations used to
reconstruct the U.S. surface temperature are unreliable and systemically biased toward recording more warming over time. Watts recruited a team of more than 650 volunteers to visually
inspect the temperature stations used by the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA) to measure changes in
temperatures in the U.S. In the researcher’s words, “using the same quality standards established by NOAA, they found found that 89 percent of the stations – nearly 9 of every 10 – fail
to meet NOAA’s own siting requirements for stations with an expected reporting error of less than 1º C. Many of them fall far short of that standard [italics in the original].” Watts
goes on to report finding stations “located next to the exhaust fans of air conditioning units, causing them to report much-higher-than-actual temperatures. We found stations surrounded
by asphalt parking lots and located near roads, sidewalks, and buildings that absorb and radiate heat. We found 68 stations located at wastewater treatment plants, where the process
of waste digestion causes temperatures to be higher than in surrounding areas.” Watts also discovered that failure to adequately account for changes in the technology used by temperature
stations over time—including moving from whitewash to latex paint and from mercury thermostats to digital technology—“have further contaminated the data, once again in the direction
of falsely raising temperature readings.” Watts is also extremely critical of adjustments to the raw data made by both NOAA and NASA, which “far from correcting the warming biases, actually
compounded the measurement errors.” The results of these several North American studies demonstrate that the impact of population growth on the urban heat island effect is very real
and can be very large, overshadowing the effects of natural temperature change. This insight is not new: more than three decades ago, Oke (1973) demonstrated that towns with as few as
a thousand inhabitants typically create a warming of the air within them that is more than twice as great as the increase in mean global air temperature believed to have occurred since
the end of the Little Ice Age, while the urban heat islands of the great metropolises of the world create warmings that rival those that occur between full-fledged ice ages and interglacials.
Extensive research conducted since then by independent scientists has confirmed Oke’s finding. Due to extensive corruption of land-based temperature data from urban heat islands, the
North
Climate Change Reconsidered 102 American temperature record cannot be cited as providing reliable data in support of the greenhouse theory of global warming. Additional information on
this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/u/uhinorthamerica.php. References Balling Jr., R.C., Cerveny,
R.S. and Idso, C.D. 2002. Does the urban CO2 dome of Phoenix, Arizona contribute to its heat island? Geophysical Research Letters 28: 4599-4601. Changnon, S.A. 1999. A rare long record
of deep soil temperatures defines temporal temperature changes and an urban heat island. Climatic Change 42: 531-538. DeGaetano, A.T. and Allen, R.J. 2002. Trends in twentiethcentury
temperature extremes across the United States. Journal of Climate 15: 3188-3205. Dow, C.L. and DeWalle, D.R. 2000. Trends in evaporation and Bowen ratio on urbanizing watersheds in eastern
United States. Water Resources Research 36: 1835-1843. George, K., Ziska, L.H., Bunce, J.A. A. and Quebedeaux, B. 2007. Elevated atmospheric CO2 concentration and temperature across
an urban-rural transect. Atmospheric Environment 41: 7654-7665. Gonzalez, J.E., Luvall, J.C., Rickman, D., Comarazamy, D., Picon, A., Harmsen, E., Parsiani, H., Vasquez, R.E., Ramirez,
N., Williams, R. and Waide, R.W. 2005. Urban heat islands developing in coastal tropical cities. EOS: Transactions, American Geophysical Union 86: 397,403. Hansen, J., Sato, M., Glascoe,
J. and Ruedy, R. 1998. A commonsense climatic index: Is climate change noticeable? Proceedings of the National Academy of Sciences USA 95: 4113-4120. Hayhoe, K., Cayan, D., Field, C.B.,
Frumhoff, P.C. et al. 2004. Emissions, pathways, climate change, and impacts on California. Proceedings of the National Academy of Sciences USA 101: 12,422-12,427. Hinkel, K.M. and Nelson,
F.E. 2007. Anthropogenic heat island at Barrow, Alaska, during winter: 2001-2005. Journal of Geophysical Research 112: 10.1029/2006JD007837. Hinkel, K.M., Nelson, F.E., Klene, A.E. and
Bell, J.H. 2003. The urban heat island in winter at Barrow, Alaska. International Journal of Climatology 23: 1889-1905. Intergovernmental Panel on Climate Change (IPCC). 1995. Climate
Change 1995, The Science of Climate Change. Cambridge University Press, Cambridge, U.K. Jáuregui, E. 2005. Possible impact of urbanization on the thermal climate of some large cities
in Mexico. Atmosfera 18: 249-252. LaDochy, S., Medina, R. and Patzert, W. 2007. Recent California climate variability: spatial and temporal patterns in temperature trends. Climate Research
33: 159-169. Leung, L.R., Qian, Y., Bian, X., Washington, W.M., Han, J. and Roads, J.O. 2004. Mid-century ensemble regional climate change scenarios for the western United States. Climatic
Change 62: 75-113. Maul, G.A. and Davis, A.M. 2001. Seawater temperature trends at USA tide gauge sites. Geophysical Research Letters 28: 3935-3937. Oke, T.R. 1973. City size and the
urban heat island. Atmospheric Environment 7: 769-779. Streutker, D.R. 2003. Satellite-measured measured growth of the urban heat island of Houston, Texas. Remote Sensing of Environment
85: 282-289. Velazquez-Lozada, A., Gonzalez, J.E. and Winter, A. 2006. Urban heat island effect analysis for San Juan, Puerto Rico. Atmospheric Environment 40: 1731-1741. Watts, A. 2009.
Is the U.S. Temperature Record Reliable? Chicago, IL: The Heartland Institute. Ziska, L.H., Bunce, J.A. and Goins, E.W. 2004. Characterization of an urban-rural CO2/temperature gradient
and associated changes in initial plant productivity during secondary succession. Oecologia 139: 454-458. 3.3.3. Asia Hasanean (2001) investigated surface air temperature trends with
data obtained from meteorological stations located in eight Eastern Mediterranean cities: Malta, Athens, Tripoli, Alexandria, Amman, Beirut, Jerusalem, and Latakia. The period of analysis
varied from station to station according to available data, with Malta having the longest temperature record (1853-1991) and Latakia the shortest (1952-1991). Four of the cities exhibited
overall warming trends and four of them cooling trends. In addition, there was an important warming around 1910 that began nearly simultaneously at all of the longer-record stations,
as well as a second warming in the 1970s; but Hasanean reports that the latter warming was “not uniform, continuous or of the same order” as the warming that began about 1910, nor was
it evident at all of the stations. One interpretation of this nonuniformity of temperature behavior in the 1970s is that it may have been the result of temporal
Observations: Temperature Records 103 differences in city urbanization histories that were accentuated about that time, which could have resulted in significantly different urban heat
island trajectories at the several sites over the latter portions of their records. In a more direct study of the urban heat island effect that was conducted in South Korea, Choi et
al. (2003) compared the mean station temperatures of three groupings of cities (one comprised of four large urban stations with a mean 1995 population of 4,830,000, one comprised of
six smaller urban stations with a mean 1995 population of 548,000, and one comprised of six “rural” stations with a mean 1995 population of 214,000) over the period 1968-1999. This analysis
revealed, in their words, that the “temperatures of large urban stations exhibit higher urban bias than those of smaller urban stations and that the magnitude of urban bias has increased
since the late 1980s.” Specifically, they note that “estimates of the annual mean magnitude of urban bias range from 0.35°C for smaller urban stations to 0.50°C for large urban stations.”
In addition, they indicate that “none of the rural stations used for this study can represent a true non-urbanized environment.” Hence, they correctly conclude that their results are
underestimates of the true urban effect, and that “urban growth biases are very serious in South Korea and must be taken into account when assessing the reliability of temperature trends.”
In a second study conducted in South Korea, Chung et al. (2004a) report there was an “overlapping of the rapid urbanization-industrialization period with the global warming era,” and
that the background climatic trends from urbanized areas might therefore be contaminated by a growing urban heat island effect. To investigate this possibility, they say “monthly averages
of daily minimum, maximum, and mean temperature at 14 synoptic stations were prepared for 1951-1980 (past normal) and 1971-2000 (current normal) periods,” after which “regression equations
were used to determine potential effects of urbanization and to extract the net contribution of regional climate change to the apparent temperature change.” Twelve of these stations
were growing urban sites of various size, while two (where populations actually decreased) were rural, one being located inland and one on a remote island. In terms of change over the
20 years that separated the two normal periods, Chung et al. report that in Seoul, where population increase was greatest, annual mean daily minimum temperature increased by 0.7°C, while
a mere 0.1°C increase was detected at one of the two rural sites and a 0.1°C decrease was detected at the other, for no net change in their aggregate mean value. In the case of annual
mean daily maximum temperature, a 0.4°C increase was observed at Seoul and a 0.3°C increase was observed at the two rural sites. Hence, the change in the annual mean daily mean temperature
was an increase of 0.15°C at the two rural sites (indicative of regional background warming of 0.075°C per decade), while the change of annual mean daily mean temperature at Seoul was
an increase of 0.55°C, or 0.275°C per decade (indicative of an urban-induced warming of 0.2°C per decade in addition to the regional background warming of 0.075°C per decade). Also,
corresponding results for urban areas of intermediate size defined a linear relationship that connected these two extreme results when plotted against the logarithm of population increase
over the two-decade period. In light of the significantly intensifying urban heat island effect detected in their study, Chung et al. say it is “necessary to subtract the computed urbanization
effect from the observed data at urban stations in order to prepare an intended nationwide climatic atlas,” noting that “rural climatological normals should be used instead of the conventional
normals to simulate ecosystem responses to climatic change, because the urban area is still much smaller than natural and agricultural ecosystems in Korea.” A third study of South Korea
conducted by Chung et al. (2004b) evaluated temperature changes at 10 urban and rural Korean stations over the period 1974-2002. They found “during the last 29 years, the increase in
annual mean temperature was 1.5°C for Seoul and 0.6°C for the rural and seashore stations,” while increases in mean January temperatures ranged from 0.8 to 2.4°C for the 10 stations.
In addition, they state that “rapid industrialization of the Korean Peninsula occurred during the late 1970s and late 1980s,” and that when plotted on a map, “the remarkable industrialization
and expansion ... correlate with the distribution of increases in temperature.” Consequently, as in the study of Chung et al. (2004a), Chung et al. (2004b) found that over the past several
decades, much (and in many cases most) of the warming experienced in the urban areas of Korea was the result of local urban influences that were not indicative of regional background
warming. Shifting attention to China, Weng (2001) evaluated the effect of land cover changes on surface temperatures of the Zhujiang Delta (an area of slightly more than 17,000 km2)
via a series of
Climate Change Reconsidered 104 analyses of remotely sensed Landsat Thematic Mapper data. They found that between 1989 and 1997, the area of land devoted to agriculture declined by nearly
50 percent, while urban land area increased by close to the same percentage. Then, upon normalizing the surface radiant temperature for the years 1989 and 1997, they used image differencing
to produce a radiant temperature change image that they overlaid with images of urban expansion. The results indicated, in Weng’s words, that “urban development between 1989 and 1997
has given rise to an average increase of 13.01°C in surface radiant temperature.” In Shanghai, Chen et al. (2003) evaluated several characteristics of that city’s urban heat island,
including its likely cause, based on analyses of monthly meteorological data from 1961 to 1997 at 16 stations in and around this hub of economic activity that is one of the most flourishing
urban areas in all of China. Defining the urban heat island of Shanghai as the mean annual air temperature difference between urban Longhua and suburban Songjiang, Chen et al. found
that its strength increased in essentially linear fashion from 1977 to 1997 by 1°C. Commenting on this finding, Chen et al. say “the main factor causing the intensity of the heat island
in Shanghai is associated with the increasing energy consumption due to economic development,” noting that in 1995 the Environment Research Center of Peking University determined that
the annual heating intensity due to energy consumption by human activities was approximately 25 Wm-2 in the urban area of Shanghai but only 0.5 Wm-2 in its suburbs. In addition, they
point out that the 0.5°C/decade intensification of Shanghai’s urban heat island is an order of magnitude greater than the 0.05°C/decade global warming of the earth over the past century,
which is indicative of the fact that ongoing intensification of even strong urban heat islands cannot be discounted. Simultaneously, Kalnay and Cai (2003) used differences between trends
in directly observed surface air temperature and trends determined from the NCEP-NCAR 50-year Reanalysis (NNR) project (based on atmospheric vertical soundings derived from satellites
and balloons) to estimate the impact of land-use changes on surface warming. Over undisturbed rural areas of the United States, they found that the surface-and reanalysis-derived air
temperature data yielded essentially identical trends, implying that differences between the two approaches over urban areas would represent urban heat island effects. Consequently,
Zhou et al. (2004) applied the same technique over southeast China, using an improved version of reanalysis that includes newer physics, observed soil moisture forcing, and a more accurate
characterization of clouds. For the period January 1979 to December 1998, the eight scientists involved in the work derived an “estimated warming of mean surface [air] temperature of
0.05°C per decade attributable to urbanization,” which they say “is much larger than previous estimates for other periods and locations, including the estimate of 0.027°C for the continental
U.S. (Kalnay and Cai, 2003).” They note, however, that because their analysis “is from the winter season over a period of rapid urbanization and for a country with a much higher population
density, we expect our results to give higher values than those estimated in other locations and over longer periods.” In a similar study, Frauenfeld et al. (2005) used daily surface
air temperature measurements from 161 stations located throughout the Tibetan Plateau (TP) to calculate the region’s mean annual temperature for each year from 1958 through 2000, while
in the second approach they used 2-meter temperatures from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40), which temperatures, in their words, “are
derived from rawinsonde profiles, satellite retrievals, aircraft reports, and other sources including some surface observations.” This approach, according to them, results in “more temporally
homogeneous fields” that provide “a better assessment of large-scale temperature variability across the plateau.” Frauenfeld et al. report that over the period 1958-2000, “time series
based on aggregating all station data on the TP show a statistically significant positive trend of 0.16°C per decade,” as has also been reported by Liu and Chen (2000). However, they
say that “no trends are evident in the ERA-40 data for the plateau as a whole.” In discussing this discrepancy, the three scientists suggest that “a potential explanation for the difference
between reanalysis and station trends is the extensive local and regional land use change that has occurred across the TP over the last 50 years.” They note, for example, that “over
the last 30 years, livestock numbers across the TP have increased more than 200% due to inappropriate land management practices and are now at levels that exceed the carrying capacity
of the region (Du et al., 2004).” The resultant overgrazing, in their words, “has caused land degradation and desertification at an alarming rate (Zhu and Li, 2000; Zeng et al., 2003),”
and they note
Observations: Temperature Records 105 that “in other parts of the world, land degradation due to overgrazing has been shown to cause significant local temperature increases (e.g., Balling
et al., 1998).” Another point they raise is that “urbanization, which can result in 8°-11°C higher temperatures than in surrounding rural areas (e.g., Brandsma et al., 2003), has also
occurred extensively on the TP,” noting that “construction of a gas pipeline in the 1970s and highway expansion projects in the early 1980s have resulted in a dramatic population influx
from other parts of China, contributing to both urbanization and a changed landscape.” In this regard, they say that “the original Tibetan section of Lhasa (i.e., the pre-1950 Lhasa)
now only comprises 4% of the city, suggesting a 2400% increase in size over the last 50 years.” And they add that “similar population increases have occurred at other locations across
the TP,” and that “even villages and small towns can exhibit a strong urban heat island effect.” In concluding their analysis of the situation, Frauenfeld et al. contend that “these
local changes are reflected in station temperature records.” We note that when the surface-generated anomalies are removed, as in the case of the ERA-40 reanalysis results they present,
it is clear there has been no warming of the Tibetan Plateau since at least 1958. Likewise, we submit that the other results reported in this section imply much the same about other
parts of China and greater Asia. In conclusion, a large body of research conducted by scores of scientists working in countries around the world reveals that the twentieth century warming
claimed by the IPCC, Mann et al. (1998, 1999), and Mann and Jones (2003) to represent mean global background conditions is likely significantly biased towards warming over the past 30
years and is therefore not a true representation of earth’s recent thermal history. Additional information on this topic, including reviews of newer publications as they become available,
can be found at http://www.co2science.org/subject/u/uhiasia.php References Balling Jr., R.C., Klopatek, J.M., Hildebrandt, M.L., Moritz, C.K. and Watts, C.J. 1998. Impacts of land degradation
on historical temperature records from the Sonoran desert. Climatic Change 40: 669-681. Brandsma, T., Konnen, G.P. and Wessels, H.R.A. 2003. Empirical estimation of the effect of urban
heat advection on the temperature series of DeBilt (the Netherlands). International Journal of Climatology 23: 829-845. Chen, L., Zhu, W., Zhou, X. and Zhou, Z. 2003. Characteristics
of the heat island effect in Shanghai and its possible mechanism. Advances in Atmospheric Sciences 20: 991-1001. Choi, Y., Jung, H.-S., Nam, K.-Y. and Kwon, W.-T. 2003. Adjusting urban
bias in the regional mean surface temperature series of South Korea, 1968-99. International Journal of Climatology 23: 577-591. Chung, U., Choi, J. and Yun, J.I. 2004a. Urbanization
effect on the observed change in mean monthly temperatures between 1951-1980 and 1971-2000. Climatic Change 66: 127-136. Chung, Y.-S., Yoon, M.-B. and Kim, H.-S. 2004b. On climate variations
and changes observed in South Korea. Climatic Change 66: 151-161. Du, M., Kawashima, S., Yonemura, S., Zhang, X. and Chen, S. 2004. Mutual influence between human activities and climate
change in the Tibetan Plateau during recent years. Global and Planetary Change 41: 241-249. Frauenfeld, O.W., Zhang, T. and Serreze, M.C. 2005. Climate change and variability using European
Centre for Medium-Range Weather Forecasts reanalysis
(ERA-40) temperatures on the Tibetan Plateau. Journal of Geophysical Research 110: 10.1029/2004JD005230. Hasanean, H.M. 2001. Fluctuations of surface air temperature in the Eastern
Mediterranean. Theoretical and Applied Climatology 68: 75-87. Kalnay, E. and Cai, M. 2003. Impact of urbanization and land-use change on climate. Nature 423: 528-531. Liu, X. and Chen,
B. 2000. Climatic warming in the Tibetan Plateau during recent decades. International Journal of Climatology 20: 1729-1742. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale
temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787. Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the
past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762. Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two
millennia. Geophysical Research Letters 30: 10.1029/2003GL017814. Weng, Q. 2001. A remote sensing-GIS evaluation of urban expansion and its impact on surface temperature in the
Climate Change Reconsidered 106 Zhujiang Delta, China. International Journal of Remote Sensing 22: 1999-2014. Zeng, Y., Feng, Z. and Cao, G. 2003. Land cover change and its environmental
impact in the upper reaches of the Yellow River, northeast Qinghai-Tibetan Plateau. Mountain Research and Development 23: 353-361. Zhou, L., Dickinson, R.E., Tian, Y., Fang, J., Li,
Q., Kaufmann, R.K., Tucker, C.J. and Myneni, R.B. 2004. Evidence for a significant urbanization effect on climate in China. Proceedings of the National Academy of Sciences USA 101: 9540-9544.
Zhu, L. and Li, B. 2000. Natural hazards and environmental issues. In: Zheng, D., Zhang, Q. and Wu, S. (Eds.) Mountain Genecology and Sustainable Development of the Tibetan Plateau,
Springer, New York, New York, USA, pp. 203-222. 3.4. Fingerprints Is there a method that can distinguish anthropogenic global warming from natural warming? The IPCC (IPCC-SAR, 1996,
p. 411; IPCC, 2007-I, p. 668) and many scientists believe the “fingerprint” method is the only reliable one. It compares the observed pattern of warming with a pattern calculated from
greenhouse models. While an agreement of such fingerprints cannot prove an anthropogenic origin for warming, it would be consistent with such a conclusion. A mismatch would argue strongly
against any significant contribution from greenhouse gas (GHG) forcing and support the conclusion that the observed warming is mostly of natural origin. Climate models all predict that,
if GHG is driving climate change, there will be a unique fingerprint in the form of a warming trend increasing with altitude in the tropical troposphere, the region of the atmosphere
up to about 15 kilometers. (See Figure 3.4.1.) Climate changes due to solar variability or other known natural factors will not yield this pattern; only sustained greenhouse warming
will do so. The fingerprint method was first attempted in the IPCC’s Second Assessment Report (SAR) (IPCCSAR, 1996, p. 411). Its Chapter 8, titled “Detection and Attribution,” attributed
observed temperature changes to anthropogenic factors—greenhouse gases and aerosols. The attempted match of warming trends with altitude turned out to be spurious, since it depended
entirely on a particular choice of time interval for the comparison (Michaels and Knappenberger, 1996). Similarly, an attempt to correlate the observed and calculated geographic distribution
of surface temperature trends (Santer et al. 1996) involved making changes on a published graph that could and did mislead readers (Singer, 1999, p. 9; Singer, 2000, pp. 15, 43-44).
In spite of these shortcomings, IPCC-SAR concluded that “the balance of evidence” supported AGW. With the availability of higher-quality temperature data, especially from balloons and
satellites, and with improved GH models, it has become possible to apply the fingerprint method in a more realistic way. This was done in a report issued by the U.S. Climate Change Science
Program (CCSP) in April 2006—making it readily available to the IPCC for its Fourth Assessment Report—and and it permits the most realistic comparison of fingerprints (Karl et al., 2006).
The CCSP report is an outgrowth of an NAS report “Reconciling Observations of Global Temperature Change” issued in January 2000 (NAS, 2000). That NAS report compared surface and troposphere
temperature trends and concluded they cannot be reconciled. Six years later, the CCSP report expanded considerably on the NAS study. It is Figure 3.4.1. Model-calculated zonal mean atmospheric
temperature change from 1890 to 1999 (degrees C per century) as simulated by climate models from [A] well-mixed greenhouse gases, [B] sulfate aerosols (direct effects only), [C] stratospheric
and tropospheric ozone, [D] volcanic aerosols, [E] solar irradiance, and [F] all forcings (U.S. Climate Change Science Program 2006, p. 22). Note the pronounced increase in warming trend
with altitude in figures A and F, which the IPCC identified as the ‘fingerprint’ of greenhouse forcing.
Observations: Temperature Records 107 essentially a specialized report addressing the most crucial issue in the global warming debate: Is current global warming anthropogenic or natural?
The CCSP result is unequivocal. While all greenhouse models show an increasing warming trend with altitude, peaking around 10 km at roughly two times the surface value, the temperature
data from balloons give the opposite result: no increasing warming, but rather a slight cooling with altitude in the tropical zone. See Figures 3.4.2 and 3.4.3, taken directly from the
CCSP report. The CCSP executive summary inexplicably claims agreement between observed and calculated patterns, the opposite of what the report itself documents. It tries to dismiss
the obvious disagreement shown in the body of the report by suggesting there might be something wrong with both balloon and satellite data. Unfortunately, many people do not read beyond
the summary and have therefore been misled to believe the CCSP report supports anthropogenic warming. It does not. The same information also can be expressed by plotting the difference
between surface trend and troposphere trend for the models and for the data (Singer, 2001). As seen in Figure 3.4.4 and 3.4.5, the models show a histogram of negative values (i.e. surface
trend less than troposphere trend) indicating that atmospheric warming will be greater than surface warming. By contrast, the data show mainly positive values for the difference in trends,
demonstrating that measured warming is occurring principally on the surface and not in the atmosphere. Figure 3.4.2. Greenhouse-model-predicted temperature trends versus latitude and
altitude; this is figure 1.3F from CCSP 2006, p. 25. Note the increased temperature trends in the tropical mid_troposphere, in agreement also with the IPCC result (IPCC-AR4 2007, p.
675). Figure 3.4.3. By contrast, observed temperature trends versus latitude and altitude; this is figure 5.7E from CCSP 2006, p. 116. These trends are based on the analysis of radiosonde
data by the Hadley Centre and are in good agreement with the corresponding U.S. analyses. Notice the absence of increased temperature trends in the tropical mid-troposphere. Figure 3.4.4.
Another way of presenting the difference between temperature trends of surface and lower troposphere; this is figure 5.4G from CCSP 2006, p. 111. The model results show a spread of values
(histogram); the data points show balloon and satellite trend values. Note the model results hardly overlap with the actual observed trends. (The apparent deviation of the RSS analysis
of the satellite data is as yet unexplained.) Figure 3.4.5. By contrast, the executive summary of the CCSP report presents the same information as Figure 3.4.4 in terms of ‘range’ and
shows a slight overlap between modeled and observed temperature trends (Figure 4G, p. 13). However, the use of ‘range’ is clearly inappropriate (Douglass et al. 2007) since it gives
undue weight to ‘outliers.’
Climate Change Reconsidered 108 The same information can be expressed in yet a different way, as seen in research papers by Douglass et al. (2004, 2007), as shown in Figure 3.4.6. The
models show an increase in temperature trend with altitude but the observations show the opposite. Figure 3.4.6. A more detailed view of the disparity of temperature trends is given
in this plot of trends (in degrees C/decade) versus altitude in the tropics [Douglass et al. 2007]. Models show an increase in the warming trend with altitude, but balloon and satellite
observations do not. This mismatch of observed and calculated fingerprints clearly falsifies the hypothesis of anthropogenic global warming (AGW). We must conclude therefore that anthropogenic
greenhouse gases can contribute only in a minor way to the current warming, which is mainly of natural origin. The IPCC seems to be aware of this contrary evidence but has tried to ignore
it or wish it away. The summary for policymakers of IPCC’s Fourth Assessment Report (IPCC IPCC 2007-I, p. 5) distorts the key result of the CCSP report: “New analyses of balloon-borne
and satellite measurements of lowerand mid-tropospheric temperature show warming rates that are similar to those of the surface temperature record, and are consistent within their respective
uncertainties, largely reconciling a discrepancy noted in the TAR.” How is this possible? It is done partly by using the concept of “range” instead of the statistical distribution shown
in Figure 12a. But “range” is not a robust statistical measure because it gives undue weight to “outlier” results (Figure 12b). If robust probability distributions were used they would
show an exceedingly low probability of any overlap of modeled and the observed temperature trends. If one takes greenhouse model results seriously, then the greenhouse fingerprint would
suggest the true surface trend should be only 30 to 50 percent of the observed balloon/satellite trends in the troposphere. In that case, one would end up with a much-reduced surface
warming trend, an insignificant AGW effect, and a minor greenhouse-induced warming in the future. References Douglass, D.H., Pearson, B. and Singer, S.F. 2004. Altitude dependence of
atmospheric temperature trends: Climate models versus observations. Geophysical Research Letters 31. Douglass, D.H., Christy, J.R. , Pearson, B.D. and Singer, S.F. 2007. A comparison
of tropical temperature trends with model predictions. International Journal of Climatology (Royal Meteorol Soc). DOI:10.1002/joc.1651. IPCC. 2007-I. Climate Change 2007: The Physical
Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis,
K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge, UK. IPCC-SAR 1996. Climate Change 1995: The Science of Climate Change. Contribution of Working Group
I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK. Karl, T.R., Hassol, S.J. , Miller, C.D. and Murray, W.L.
(Eds.) 2006. Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. A report by the Climate Change Science Program and Subcommittee on Global
Change Research, http://www.climatescience.gov/Library/sap/sap1-1/final report/default.htm. Michaels, P.J. and Knappenberger, P.C. 1996. Human effect on global climate? Nature 384: 522-523.
NAS 2000. Reconciling Observations of Global Temperature Change. National Academy of Sciences. National Academy Press, Washington, DC. Santer, B.D., et al. 1996. Towards the detection
and attribution of an anthropogenic effect on climate. Climate Dynamics 12: 79-100. Singer, S.F. 1999. Human contribution to climate change remains questionable. Also, Reply. EOS: Transactions,
American Geophysical Union 80: 33, 186-187 and 372-373.
Observations: Temperature Records 109 Singer, S.F. 2000. Climate policy—From Rio to Kyoto a political issue for 2000 and beyond. Essays in Public Policy 102. Hoover Institution, Stanford
University, Stanford, CA. Singer, S.F. 2001. Disparity of temperature trends of atmosphere and surface. Paper presented at 12th Symposium on Global Climate Change, American Meteorological
Society, Albuquerque, NM. 3.5. Satellite Data The IPCC claims that data collected by satellitemounted microwave sounding units (MSU) and advanced MSU measurements since 1979 reveal a
warming trend of 0.12º C to 0.19º C per decade, which it says “is broadly consistent with surface temperature trends” (IPCC, 2007-I, p. 237). This would be surprising, since we indicated
in the previous section that the surface-based temperature record is unreliable and biased toward a spurious warming trend. In this section we investigate the truth of the IPCC’s claim
in this regard and report other findings based on satellite data. Most climate models predict that the troposphere should warm about 1.2 times more than the surface globally, and about
1.5 times more in the tropics. Although the MSUs mounted on satellites sent into orbit by NASA for the National Oceanic and Atmospheric Administration (NOAA) were not originally intended
to be used to measure temperatures in the troposphere, they have been used for this purpose since 1979 and, despite some ongoing debate, are acknowledged to be a reliable source of information
about temperatures in the troposphere (Christy et al., 2003, 2007; Santer et al., 2005). As Wentz and Schabel observed in an article in Nature in 1998, “the detection and measurement
of small changes in the Earth’s climate require extremely precise global observations of a broad spectrum of complementary physical variables. In this endeavour, satellite observations
are playing an increasingly important role. As compared to conventional in situ observations, satellites provide daily near-global coverage with a very high statistical precision that
results from averaging millions of individual observations” (Wentz and Schabel, 1998). Four groups currently report MSU measurements: the University of Alabama in Huntsville (UAH), Remote
Sensing System (RSS) (a small private weather forecasting firm led by the previously cited Frank Wentz), the University of Maryland (UMd), and a group from NOAA whose data series begins
in 1987. RSS and UAH produce estimates of temperatures for the lower troposphere (LT), midtroposphere (MT), and lower stratosphere (LS). UMd produces estimates only for MT (Christy and
Norris, 2006). New data for the UAH series is posted every month on a Web site maintained at the University of Alabama at Huntsville. The first satellite record was produced by Roy Spencer,
then with NASA and now the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer flying on NASA’s Aqua satellite, and John Christy, distinguished professor of atmospheric
science and director of the Earth System Science Center at the University of Alabama in Huntsville. Published in Science in 1990 (Spencer and Christy, 1990), the article presented the
first 10 years of satellite measurements of lower atmospheric temperature changes (from 1979 to 1988) and found “no obvious trend for the 10-year period.” Although this finding covered
too short a period of time to prove a trend, it seemed to contradict claims by some scientists at the time that a warming trend was underway. It triggered a long-running debate, which
continues to this day, over the accuracy of the satellite data. In 1997, Kevin Trenberth, of the U.S. National Center for Atmospheric Research (NCAR) and later a lead author of the IPCC’s
Third Assessment Report, along with coauthors challenged the reliability of the satellite data (Trenberth, 1997; Trenberth and Hurrell, 1997; Hurrell and Trenberth, 1997). Trenberth
argued that Spencer and Christy had failed to properly calibrate the sensors on each new satellite as older satellites were retired and new ones launched into orbit, based on a surface-satellite
comparison. Spencer and Christy, however, showed that the surface and tropospheric discrepancy was real, as it also was found in independent balloon comparisons (Christy et al., 1997).
Critics of the satellite data pointed to other possible and actual errors in the satellite record, and Spencer and Christy made two adjustments based on these external criticisms for
such things as orbit decay and changes in technology. One of the larger changes was made to correct for drift in local crossing time (i.e., change in the time-of-day that the measures
are taken), an error discovered by Mears and Wentz (2005) and subsequently corrected by Christy and Spencer (2005). Many of the adjustments made by Christy and Spencer resulted in the
satellite record
Climate Change Reconsidered 110 showing a small warming trend of 0.123 ºC between 1979 and 2005. In 2006, a panel of the U.S. Climate Change Science Program (CCSP) attempted to reconcile
differences between satellite and surface-station data. While the executive summary of the report claimed (as the IPCC does) that “this significant discrepancy [between surface station
records and satellite records] no longer exists because errors in the satellite and radiosonde data have been identified and corrected,” in fact significant differences in some values
(especially in the important region of the tropics, as discussed in Section 3.4. Fingerprints) remained unsolved (CCSP, 2006). Satellite data allow us to check the accuracy of the warming
trend during the last three decades reported by the three combined land-surface air temperature and sea-surface temperature (SST) records used by the IPCC: CRU (from the Climate Research
Unit (CRU) at the University of East Anglia, in Norwich, England), NCDC (from the National Climatic Data Center), and GISS (from NASA’s Goddard Institute for Space Studies). The IPCC
lists temperature trends (ºC /decade) for each record for the periods 1850-2005, 1901-2005, and 1979-2005 (IPCC, 2007-I, Table 3.3, p. 248). All three surface temperature records used
by the IPCC show positive trends in global temperatures during the 1979-2005 period (the period that can be checked against satellite data) of between 0.163º C/decade and 0.174 ºC /decade,
compared to the UAH record of only 0.123º C/decade. This means the IPCC’s estimates of warming are between 33 percent and 41.5 percent more rapid than the most scientifically accurate
record we have of global temperatures during this 26-year period. The IPCC claims an even higher estimate of 0.177º C/decade (44 percent higher than UAH) in a graph on page 253, a variation
of which appears in the Summary for Policymakers (p. 6). Finally, we note that none of the warming rates reported in the IPCC’s Table 3.3 reaches the 0.19º C that the IPCC claimed to
be the upper end of the range of credible estimates, while the UAH record of 0.128º C/decade sits very close to the lowest estimate of 0.12º C. Similarly, the IPCC’s temperature records
for the Northern Hemisphere are CRU’s 0.234º C and NCDC’s 0.245º C, approximately 17 percent to 22.5 percent higher than the UAH’s record of 0.20º C. For the Southern Hemisphere, the
IPCC’s estimates are 0.092º C (CRU) and 0.096 ºC (NCDC), or a very large 84 percent and 92 percent higher than the UAH’s 0.05º C. To say this is “broadly consistent,” as the IPCC does,
is not accurate. In light of the large discrepancy between satellite and surface records for the Southern Hemisphere, it is notable that Christy has been using the UAH database to detect
and correct errors in the Australian radiosondes record (Christy and Norris, 2009) and the surface station record in East Africa (Christy et al., 2009). Satellite data also allow us
to compare real-world temperatures to the predictions (or “scenarios”) offered by those who have been been predicting warming since the 1980s. Figure 3.5.1. compares the UAH and RSS
temperature records “adjusted to mimic surface temperature variations for an apples to apples comparison with the model projections (factor of 1.2, CCSP SAP 1.1)” to three model projections
of global surface temperature presented by NASA’s James Hansen in Senate testimony in 1988 (Christy, 2009). “GISS-A 88” and “GISS-B 88,” at the top of the graph, are Hansen’s two “business-as-usual”
model projections of temperature which assumed greenhouse gas emissions would be similar to what actually has happened. “GISS-C 88” is Hansen’s temperature forecast if drastic GHG reductions
were made. Obviously, real-world temperatures have failed to rise as Hansen had predicted, and indeed, global temperatures in 2009 were no higher than when Hansen testified in 1988.
As Christy comments, “Even the model projection for drastic CO2 cuts still overshot the observations. This would be considered a failed hypothesis test for the models from 1988” (Christy,
2009). By 2008, the UAH data series indicated that global temperatures in the lower atmosphere had warmed at the slightly higher rate of about 0.14º C/decade from January 1979 through
December 2007, while the RSS data series showed a warming rate of 0.17º C/decade. Recent research by Randall and Herman (2009) using data collected from a subset of weather balloon observations
thought to be most reliable suggests the RSS data incorporate an improper handling of diurnal cycle effects that causes a small warming bias over global land areas, thus suggesting that
the lower UAH estimate of 0.14º C/decade may have been closer to correct. Graphs showing both data sets and a third graph showing the difference between the two data sets appear in Figure
3.5.2. Explanations exist for two of the biggest differences between the two datasets. The first is a sudden warming in RSS relative to UAH in January 1992. This feature has been found
in comparison with
Observations: Temperature Records 111 every other surface and tropospheric temperature dataset, indicating RSS contains a spurious warming shift at that time (Christy et al., 2007).
The second feature is the relative cooling of RSS vs. UAH since 2006. This can be explained by the fact UAH uses a spacecraft (NASA’s Aqua) that is not subject to orbital drifting, whereas
RSS relies on NOAA-15, which is drifting into warmer diurnal times. The implication here is that RSS is overcorrecting for this spurious warming by reporting too much cooling. Overall,
the shifts unique to RSS create a spurious warming in the record, which is being slowly mitigated by the more recent spurious cooling. The graphs show that the temperature anomalies
in the RSS dataset for November 2007 and December 2007 were below the 1979-1998 mean average for the first time since 2000. Both data series show the rate of warming has slowed dramatically
during the past seven to 12 years. Between the end of 2007 and early 2009, global temperature anomalies fell even further, effectively returning the world to the temperatures that prevailed
in the late 1980s and early 1990s. See Figure 3.5.3. below. The new trend toward less warming has prompted some scientists to wonder if the world’s climate experienced a trend break
in 2001-2002, similar to ones that occurred around 1910 (ending a cooling trend), the early 1940s (ending a warming trend), and the mid-1970s (ending a cooling trend). Swanson and Anastasios
(2009), writing in Geophysical Research Letters, say “a break in the global mean temperature trend from the consistent warming over the 1976/77– 2001/02 period may have occurred.” Moreover,
the episodic nature of temperature changes during the Figure 3.5.1. Actual temperature changes from UAH and RSS satellite data, adjusted to mimic surface temperatures, compared to predictions
made by James Hansen to Congress in 1988. Source: Christy, J.R. 2009. Written testimony to House Ways and Means Committee. 25 February. http://waysandmeans.house.gov/media/pdf/111/ctest.pdf,
last accessed May 10, 2009
Climate Change Reconsidered 112 past century is “difficult to reconcile with the presumed smooth evolution of anthropogenic greenhouse gas and aerosol radiative forcing with respect
to time” and “suggests that an internal reorganization of the climate system may underlie such shifts.” Noel Keenlyside, a scientist with Germany’s Leibniz Institute of Marine Science,
writing with colleagues in a letter published in Nature, said “the climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences”
and “these multidecadal Figure 3.5.2. Global temperature anomalies from the lower troposphere. January 1979 through December 2007. (top) Data compiled at the University of Alabama, (middle)
data compiled by Remote Sensing Systems, (bottom) difference between the two datasets (UAH minus RSS). Graphs were produced by Patrick Michaels using UAH and RSS data and first appeared
in World Climate Report on February 7, 2008, http://www.worldclimatereport.com/index.php/category/temperaturehistory/satelliteballoons/.
Observations: Temperature Records 113 variations are potentially predictable if the current state of the ocean is known” (Keenlyside et al., 2008). Using a database of sea-surface temperature
(SST) observations, they “make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North
Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. Our results suggest that global surface
temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.” We
predict more predictions of this kind as more scientists recognize, first, that estimates of past warming have been exaggerated by reliance on surface-station data that have been discredited
by physical observation and by testing against superior satellite data; second, that recent temperature trends contradict past and recent forecasts by the IPCC and other prominent advocates
of the theory that temperatures will steadily rise in response to increasing forcing by rising CO2 levels in the atmosphere; and third, as attention turns to natural cycles like those
modeled by Keenlyside et al., as most scientists have known all along are more influential than the small effects of rising CO2 in the atmosphere. References CCSP. 2006. Temperature
Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. Karl, T.R., Hassol, S.J., Miller, C.D., and Murray, W.L. (Eds.) A Report by the Climate Change Science
Program and the Subcommittee on Global Change Research, Washington, DC. Christy, J.R. 2009. Written testimony to House Ways and Means Committee. 25 February. http://waysandmeans. house.gov/media/pdf/
11/ctest.pdf, last accessed May 10, 2009. Christy, J.R. and Norris, W.B. 2006. Satellite and VIZradiosonde intercomparisons for diagnosis of nonclimatic influences. Journal of Atmospheric
and Oceanic Technology 23: 1181-1194. Christy, J.R. and Norris, W.B. 2009. Discontinuity issues with radiosondes and satellite temperatures in the Australia region 1979-2006. Journal
of Atmospheric and Oceanic Technology 25: OI:10.1175/2008JTECHA1126.1. Figure 3.5.3. UAH Globally Averaged Satellite-Based Temperature of the Lower Atmosphere, January 1979 – April 2009.
Source: Roy Spencer, http://www.drroyspencer.com/latest-global-temperatures/, last accessed May 10, 2009.
Climate Change Reconsidered 114 Christy, J.R., Norris, W.B. and McNider, R.T. 2009. Surface temperature variations in East Africa and possible causes. Journal of Climate. 22 (in press).
Christy, J.R., Norris, W.B., Spencer, R.W. and Hnilo, J.J. 2007. Tropospheric temperature change since 1979 from tropical radiosonde and satellite measurements. Journal of Geophysical
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accurate are satellite “thermometers”? Nature 389: 342-3. Christy, J.R., Spencer, R.W., Norris, W.B., Braswell, W.D. and Parker, D.E. 2003. Error estimates of version 5.0 of MSU-AMSU
bulk atmospheric temperatures. Journal of Atmospheric and Oceanic Technology 20: 613-629. Douglass, D.H., Christy, J.R., Pearson, B.D. and Singer, S.F. 2007. A comparison of tropical
temperature trends with model predictions. International Journal of Climatology. DOI:10.1002/joc.1651. 1651. Hurrell,
J.W. and Trenberth, K.E. 1997. Spurious trends in the satellite MSU temperature record arising from merging different satellite records. Nature 386: 164-167. IPCC. 2007-I. Climate Change
2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning, Z.
Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge, UK. Keenlyside, N.S., Latif, M., Jungclaus, J., Kornblueh, L. and Roeckner, E.
2008. Advancing decadal-scale climate prediction in the North Atlantic sector. Nature 453: 84-88. Mears, C.A. and Wentz, F.J. 2005. The effect of diurnal correction on satellite-derived
lower tropospheric temperature. Science 309: 1548-1551. Randall, R.M. and Herman, B.M. 2008. Using limited time period trends as a means to determine attribution of discrepancies in
microwave sounding unit derived tropospheric temperature time series. Journal of Geophysical Research: doi:10.1029/2007JD008864. Santer, B.D. et al. 2005. Amplification of surface temperature
trends and variability in the tropical atmosphere. Science 309: 1551-1556. Spencer, R.W., Braswell, W.D., Christy, J.R. and Hnilo, J. 2007. Cloud and radiation budget changes associated
with tropical intraseasonal oscillations. Geophysical Research Letters 34: L15707, doi:10.1029/2007GLO296998. Spencer, R.W. and Christy, J.R. 1990. Precise monitoring of global temperature
trends from satellites. Science 247: 1558-1562. Swanson, K.L. and Tsonis, A.A. 2009. Has the climate shifted? Geophysical Research Letters 36: L06711, doi:10.1029/2008GL037022. Trenberth,
K.E. 1997. The use and abuse of climate models in climate change research. Nature 386: 131-133. Trenberth, K.E. and Hurrell, J.W. 1997. How accurate are satellite “thermometers.” Nature
389: 342-343. Wentz, F.J. and Schabel, M. 1998. Effects of satellite orbital decay on MSU lower tropospheric temperature trends. Nature 394: 661-664. 3.6. Arctic The IPCC claims “average
arctic temperatures increased at almost twice the global average rate in the past 100 years,” though it then acknowledges that “arctic temperatures have high decadal variability, and
a warm period was also observed from 1925 to 1945” (IPCC 2007-I, p. 7). Later in the report, the IPCC says “the warming over land in the Arctic north of 65°C is more than double the
warming in the global mean from the 19th century to the 21st century and also from about the late 1960s to the present. In the arctic series, 2005 is the warmest year” (p. 248). But
the IPCC then admits that “a few areas have cooled since 1901, most notably the northern North Atlantic near southern Greenland” (p. 252). So has the Artic really experienced the so-called
unprecedented warming of the twentieth century? References IPCC. 2007-I. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report
of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge,
UK.
Observations: Temperature Records 115 3.6.1. Greenland Dahl-Jensen et al. (1998) used data from two ice sheet boreholes to reconstruct the temperature history of Greenland over the past
50,000 years. Their analysis indicated that temperatures on the Greenland Ice Sheet during the Last Glacial Maximum (about 25,000 years ago) were 23 ± 2 °C colder than at present. After
the termination of the glacial period, however, temperatures increased steadily to a value that was 2.5°C warmer than at present, during the Climatic Optimum of 4,000 to 7,000 years
ago. The Medieval Warm Period and Little Ice Age were also evident in the borehole data, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present, respectively. Then, after
the Little Ice Age, the scientists report “temperatures reached a maximum around 1930 AD” and that “temperatures have decreased [our italics] during the last decades.” The results of
this study stand in stark contrast to the predictions of general circulation models of the atmosphere, which consistently suggest there should have been a significant CO2-induced warming
in high northern latitudes over the past several decades. They also depict large temperature excursions over the past 10,000 years, when the air’s CO2 content was relatively stable.
Each of these observations raises serious doubts about the models’ ability to correctly forecast earth’s climatic response to the ongoing rise in the air’s CO2 content. In another study
of Greenland climate that included both glacial and interglacial periods, Bard (2002) reviews the concept of rapid climate change. Of this phenomenon, he writes that “it is now recognized
that the ocean-atmosphere system exhibits several stable regimes under equivalent external forcings,” and that “the transition from one state to another occurs very rapidly when certain
climatic parameters attain threshold values.” Specifically, he notes that in the models “a slight increase in the freshwater flux above the modern level F produces a decrease in the
NADW [North Atlantic Deep Water] convection and a moderate cooling in the North Atlantic,” but that “the system flips to another state once the flux reaches a threshold value F + deltaF,”
which state has no deep convection and “is characterized by surface temperatures up to 6°C lower in and around the North Atlantic.” With respect to what has been learned from observations,
Bard concentrates on the region of the North Atlantic, describing glacial-period millennialscale episodes of dramatic warming called Dansgaard-Oeschger events (with temperature increases
“of more than 10°C”), which are evident in Greenland ice core records, as well as episodes of “drastic cooling” called Heinrich events (with temperature drops “of up to about 5°C”),
which are evident in sea surface temperature records derived from the study of North Atlantic deep-sea sediment cores. In the Greenland record, according to Bard, the progression of
these events is such that “the temperature warms abruptly to reach a maximum and then slowly decreases for a few centuries before reaching a threshold, after which it drops back to the
cold values that prevailed before the warm event.” He also reports that “models coupling the atmosphere, ocean, and ice sheets are still unable to correctly simulate that variability
on all scales in both time and space,” which suggests we do not fully understand the dynamics of these rapid climate changes. Bard states, “all the studies so far carried out fail to
answer the crucial question: How close are we to the next bifurcation [which could cause a rapid change-ofstate in earth’s climate system]?” In this regard, he notes that “an intense
debate continues in the modeling community about the reality of such instabilities under warm conditions,” which is a particularly important point, since all dramatic warming and cooling
events that have been detected to date have occurred in either full glacials or transitional periods between glacials and interglacials. This latter real-world fact clearly suggests
we are unlikely to experience any dramatic warming or cooling surprises in the near future, as long as the earth does not begin drifting towards glacial conditions, which is another
reason to not be concerned about the ongoing rise in the air’s CO2 content. In fact, it suggests that allowing more CO2 to accumulate in the atmosphere provides an “insurance policy”
against abrupt climate change; interglacial warmth seems to inoculate the planet against climatic instabilities, allowing only the mild millennial-scale climatic oscillation that alternately
brings the earth slightly warmer and cooler conditions typical of the Medieval Warm Period and Little Ice Age. Focusing on the more pertinent period of the current interglacial or Holocene,
we next consider a number of papers that bear upon the reality of the Medieval Warm Period and Little Ice Age: two wellknown multi-century periods of significant climatic aberration.
These periods of modest climatic aberration, plus the analogous warm and cool periods that preceded them (the Roman Warm Period and
Climate Change Reconsidered 116 Dark Ages Cold Period), provide strong evidence for the existence of a millennial-scale oscillation of climate that is unforced by changes in the air’s
CO2 content, which in turn suggests that the global warming of the Little Ice Age-to-Current Warm Period transition was likely totally independent of the coincidental concomitant increase
in the air’s CO2 content that accompanied the Industrial Revolution. We begin with the study of Keigwin and Boyle (2000), who briefly reviewed what is known about the millennial-scale
oscillation of earth’s climate that is evident in a wealth of proxy climate data from around the world. Stating that “mounting evidence indicates that the Little Ice Age was a global
event, and that its onset was synchronous within a few years in both Greenland and Antarctica,” they remark that in Greenland it was characterized by a cooling of approximately 1.7°C.
Likewise, in an article titled “Was the Medieval Warm Period Global?” Broecker (2001) answers yes, citing borehole temperature data that reveal the magnitude of the temperature drop
over Greenland from the peak warmth of the Medieval Warm Period (800 to 1200 A.D.) to the coldest part of the Little Ice Age (1350 to 1860 A.D.) to have been approximately 2°C, and noting
that as many as six thousand borehole records from all continents of the world confirm that the earth was a significantly warmer place a thousand years ago than it is today. McDermott
et al. (2001) derived a d18O record from a stalagmite discovered in Crag Cave in southwestern Ireland, after which they compared this record with the d18O records from the GRIP and GISP2
ice cores from Greenland. In doing so, they found evidence for “centennial-scale d18O variations that correlate with subtle d18O changes in the Greenland ice cores, indicating regionally
coherent variability in the early Holocene.” They additionally report that the Crag Cave data “exhibit variations that are broadly consistent with a Medieval Warm Period at ~1000 ± 200
years ago ago and a two-stage Little Ice Age, as reconstructed by inverse modeling of temperature profiles in the Greenland Ice Sheet.” Also evident in the Crag Cave data were the d18O
signatures of the earlier Roman Warm Period and Dark Ages Cold Period that comprised the prior such cycle of climate in that region. In concluding they reiterate the important fact that
the coherent d18O variations in the records from both sides of the North Atlantic “indicate that many of the subtle multicentury d18O variations in the Greenland ice cores reflect regional
North Atlantic margin climate signals rather than local effects.” Another study that looked at temperature variations on both sides of the North Atlantic was that of Seppa and Birks
(2002), who used a recently developed pollen-climate reconstruction model and a new pollen stratigraphy from Toskaljavri, a tree-line lake in the continental sector of northern Fenoscandia
(located just above 69°N latitude), to derive quantitative estimates of annual precipitation and July July mean temperature. The two scientists say their reconstructions “agree with
the traditional concept of a ‘Medieval Warm Period’ (MWP) and ‘Little Ice Age’ in the North Atlantic region (Dansgaard et al., 1975).” Specifically, they report there is “a clear correlation
between our MWP reconstruction and several records from Greenland ice cores,” and that “comparisons of a smoothed July temperature record from Toskaljavri with measured borehole temperatures
of the GRIP and Dye 3 ice cores (Dahl-Jensen et al., 1998) and the d18O record from the Crete ice core (Dansgaard et al., 1975) show the strong similarity in timing of the MWP between
the records.” Last of all, they note that “July temperature values during the Medieval Warm Period (ca. 1400-1000 cal yr B.P.) were ca. 0.8°C higher than at present,” where present means
the last six decades of the twentieth century. Concentrating solely on Greenland and its immediate environs are several other papers, among which is the study of Wagner and Melles (2001),
who retrieved a sediment core from a lake on an island situated just off Liverpool Land on the east coast of Greenland. Analyzing it for a number of properties related to the past presence
of seabirds there, they obtained a 10,000-year record that tells us much about the region’s climatic history. Key to the study were certain biogeochemical data that reflected variations
in seabird breeding colonies in the catchment area of the lake. These data revealed high levels of the various parameters measured by Wagner and Melles between about 1,100 and 700 years
before present (BP) that were indicative of the summer presence of significant numbers of seabirds during that “medieval warm period,” as they describe it, which had been preceded by
a several-hundred-year period of little to no inferred bird presence. Then, after the Medieval Warm Period, the data suggested another absence of birds during what they refer to as “a
subsequent Little Ice Age,” which they note was “the coldest period since the early Holocene in East Greenland.” Their
Observations: Temperature Records 117 data also showed signs of a “resettlement of seabirds during the last 100 years, indicated by an increase of organic matter in the lake sediment
and confirmed by bird observations.” However, values of the most recent data were not as great as those obtained from the earlier Medieval Warm Period; and temperatures derived from
two Greenland ice cores led to the same conclusion: it was warmer at various times between 1,100 to 700 years BP than it was over the twentieth century. Kaplan et al. (2002) also worked
with data obtained from a small lake, this one in southern Greenland, analyzing sediment physical-chemical properties, including magnetic susceptibility, density, water content, and
biogenic silica and organic matter concentrations. They discovered that “the interval from 6000 to 3000 cal yr BP was marked by warmth and stability.” Thereafter, however, the climate
cooled “until its culmination during the Little Ice Age,” but from 1,300-900 years BP, there was a partial amelioration of climate (the Medieval Warm Period) that was associated with
an approximate 1.5°C rise in temperature. Following another brief warming between AD 1500 and 1750, the second and more severe portion of the Little Ice Age occurred, which was in turn
followed by “naturally initiated post-Little Ice Age warming since AD 1850, which is recorded throughout the Arctic.” They report that Viking “colonization around the northwestern North
Atlantic occurred during peak Medieval Warm Period conditions that ended in southern Greenland by AD 1100,” noting that Norse movements around the region thereafter “occurred at perhaps
the worst time in the last 10,000 years, in terms of the overall stability of the environment for sustained plant and animal husbandry.” We can further explore these aspects of Greenland’s
climatic history from three important papers that reconstructed environmental conditions in the vicinity of Igaliku Fjord, South Greenland, before, during, and after the period of Norse
habitation of of this and other parts of the ice-covered island’s coast, beginning with the study of Lassen et al. (2004), who provide some historical background to their palaeoclimatic
work by reporting that “the Norse, under Eric the Red, were able to colonize South Greenland at AD 985, according to the Icelandic Sagas, owing to the mild Medieval Warm Period climate
with favorable open-ocean conditions.” They also mention, in this regard, that the arrival of the gritty Norsemen was “close to the peak of Medieval warming recorded in the GISP2 ice
core which was dated at AD 975 (Stuiver et al., 1995),” while we additionally note that Esper et al. (2002) independently identified the peak warmth of this period throughout North American
extratropical latitudes as “occurring around 990.” Hence, it would appear that the window of climatic opportunity provided by the peak warmth of the Medieval Warm Period was indeed a
major factor enabling seafaring Scandinavians to establish long-enduring settlements on the coast of Greenland. As time progressed, however, the glowing promise of the apex of Medieval
warmth gave way to the debilitating reality of the depth of Little Ice Age cold. Jensen et al. (2004), for example, report that the diatom record of Igaliku Fjord “yields evidence of
a relatively moist and warm climate at the beginning of settlement, which was crucial for Norse land use,” but that “a regime of more extreme climatic fluctuations began soon after AD
1000, and after AD c. 1350 cooling became more severe.” Lassen et al. additionally note that “historical documents on Iceland report the presence of the Norse in South Greenland for
the last time in AD 1408,” during what they describe as a period of “unprecedented influx of (ice-loaded) East Greenland Current water masses into the innermost parts of Igaliku Fjord.”
They also report that “studies of a Canadian high-Arctic ice core and nearby geothermal data (Koerner and Fisher, 1990) correspondingly show a significant temperature lowering at AD
1350-1400,” when, in their words, “the Norse society in Greenland was declining and reaching its final stage probably before the end of the fifteenth century.” Consequently, what the
relative warmth of the Medieval Warm Period provided the Norse settlers, the relative cold of the Little Ice Age took from them: the ability to survive on Greenland. More details of
the saga of five centuries of Nordic survival at the foot of the Greenland Ice Cap are provided by the trio of papers addressing the palaeohistory of Igaliku Fjord. Based on a highresolution
record of the fjord’s subsurface water-mass properties derived from analyses of benthic foraminifera, Lassen et al. conclude that stratification of the water column, with Atlantic water
masses in its lower reaches, appears to have prevailed throughout the last 3,200 years, except for the Medieval Warm Period. During this period, which they describe as occurring between
AD 885 and 1235, the outer part of Igaliku Fjord experienced enhanced vertical mixing (which they attribute to increased wind wind stress) that would have been expected to increase nutrient
Climate Change Reconsidered 118 availability there. A similar conclusion was reached by Roncaglia and Kuijpers (2004), who found evidence of increased bottom-water ventilation between
AD 960 and 1285. Hence, based on these findings, plus evidence of the presence of Melonis barleeanus during the Medieval Warm Period (the distribution of which is mainly controlled by
the presence of partly decomposed organic matter), Lassen et al. conclude that surface productivity in the fjord during this interval of unusual relative warmth was “high and thus could
have provided a good supply of marine food for the Norse people.” Shortly thereafter, the cooling that led to the Little Ice Age was accompanied by a gradual restratification of the
water column, which curtailed nutrient upwelling and reduced the high level of marine productivity that had prevailed throughout the Medieval Warm Period. These linked events, according
to Lassen et al., “contributed to the loss of the Norse settlement in Greenland.” Indeed, with deteriorating growing conditions on land and simultaneous reductions in oceanic productivity,
the odds were truly stacked against the Nordic colonies, and it was only a matter of time before their fate was sealed. As Lassen et al. describe it, “around AD 1450, the climate further
deteriorated with further increasing stratification of the water-column associated with stronger advection of (ice-loaded) East Greenland Current water masses.” This development, in
their words, led to an even greater “increase of the ice season and a decrease of primary production and marine food supply,” which “could also have had a dramatic influence on the local
seal population and thus the feeding basis for the Norse population.” The end result of these several conjoined phenomena, in the words of Lassen et al., was that “climatic and hydrographic
changes in the area of the Eastern Settlement were significant in the crucial period when the Norse disappeared.” Also, Jensen et al. report that “geomorphological studies in Northeast
Greenland have shown evidence of increased winter wind speed, particularly in the period between AD 1420 and 1580 (Christiansen, 1998),” noting that “this climatic deterioration coincides
with reports of increased sea-ice conditions that caused difficulties in using the old sailing routes from Iceland westbound and further southward along the east coast of Greenland,
forcing sailing on more southerly routes when going to Greenland (Seaver, 1996).” In light of these observations, Jensen et al. state that “life conditions certainly became harsher during
the 500 years of Norse colonization,” and that this severe cooling-induced environmental deterioration “may very likely have hastened the disappearance of the culture.” At the same time,
it is also clear that the more favorable living conditions associated with the peak warmth of the Medieval Warm Period—which occurred between approximately AD 975 (Stuiver et al., 1995)
and AD 990 (Esper et al., 2002)—were what originally enabled the Norse to successfully colonize colonize the region. In the thousand-plus subsequent years, there has never been a sustained
period of comparable warmth, nor of comparable terrestrial or marine productivity, either locally or hemispherically (and likely globally, as well), the strident protestations of Mann
et al. (2003) notwithstanding. Concentrating on the twentieth century, Hanna and Cappelen (2003) determined the air temperature history of coastal southern Greenland from 1958-2001,
based on data from eight Danish Meteorological Institute stations in coastal and nearcoastal southern Greenland, as well as the concomitant sea surface temperature (SST) history of the
Labrador Sea off southwest Greenland, based on three previously published and subsequently extended SST datasets (Parker et al., 1995; Rayner et al., 1996; Kalnay et al., 1996). The
coastal temperature data showed a cooling of 1.29°C over the period of study, while two of the three SST databases also depicted cooling: by 0.44°C in one case and by 0.80°C in the other.
Both the the land-based air temperature and SST series followed similar patterns and were strongly correlated, but with no obvious lead/lag either way. In addition, it was determined
that the cooling was “significantly inversely correlated with an increased phase of the North Atlantic Oscillation (NAO) over the past few decades.” The two researchers say this “NAO-temperature
link doesn’t explain what caused the observed cooling in coastal southern Greenland but it does lend it credibility.” In referring to what they call “this important regional exception
to recent ‘global warming’,” Hanna and Cappelen note that the “recent cooling may have significantly added to the mass balance of at least the southern half of the [Greenland] Ice Sheet.”
Consequently, since this part of the ice sheet is the portion that would likely be the first to experience melting in a warming world, it would appear that whatever caused the cooling
has not only protected the Greenland Ice Sheet against warminginduced disintegration but actually actually fortified it against that possibility.
Observations: Temperature Records 119 Several other studies have also reported latetwentieth century cooling on Greenland. Based on mean monthly temperatures of 37 Arctic and seven sub-Arctic
stations, as well as temperature anomalies of 30 grid-boxes from the updated dataset of Jones, for example, Przybylak (2000) found that “the level of temperature in Greenland in the
last 10-20 years is similar to that observed in the 19th century.” Likewise, in a study that utilized satellite imagery of the Odden ice tongue (a winter ice cover that occurs in the
Greenland Sea with a length of about 1,300 km and an aerial coverage of as much as 330,000 square kilometers) plus surface air temperature data from adjacent Jan Mayen Island, Comiso
et al. (2001) determined that the ice phenomenon was “a relatively smaller feature several decades ago,” due to the warmer temperatures that were prevalent at that time. In addition,
they report that observational evidence from Jan Mayen Island indicates temperatures there cooled at a rate of 0.15 ± 0.03°C per decade during the past 75 years. Taurisano et al. (2004)
examined the temperature history of the Nuuk fjord during the last century, where their analyses of all pertinent regional data led them to conclude that “at all stations in the Nuuk
fjord, both the annual mean and the average temperature of the three summer months (June, July and August) exhibit a pattern in agreement with the trends observed at other stations in
south and west Greenland (Humlum 1999; Hanna and Cappelen, 2003).” As they describe it, the temperature data “show that a warming trend occurred in the Nuuk fjord during the first 50
years of the 1900s, followed by a cooling over the second part of the century, when the average annual temperatures decreased by approximately 1.5°C.” Coincident with this cooling trend
there was also what they describe as “a remarkable increase in the number of snowfall days (+59 days).” What is more, they report that “not only did the cooling affect the winter months,
as suggested by Hannna and Cappelen (2002), but also the summer mean,” noting that “the summer cooling is rather important information for glaciological studies, due to the ablation-temperature
relations.” In a study of three coastal stations in southern and central Greenland that possess almost uninterrupted temperature records between 1950 and 2000, Chylek et al. (2004) discovered
that “summer temperatures, which are most relevant to Greenland ice sheet melting rates, do not show any persistent increase during the last fifty years.” In fact, working with the two
stations with the longest records (both over a century in length), they determined that coastal Greenland’s peak temperatures occurred between 1930 and 1940, and that the subsequent
decrease in temperature was so substantial and sustained that current coastal temperatures “are about 1°C below their 1940 values.” Furthermore, they note that “at the summit of the
Greenland ice sheet the summer average temperature has decreased at the rate of 2.2°C per decade since the beginning of the measurements in 1987.” Hence, as with the Arctic as a whole,
it would appear that Greenland has not experienced any net warming over the most dramatic period of atmospheric CO2 increase on record. In fact, it has cooled during this period. At
the start of the twentieth century, however, Greenland was warming, as it emerged, along with the rest of the world, from the depths of the Little Ice Age. Between 1920 and 1930, when
the atmosphere’s CO2 concentration rose by a mere 3 to 4 ppm, there was a phenomenal warming at all five coastal locations for which contemporary temperature records are available. In
the words of Chylek et al., “average annual temperature rose between 2 and 4°C [and by as much as 6°C in the winter] in less than ten years.” And this warming, as they note, “is also
seen in the 18O/16O record of the Summit ice core (Steig et al., 1994; Stuiver et al., 1995; White et al., 1997).” In commenting on this dramatic temperature rise, which they call the
“great Greenland warming of the 1920s,” Chylek et al. conclude that “since there was no significant increase in the atmospheric greenhouse gas concentration during that time, the Greenland
warming of the 1920s demonstrates that a large and rapid temperature increase can occur over Greenland, and perhaps in other regions of the Arctic, due to internal climate variability
such as the NAM/NAO [Northern Annular Mode/North Atlantic Oscillation], without a significant anthropogenic influence.” These facts led them to speculate that “the NAO may play a crucial
role in determining local Greenland climate during the 21st century, resulting in a local climate that may defy the global climate change.” Clearly, there is no substance to the claim
that Greenland provides evidence for an impending CO2-induced warming. These many studies of the temperature history of Greenland depict long-term oscillatory
cooling ever since the Climatic Optimum of the mid-Holocene, when it was perhaps 2.5°C warmer than it is now, within which cooling trend is included the Medieval Warm Period, when it
was about 1°C warmer than it is currently, and the Little Ice Age, when it was 0.5 to 0.7°C cooler than now,
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A century of climate variability and climate gradients from coast to ice sheet in West Greenland. Geografiska Annaler 86A: 217-224. Wagner, B. and Melles, M. 2001. A Holocene seabird
record from Raffles So sediments, East Greenland, in response to climatic and oceanic changes. Boreas 30: 228-239. White, J.W.C., Barlow, L.K., Fisher, D., Grootes, P.M., Jouzel, J.,
Johnsen, S.J., Stuiver, M. and Clausen, H.B. 1997. The climate signal in the stable isotopes of snow from Summit, Greenland: Results of comparisons with modern climate observations.
Journal of Geophysical Research 102: 26,425-26,439. 3.6.1.2. Rest of Arctic Overpeck et al. (1997) combined paleoclimatic records obtained from lake and marine sediments, trees, and
glaciers to develop a 400-year history of circum-Arctic surface air temperature. From this record they determined that the most dramatic warming of the last four centuries of the past
millennium (1.5°C) occurred between 1840 and 1955, over which period the air’s CO2 concentration rose from approximately 285 ppm to 313 ppm, or by 28 ppm. Then, from 1955 to the end
of the record (about 1990), the mean circum-Arctic air temperature declined by 0.4°C, while the air’s CO2 concentration rose from 313 ppm to 354 ppm, or by 41 ppm. On the basis of these
observations, which apply to the entire Arctic, it is not possible to assess the influence of atmospheric CO2 on surface air temperature or even to conclude it has any effect at all.
Why? Because over the first 115 years of warming, as the air’s CO2 concentration rose by an average of 0.24 ppm/year, air temperature rose by an average of 0.013°C/year; over the final
35 years of the record, when the air’s CO2 content rose at a mean rate of 1.17 ppm/year (nearly five times the rate at which it had risen in the prior period), the rate-of-rise of surface
air temperature decelerated, to a mean value (0.011°C/year) that was nearly the same as the rate at which it had previously risen. Naurzbaev and Vaganov (2000) developed a 2,200-year
temperature history using tree-ring data obtained from 118 trees near the upper-timberline in Siberia for the period 212 BC to AD 1996, as well as a similar history covering the period
of the Holocene Climatic Optimum (3300 to 2600 BC). They compared their results with those obtained from an analysis of isotopic oxygen data extracted from a Greenland ice core. This
work revealed that fluctuations in average annual temperature derived from the Siberian record agreed well with air temperature variations reconstructed from the Greenland data, suggesting
to the two researchers that “the tree ring chronology of [the Siberian] region can be used to analyze both regional peculiarities and global temperature variations in the Northern Hemisphere.”
Naurzbaev and Vaganov reported that several warm and cool periods prevailed for several multicentury periods throughout the last two millennia: a cool period in the first two centuries
AD, a warm period from AD 200 to 600, cooling again from 600 to 800 AD, followed by the Medieval Warm Period from about AD 850 to 1150, the cooling of the Little Ice Age from AD 1200
though 1800, followed by the recovery recovery warming of the twentieth century. In regard to this latter temperature rise, however, the two scientists say it was “not extraordinary,”
and that “the warming at the border of the first and second millennia [AD 1000] was longer in time and similar in amplitude.” In addition, their reconstructed temperatures for the Holocene
Climatic Optimum revealed there was an even warmer period about 5,000 years ago, when temperatures averaged 3.3°C more than they did over the past two millennia. Contemporaneously, Vaganov
et al. (2000) also used tree-ring width as a temperature proxy, reporting temperature variations for the Asian subarctic region over the past 600 years. Their graph of these data reveals
that temperatures in this region exhibited a
Climate Change Reconsidered 122 small positive trend from the beginning of the record until about AD 1750. Thereafter, a severe cooling trend ensued, followed by a 130-year warming trend
from about 1820 through 1950, after which temperatures fell once again. In analyzing the entire record, the researchers determined that the amplitude of twentieth century warming “does
not go beyond the limits of reconstructed natural temperature fluctuations in the Holocene subarctic zone.” And in attempting to determine the cause or causes of the temperature fluctuations,
they report finding a significant correlation with solar radiation and volcanic activity over the entire 600-year period (r = 0.32 for solar radiation, r = -0.41 for volcanic activity),
which correlation improved over the shorter interval (1800-1990) of the industrial period (r = 0.68 for solar radiation, r = -0.59 for volcanic activity). It is also enlightening to
note, in this regard, that in this region of the world, where climate models predict large large increases in temperature as a result of the historical rise in the air’s CO2 concentration,
real-world data show an actual cooling trend since around 1940. Where warming does exist in the record—between about 1820 and 1940—much of it correlates with changes in solar irradiance
and volcanic activity, two factors that are free of anthropogenic influence. One year later, Moore et al. (2001) analyzed sediment cores extracted from Donard Lake, Baffin Island, Canada
(~66.25°N, 62°W), to produce a 1,240-year record of mean summer temperature for this region that averaged 2.9°C over the period AD 750-1990. Within this period there were several anomalously
warm decades with temperatures that were as high as 4°C around AD 1000 and 1100, while at the beginning of the thirteenth century Donard Lake witnessed what they called “one of the largest
climatic transitions in over a millennium,” as “average summer temperatures rose rapidly by nearly 2°C from AD 1195-1220, ending in the warmest decade in the record,” with with temperatures
near 4.5°C. This latter temperature rise was then followed by a period of extended warmth that lasted until an abrupt cooling event occurred around AD 1375, resulting in the following
decade being one of the coldest in the record and signaling the onset of the Little Ice Age on Baffin Island, which lasted 400 years. At the modern end of the record, a gradual warming
trend occurred over the period 1800-1900, followed by a dramatic cooling event that brought temperatures back to levels characteristic of the Little Ice Age, which chilliness lasted
until about 1950. Thereafter, temperatures rose once more throughout the 1950s and 1960s, whereupon they trended downwards toward cooler conditions to the end of the record in 1990.
Gedalof and Smith (2001) compiled a transect of six tree ring-width chronologies from stands of mountain hemlock growing near the treeline that extends from southern Oregon to the Kenai
Peninsula, Alaska. Over the period of their study (AD 1599-1983), they determined that “much of the preinstrumental record in the Pacific Northwest region of North America [was] characterized
by alternating regimes of relatively warmer and cooler SST [sea surface temperature] in the North Pacific, punctuated by abrupt shifts in the mean background state,” which were found
to be “relatively common occurrences.” They concluded that “regime shifts in the North Pacific have occurred 11 times since 1650.” A significant aspect of these findings is the fact
that the abrupt 1976-77 shift in this Pacific Decadal Oscillation, as it is generally called, is what was responsible for the vast majority of the past halfcentury’s warming in Alaska,
which some commentators wrongly point to as evidence of CO2-induced global warming. About the same time, Kasper and Allard (2001) examined soil deformations caused by ice wedges (a widespread
and abundant form of ground ice in permafrost regions that can grow during colder periods and deform and crack the soil). Working near Salluit, northern Quebéc (approx. 62°N, N, 75.75°W),
they found evidence of ice wedge activity prior to AD 140, reflecting cold climatic conditions. Between AD 140 and 1030, however, this activity decreased, reflective of warmer conditions.
Then, from AD 1030 to 1500, conditions cooled; and from 1500 to 1900 ice wedge activity was at its peak, when the Little Ice Age ruled, suggesting this climatic interval exhibited the
coldest conditions of the past 4,000 years. Thereafter, a warmer period prevailed, from about 1900 to 1946, which was followed by a return to cold conditions during the last five decades
of the twentieth century, during which time more than 90 percent of the ice wedges studied reactivated and grew by 20-30 cm, in harmony with a reported temperature decline of 1.1°C observed
at the meteorological station in Salluit. In another study from the same year, Zeeberg and Forman (2001) analyzed twentieth century changes in glacier terminus positions on north Novaya
Zemlya, a Russian island located between the Barents and Kara Seas in the Arctic Ocean, providing in the process a quantitative assessment of the effects of temperature
Observations: Temperature Records 123 and precipitation on glacial mass balance. This work revealed a significant and accelerated post-Little Ice Age glacial retreat in the first and
second decades of the twentieth century; but by 1952, the region’s glaciers had experienced between 75 to 100 percent of their net twentieth century retreat. During the next 50 years,
the recession of more than half of the glaciers stopped, and many tidewater glaciers actually began to advance. These glacial stabilizations and advances were attributed by the two scientists
to observed increases in precipitation and/or decreases in temperature. In the four decades since 1961, for example, weather stations at Novaya Zemlya show summer temperatures to have
been 0.3° to 0.5°C colder than they were over the prior 40 years, while winter temperatures were 2.3° to 2.8°C colder than they were over the prior 40-year period. Such observations,
in Zeeberg and Forman’s words, are “counter to warming of the Eurasian Arctic predicted for the twenty-first century by climate models, particularly for the winter season.” Comiso et
al. (2000) utilized satellite imagery to analyze and quantify a number of attributes of the Odden ice, including its average concentration, maximum area, and maximum extent over the
period 1979-1998. They used surface air temperature data from Jan Mayen Island, located within the region of study, to infer the behavior of the phenomenon over the past 75 years. The
Odden ice tongue was found to vary in size, shape, and length of occurrence during the 20-year period, displaying a fair amount of interannual variability. Quantitatively, trend analyses
revealed that the ice tongue had exhibited no statistically significant change in any of the parameters studied over the short 20-year period. However, a proxy reconstruction of the
Odden ice tongue for the past 75 years revealed the ice phenomenon to have been “a relatively smaller feature several decades ago,” due to the significantly warmer temperatures that
prevailed at that time. The fact that the Odden ice tongue has persisted, virtually unchanged in the mean during the past 20 years, is in direct contrast with predictions of rapid and
increasing warmth in earth’s polar regions as a result of CO2-induced global warming. This observation, along with the observational evidence from Jan Mayen Island that temperatures
there actually cooled at a rate of 0.15 ± 0.03°C per decade during the past 75 years, bolsters the view that there has been little to no warming in this part of the Arctic, as well as
most of its other parts, over the past seven decades. Polyakov et al. (2002b) used newly available long-term Russian observations of surface air temperature from coastal stations to
gain new insights into trends and variability in the Arctic environment poleward of 62°N. Throughout the 125-year history they developed, they identified “strong intrinsic variability,
dominated by multi-decadal fluctuations with a timescale of 60-80 years”; they found temperature trends in the Arctic to be highly dependent on the particular time period selected for
analysis. They found they could “identify periods when Arctic trends were actually smaller or of different sign [our italics] than Northern Hemisphere trends.” Over the bulk of the twentieth
century, when they say “multi-decadal variability had little net effect on computed trends,” the temperature histories of the two regions were “similar,” but they did “not support amplified
warming in polar regions predicted by GCMs.” In a concomitant study, Naurzbaev et al. (2002) developed a 2,427-year proxy temperature history for the part of the Taimyr Peninsula, northern
Russia, lying between 70°30’ and 72°28’ North latitude, based on a study of ring-widths of living and preserved larch trees, noting that it has been shown that “the main driver of tree-ring
variability at the polar timber-line [where they worked] is temperature (Vaganov et al., 1996; Briffa et al., 1998; Schweingruber and Briffa, 1996).” This work revealed that “the warmest
periods over the last two millennia in this region were clearly in the third [Roman Warm Period], tenth to twelfth [Medieval Warm Period] and during the twentieth [Current Warm Period]
centuries.” With respect to the second of these three periods, they emphasize that “the warmth of the two centuries AD 1058-1157 and 950-1049 attests to the reality of relative mediaeval
warmth in this region.” Their data also reveal three other important pieces of information: (1) the Roman and Medieval Warm Periods were both warmer than the Current Warm Period has
been to date, (2) the beginning of the end of the Little Ice Age was somewhere in the vicinity of 1830, and (3) the Current Warm Period peaked somewhere in the vicinity of 1940. All
of these observations are at odds with what is portrayed in the Northern Hemispheric “hockey stick” temperature history of Mann et al. (1998, 1999) and its thousand-year global extension
developed by Mann and Jones (2003), wherein (1) the Current
Climate Change Reconsidered 124 Warm Period is depicted as the warmest such era of the past two millennia, (2) recovery from the Little Ice Age does not begin until after 1910, and (3)
the Current Warm Period experiences it highest temperatures in the latter part of the twentieth century’s final decade. Przybylak (2002) conducted a detailed analysis of intraseasonal
and interannual variability in maximum, minimum, and average air temperature and diurnal air temperature range for the entire Arctic—as delineated by Treshnikov (1985)—for the period
1951-1990, based on data from 10 stations “representing the majority of the climatic regions in the Arctic.” This work indicated that trends in both the intraseasonal and interannual
variability of the temperatures studied did not show any significant changes, leading Przybylak to conclude that “this aspect of climate change, as well as trends in average seasonal
and annual values of temperature investigated earlier (Przybylak, 1997, 2000), proves that, in the Arctic in the period 1951-90, no tangible manifestations of the greenhouse effect can
be identified.” Isaksson et al. (2003) retrieved two ice cores (one from Lomonosovfonna and one from Austfonna) far above the Arctic Circle in Svalbard, Norway, after which the 12 cooperating
scientists from Norway, Finland, Sweden, Canada, Japan, Estonia, and the Netherlands used d18O data to reconstruct a 600-year temperature history of the region. As would be expected—in
light of the earth’s transition from the Little Ice Age to the Current Warm Period—the international group of scientists reported that “the d18O data from both Lomonosovfonna and Austfonna
ice cores suggest that the twentieth century was the warmest during at least the past 600 years.” However, the warmest decade of the twentieth century was centered on approximately 1930,
while the instrumental temperature record at Longyearbyen also shows the decade of the 1930s to have been the warmest. In addition, the authors remark that, “as on Svalbard, the 1930s
were the warmest decade in the Trondheim record.” Consequently, there was no net warming over the last seven decades of the twentieth century in the parts of Norway cited in this study.
In the same year, Polyakov et al. (2003) derived a surface air temperature history that stretched from 1875 to 2000, based on measurements carried out at 75 land stations and a number
of drifting buoys located poleward of 62°N latitude. From 1875 to about 1917, the team of eight U.S. and Russian scientists found the surface air temperature of the huge northern region
rose hardly at all; but then it climbed 1.7°C in just 20 years to reach a peak in 1937 that was not eclipsed over the remainder of the record. During this 20-year period of rapidly rising
air temperature, the atmosphere’s CO2 concentration rose by a mere 8 ppm. But then, over the next six decades, when the air’s CO2 concentration rose by approximately 55 ppm, or nearly
seven times more than it did throughout the 20-year period of dramatic warming that preceded it, the the surface air temperature of the region poleward of 62°N experienced no net warming
and, in fact, may have cooled. Briffa et al. (2004) reviewed several prior analyses of maximum latewood density data obtained from a widespread network of tree-ring chronologies that
spanned three to six centuries and were derived from nearly 400 locations. For the land area of the globe poleward of 20°N latitude, they too found that the warmest period of the past
six centuries occurred in the 1930s and early 1940s. Thereafter, the region’s temperature dropped dramatically, although it did recover somewhat over the last two decades of the twentieth
century. Nevertheless, its final value was still less than the mean value of the entire 1400s and portions of the 1500s. Averaged across all land area poleward of 50°N latitude, there
was a large divergence of reconstructed and instrumental temperatures subsequent to 1960, with measured temperatures rising and reconstructed temperatures falling, such that by the end
of the record there was an approximate 1.5°C difference between them. Briffa et al. attempted to relate this large temperature differential to a hypothesized decrease in tree growth
that was caused by a hypothesized increase in ultraviolet radiation that they hypothesized to have been caused by declining stratospheric ozone concentrations over this period. The results
of their effort, however, proved “equivocal,” as they themselves described it, leaving room for a growing urban heat island effect in the instrumental temperature record to be the principal
cause of the disconcerting data divergence. The three researchers wrote that these unsettled questions prevented them “from claiming unprecedented hemispheric warming during recent decades
on the basis of these tree-ring density data.” About the same time that Briffa et al. were struggling with this perplexing problem, Polyakov et al. (2004) were developing a long-term
history of Atlantic Core Water Temperature (ACWT) in the Arctic Ocean using high-latitude hydrographic hydrographic measurements that were initiated in the late nineteenth century, after
which they compared the
Observations: Temperature Records 125 results of this exercise with the long-term history of Arctic Surface Air Temperature (SAT) developed by Polyakov et al. (2003). Their ACWT record,
to quote them, revealed the existence of “two distinct warm periods from the late 1920s to 1950s and in the late 1980s-90s and two cold periods, one at the beginning of the record (until
the 1920s) and another in the 1960s-70s.” The SAT record depicted essentially the same thing, with the peak temperature of the latter warm period being not quite as high as the peak
temperature of the former warm period. In the case of the ACWT record, however, this relationship was reversed, with the peak temperature of the latter warm period slightly exceeding
the peak temperature of the former warm period. But the most recent temperature peak was very short-lived; and it rapidly declined to hover around a value that was approximately 1°C
cooler over the last few years of the record. In discussing their findings, Polyakov et al. say that, like Arctic SATs, Arctic ACWTs are dominated, in their words, “by multidecadal fluctuations
with a time scale of 50-80 years.” In addition, both records indicate that late twentieth century warmth was basically no different from that experienced in the late 1930s and early
1940s, a time when the air’s CO2 concentration was fully 65 ppm less than it is today. Knudsen et al. (2004) documented climatic changes over the past 1,200 years via high-resolution
multi-proxy studies of benthic and planktonic foraminiferal assemblages, stable isotopes, and icerafted debris found in three sediment cores retrieved from the North Icelandic shelf.
These efforts resulted in their learning that “the time period between 1200 and around 7-800 cal. (years) BP, including the Medieval Warm Period, was characterized by relatively high
bottom and surface water temperatures,” after which “a general temperature decrease in the area marks the transition to ... the Little Ice Age.” They also found that “minimum seasurface
temperatures were reached at around 350 cal. BP, when very cold conditions were indicated by several proxies.” Thereafter, they report that “a modern warming of surface waters ... is
not registered in the proxy data,” and that “there is no clear indication of warming of water masses in the area during the last decades,” even in sea surface temperatures measured over
the period 1948-2002. Raspopov et al. (2004) presented and analyzed two temperature-related datasets. The first was “a direct and systematic air temperature record for the Kola Peninsula,
in the vicinity of Murmansk,” which covered the period 1880-2000, while the second was an “annual tree-ring series generalized for 10 regions (Lovelius, 1997) along the northern timberline,
from the Kola Peninsula to Chukotka, for the period 1458-1975 in the longitude range from 30°E to 170°E,” which included nearly all of northern Eurasia that borders the Arctic Ocean.
The researchers’ primary objectives in this work were to identify any temporal cycles that might be present present in the two datasets and to determine what caused them. With respect
to this dual goal, they report discovering “climatic cycles with periods of around 90, 22-23 and 11-12 years,” which were found to “correlate well with the corresponding solar activity
cycles.” Of even more interest, however, was what they learned about the temporal development of the Current Warm Period
(CWP). Raspopov et al.’s presentation of the mean annual tree-ring series for the northern Eurasia timberline clearly shows that the region’s thermal recovery from the coldest temperatures
of the Little Ice Age (LIA) may be considered to have commenced as early as 1820 and was in full swing by at least 1840. In addition, it shows that the rising temperature peaked just
prior to 1950 and then declined to the end of the record in 1975. Thereafter, however, the Kola-Murmansk instrumental record indicates a significant temperature rise that peaked in the
early 1990s at about the same level as the pre-1950 peak; but after that time, the temperature once again declined to the end of the record in 2000. The latter of these findings (that
there has been no net warming of this expansive high-latitude region over the last half of the twentieth century) is in harmony with the findings of the many studies reviewed above,
while the former finding (that the thermal recovery of this climatically sensitive region of the planet began in the first half of the nineteenth century) is also supported by a number
of other studies (Esper et al., 2002; Moore et al., 2002; Yoo and D’Odorico, 2002; Gonzalez-Rouco et al., 2003; Jomelli and Pech, 2004), all of which demonstrate that the Little Ice
Age-to-Current Warm Period transition began somewhere in the neighborhood of 1820 to 1850, well before the date (~1910) that is indicated in the Mann et al. (1998, 1999) “hockey stick”
temperature history. One further study from 2004 yields much the same conclusion, but arrives at it by very different means. Benner et al. (2004) set the stage for what they did by stating
stating that “thawing of the permafrost which underlies a substantial fraction of the Arctic could accelerate carbon losses from soils (Goulden et
Climate Change Reconsidered 126 al., 1998).” In addition, they report that “freshwater discharge to the Arctic Ocean is expected to increase with increasing temperatures (Peterson et
al., 2002), potentially resulting in greater riverine export of terrigenous organic carbon to the ocean.” And since the organic carbon in Arctic soils, in their words, “is typically
old, with average radiocarbon ages ranging from centuries to millennia (Schell, 1983; Schirrmeister et al., 2002),” they set about to measure the age of dissolved organic carbon (DOC)
in Arctic rivers to see if there were any indications of increasing amounts of older carbon being transported to the ocean, which (if there were) would be indicative of enhanced regional
warming. Specifically, they sampled two of the largest Eurasian rivers, the Yenisey and Ob’ (which drain vast areas of boreal forest and extensive peat bogs, accounting for about a third
of all riverine DOC discharge to the Arctic Ocean), as well as two much smaller rivers on the north north slope of Alaska, the Ikpikpuk and Kokolik, whose watersheds are dominated by
Arctic tundra. In doing so, they found modern radiocarbon ages for all samples taken from all rivers, which indicates, in their words, that Arctic riverine DOC “is derived primarily
from recently fixed plant litter and near-surface soil horizons.” Thus, because warming should have caused the average radiocarbon age of the DOC of Arctic rivers to increase, the absence
of aging implied by their findings provides strong evidence for the absence of recent large-scale warming there. Laidre and Heide-Jorgensen (2005) published a most unusual paper, in
that it dealt with the danger of oceanic cooling. Using a combination of long-term satellite tracking data, climate data, and remotely sensed sea ice concentrations to detect localized
habitat trends of narwhals—a species of whale with a long spear-like tusk—in Baffin Bay between Greenland and Canada, home to the largest narwhal population in the world. They studied
the species’ vulnerability to recent and possible future climate trends. They found “since 1970, the climate in West Greenland has cooled, reflected in both oceanographic and biological
conditions (Hanna and Cappelen, 2003),” with the result that “Baffin Bay and Davis Strait display strong significant increasing trends in ice concentrations and extent, as high as 7.5
percent per decade between 1979 and 1996, with comparable increases detected back to 1953 (Parkinson et al., 1999; Deser et al., 2000; Parkinson, 2000a,b; Parkinson and Cavalieri, 2002;
Stern and Heide-Jorgensen, 2003).” Humlum et al. (2005) noted that state-of-the-art climate models were predicting that “the effect of any present and future global climatic change will
be amplified in the polar regions as a result of feedbacks in which variations in the extent of glaciers, snow, sea ice and permafrost, as well as atmospheric greenhouse gases, play
key roles.” However, they also said Polyakov et al. (2002a,b) had “presented updated observational trends and variations in Arctic climate and sea-ice cover during the twentieth century,
which do not support the modeled polar amplification of surface air-temperature changes observed by surface stations at lower latitudes,” and “there is reason, therefore, to evaluate
climate dynamics and their respective impacts on highlatitude glaciers.” They proceeded to do just that for the Archipelago of Svalbard, focusing on Spitsbergen (the Archipelago’s main
island) and the Longyearbreen glacier located in its relatively dry central region at 78°13’N latitude. In reviewing what was already known about the region, Humlum et al. report that
“a marked warming around 1920 changed the mean annual air temperature (MAAT) at sea level within only 5 years from about -9.5°C to -4.0°C,” which change, in their words, “represents
the most pronounced increase in MAAT documented anywhere in the world during the instrumental period.” Then, they report that “from 1957 to 1968, MAAT dropped about 4°C, followed by
a more gradual increase towards the end of the twentieth century.” With respect to the Longyearbreen glacier, their own work revealed that it had “increased in length from about 3 km
to its present size of about 5 km during the last c. 1100 years,” and they stated that “this example of late-Holocene glacier growth represents a widespread phenomenon in Svalbard and
in adjoining Arctic regions,” which they describe as a “development towards cooler conditions in the Arctic” that “may explain why the Little Ice Age glacier advance in Svalbard usually
represents the Holocene maximum glacier extension.” As for what it all means, climate change in Svalbard over the twentieth century appears to have been a real rollercoaster ride, with
temperatures rising more rapidly in the early 1920s than has been documented anywhere else before or since, only to be followed by a nearly equivalent temperature drop four decades later,
both of which transitions were totally out of line with what climate models suggest should have occurred. In addition, the current location of the terminus of the Longyearbreen glacier
suggests that,
Observations: Temperature Records 127 even now, Svalbard and “adjoining Arctic regions” are still experiencing some of the lowest temperatures of the entire Holocene, and at a time when
atmospheric CO2 concentrations are higher than they have been for millions of years. In one final paper from 2005, Soon (2005) explores the question of what was the more dominant driver
of twentieth century temperature change in the Arctic: the rising atmospheric CO2 concentration or variations in solar irradiance. This he did by examining the roles the two variables
may have played in forcing decadal, multi-decadal, and longerterm variations in surface air temperature (SAT). He performed a number of statistical analyses on (1) a composite Arctic-wide
SAT record constructed by Polyakov et al. (2003), (2) global CO2 concentrations taken from estimates made by the NASA GISS climate modeling group, and (3) a total solar irradiance (TSI)
record developed by Hoyt and Schatten (1993, updated by Hoyt in 2005) over the period 1875-2000. The results of Soon’s analyses indicated a much stronger statistical relationship exists
between SAT and TSI than between SAT and atmospheric CO2 concentration. Solar forcing generally explained well over 75 percent of the variance in decadal-smoothed seasonal and annual
Arctic temperatures, while CO2 forcing explained only between 8 and 22 percent. Wavelet analysis further supported the case for solar forcing of SAT, revealing similar time-frequency
characteristics for annual and seasonally averaged temperatures at decadal and multi-decadal time scales. By contrast, wavelet analysis gave little to no indication of a CO2 forcing
of Arctic SSTs. Based on these findings, it would appear that it is the sun, and not atmospheric CO2, that has been driving temperature change in the Arctic over the twentieth century.
Hanna et al. (2006) developed a 119-year history of Icelandic Sea Surface Temperature (SST) based on measurements made at 10 coastal stations located between latitudes 63°24’N and 66°32’N.
This work revealed the existence of past “long-term variations and trends that are broadly similar to Icelandic air temperature records: that is, generally cold conditions during the
late nineteenth and early twentieth centuries; strong warming in the 1920s, with peak SSTs typically being attained around 1940; and cooling thereafter until the 1970s, followed once
again by warming—but not generally back up to the level of the 1930s/1940s warm period.” Hansen et al. (2006) analyzed meteorological data from Arctic Station (69°15’N, 53°31’W) on Disko
Island (West Greenland) for the period 1991-2004, after which their results were correlated, in the words of the researchers, “to the longest record available from Greenland at Ilulissat/Jakobshavn
(since 1873).” Once this was done, marked changes were noted over the course of the study period, including “increasing mean annual air temperatures on the order of 0.4°C per year and
50% decrease in sea ice cover.” In addition, due to “a high correlation between mean monthly air temperatures at the two stations (1991-2004),” Hansen et al. were able to place the air
temperature trend observed at Disko “in a 130 years perspective.” This exercise led them to conclude that the climate changes of the past decade were “dramatic,” but that “similar changes
in air temperatures [had] occurred previous[ly] within the last 130 years.” More specifically, they report that the changes they observed over the last decade “are on the same order
as changes [that] occurred between 1920 and 1930.” In Iceland, Bradwell et al. (2006) examined the link between late Holocene fluctuations of Lambatungnajokull (an outlet glacier of
the Vatnajokull ice cap of southeast Iceland) and variations in climate, using geomorphological evidence to reconstruct patterns of past glacier fluctuations and lichenometry and tephrostratigraphy
to date glacial landforms created by the glacier over the past four centuries. This work revealed “there is a particularly close correspondence between summer air temperature and the
rate of ice-front recession of Lambatungnajokull during periods of overall retreat,” and “between 1930 and 1950 this relationship is striking.” They also report that “ice-front recession
was greatest during the 1930s and 1940s, when retreat averaged 20 m per year.” Thereafter, however, they say the retreat “slowed in the 1960s,” and “there has been little overall retreat
since the 1980s.” The researchers also report that “the 20th-century record of reconstructed glacier-front fluctuations at Lambatungnajokull compares well with those of other similar-sized,
non-surging, outlets of southern Vatnajokull,” including Skaftafellsjokull, Fjallsjokull, Skalafellsjokull, and Flaajokull. They find “the pattern of glacier fluctuations of Lambatungnajokull
over the past 200 years reflects the climatic changes that have occurred in southeast Iceland and the wider region.” Contemporaneously, Drinkwater (2006) decided “to provide a review
of the changes to the marine
Climate Change Reconsidered 128 ecosystems of the northern North Atlantic during the 1920s and 1930s and to discuss them in the light of contemporary ideas of regime shifts,” where he
defined regime shift as “a persistent radical shift in typical levels of abundance or productivity of multiple important components of the marine biological community structure, occurring
at multiple trophic levels and on a geographical scale that is at least regional in extent.” As a prologue to this effort, he first determined that “in the 1920s and 1930s, there was
a dramatic warming of the air and ocean temperatures in the northern North Atlantic and the high Arctic, with the largest changes occurring north of 60°N,” which warming “led to reduced
ice cover in the Arctic and subarctic regions and higher sea temperatures,” as well as northward shifts of multiple marine ecosystems. This change in climate occurred “during the 1920s,
and especially after 1925,” according to Drinkwater, when he reports that “average air temperatures began to rise rapidly and continued to do so through the 1930s,” when “mean annual
air temperatures increased by approximately 0.5-1°C and the cumulative sums of anomalies varied from 1.5 to 6°C between 1920 and 1940 with the higher values occurring in West Greenland
and Iceland.” Thereafter, as he describes it, “through the 1940s and 1950s air temperatures in the northernmost regions varied but generally remained relatively high,” declining in the
late 1960s in the northwest Atlantic and slightly earlier in the northeast Atlantic, which cooling has only recently begun to be reversed in certain parts of the region. In the realm
of biology, the early twentieth century warming of North Atlantic waters “contributed to higher primary and secondary production,” in the words of Drinkwater, and “with the reduced extent
of ice-covered waters, more open water allow[ed] for higher production than in the colder periods.” As a result, cod “spread approximately 1200 km northward along West Greenland,” and
“migration migration of ‘warmer water’ species also changed with earlier arrivals and later departures.” In addition, Drinkwater notes that “new spawning sites were observed farther
north for several species or stocks while for others the relative contribution from northern spawning sites increased.” Also, he writes that “some southern species of fish that were
unknown in northern areas prior to the warming event became occasional, and in some cases, frequent visitors.” Consequently, and considering all aspects of the event, Drinkwater states
that “the warming in the 1920s and 1930s is considered to constitute the most significant regime shift experienced in the North Atlantic in the 20th century.” Groisman et al. (2006)
reported using “a new Global Synoptic Data Network consisting of 2100 stations within the boundaries of the former Soviet Union created jointly by the [U.S.] National Climatic Data Center
and Russian Institute for Hydrometeorological Information … to assess the climatology of snow cover, frozen and unfrozen ground reports, and their temporal variability for the period
from 1936 to 2004.” They determined that “during the past 69 years (1936-2004 period), an increase in duration of the period with snow on the ground over Russia and the Russian polar
region north of the Arctic circle has been documented by 5 days or 3% and 12 days or 5%, respectively,” and they note this result “is in agreement with other findings.” In commenting
on this development, plus the similar findings of others, the five researchers say “changes in snow cover extent during the 1936-2004 period cannot be linked with ‘warming’ (particularly
with the Arctic warming).” Why? Because, as they continue, “in this particular period the Arctic warming was absent.” A recent essay that appeared in Ambio: A Journal of the Human Environment,
by Karlén (2005) asks if temperatures in the Arctic are “really rising at an alarming rate,” as some have claimed. His answer is a resounding no. Focusing on Svalbard Lufthavn (located
at 78°N latitude), which he later shows to be representative of much of the Arctic, Karlén reports that “the Svalbard mean annual temperature increased rapidly from the 1910s to the
late 1930s,” that “the temperature thereafter became lower, and a minimum was reached around 1970,” and that “Svalbard thereafter became warmer, but the mean temperature in the late
1990s was still slightly cooler than it was in the late 1930s,” indicative of a cooling trend of 0.11°C per decade over the last seventy years of the twentieth century. Karlén goes on
to say “the observed warming during the 1930s is supported by data from several stations along the Arctic coasts and on islands in the Arctic, e.g. Nordklim data from Bjornoya and Jan
Mayen in the north Atlantic, Vardo and Tromso in northern Norway, Sodankylaeand Karasjoki in northern Finland, and Stykkisholmur in Iceland,” and “there is also [similar] data from other
reports; e.g. Godthaab, Jakobshavn, and Egedesmindde in Greenland, Ostrov Dikson on the north coast of Siberia, Salehard in inland Siberia, and Nome in
Observations: Temperature Records 129 western Alaska.” All of these stations, to quote him further, “indicate the same pattern of changes in annual mean temperature: a warm 1930s, a
cooling until around 1970, and thereafter a warming, although the temperature remains slightly below the level of the late 1930s.” In addition, he says “many stations with records starting
later than the 1930s also indicate cooling, e.g. Vize in the Arctic Sea north of the Siberian coast and Frobisher Bay and Clyde on Baffin Island.” Finally, Karlén reports that the 250-year
temperature record of Stockholm “shows that the fluctuations of the 1900s are not unique,” and that “changes of the same magnitude as in the 1900s occurred between 1770 and 1800, and
distinct but smaller fluctuations occurred around 1825.” Karlén notes that “during the 50 years in which the atmospheric concentration of CO2 has increased considerably, the temperature
has decreased,” which leads him to conclude that “the Arctic temperature data do not support the models predicting that there will be a critical future warming of the climate because
of an increased concentration of CO2 in the atmosphere.” And this is especially important, in Karlén’s words, because the model-based prediction “is that changes will be strongest and
first noticeable in the Arctic.” Chylek et al. (2006) provides a more up-to-date report on average summer temperatures recorded at Ammassalik, on Greenland’s southeast coast, and Godthab
Nuuk on the island’s southwestern coast, covering the period 1905 to 2005. They found “the 1955 to 2005 averages of the summer temperatures and the temperatures of the warmest month
at both Godthab Nuuk and Ammassalik are significantly lower than the corresponding averages for the previous 50 years (1905-1955). The summers at both the southwestern and the southeastern
coast of Greenland were significantly colder within the 1955-2005 period compared to the 1905-1955.” Chylek et al. also compared temperatures for the 10-year periods of 1920-1930 and
1995-2005. They found the average summer temperature for 2003 in Ammassalik was a record high since 1895, but “the years 2004 and 2005 were closer to normal being well below temperatures
reached in the 1930s and 1940s.” Similarly, the record from Godthab Nuuk showed that while temperatures there “were also increasing during the 1995-2005 period,they stayed generally
below the values typical for the 1920-1940 period.” The authors conclude that “reports of Greenland temperature changes are .... diverse suggesting a long term cooling and shorter warming
periods.” Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/arctictemptrends.php.
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extent on north Novaya Zemlya in the twentieth century. Holocene 11: 161-175. 3.7. Antarctica The study of Antarctic temperatures has provided valuable insight and spurred contentious
debate on issues pertaining to global climate change. Key among the pertinent findings has been the observation of a large-scale correlation between proxy air temperature and atmospheric
CO2 measurements obtained from ice cores drilled in the interior of the continent. In the mid-to late-1980s, this broad
Climate Change Reconsidered 132 correlation dominated much of the climate change debate. Many jumped on the global warming bandwagon, saying the correlation proved that changes in atmospheric
CO2 concentration caused changes in air temperature, and that future increases in the air’s CO2 content due to anthropogenic CO2 emissions would therefore intensify global warming. By
the late 1990s and early 2000s, however, icecoring instrumentation and techniques had improved considerably and newer studies with finer temporal resolution began to reveal that increases
(decreases) in air temperature precede increases (decreases) in atmospheric CO2 content, not vice versa (see Indermuhle et al. (2000), Monnin et al. (2001)). A recent study by Caillon
et al. (2003), for example, demonstrated that during Glacial Termination III, “the CO2 increase lagged Antarctic deglacial warming by 800 ± 200 years.” This finding, in the authors’
words, “confirms that CO2 is not the forcing that initially drives the climatic system during a deglaciation.” A second major blow to the CO2-induced global warming hypothesis comes
from the contradiction between observed and model-predicted Antarctic temperature trends of the past several decades. According to nearly all climate models, CO2-induced global warming
should be most evident in earth’s polar regions, but analyses of Antarctic near-surface and tropospheric air temperatures contradict this prediction. Doran et al. (2002) examined temperature
trends in the McMurdo Dry Valleys of Antarctica over the period 1986 to 2000, reporting a cooling rate of approximately 0.7°C per decade. This dramatic rate of cooling, they state, “reflects
longer term continental Antarctic cooling between 1966 and 2000.” In addition, the 14-year temperature decline in the dry valleys occurred in the summer and autumn, just as most of the
35-year cooling over the continent as a whole (which did not include any data from the dry valleys) also occurred in the summer and autumn. Comiso (2000) assembled and analyzed Antarctic
temperature data obtained from 21 surface stations and from infrared satellites operating since 1979. He found that for all of Antarctica, temperatures had declined by 0.08°C and 0.42°C
per decade, respectively. Thompson and Solomon (2002) also report a cooling trend for the interior of Antarctica. In spite of the decades-long cooling that has been observed for the
continent as a whole, one region of Antarctica has actually bucked the mean trend and warmed over the same time period: the Antarctic Peninsula/Bellingshausen Sea region. But is the
temperature increase that has occurred there evidence of CO2-induced global warming? According to Vaughan et al. (2001), “rapid regional warming” has led to the loss of seven ice shelves
in this region during the past 50 years. However, they note that sediment cores from 6,000 to 1,900 years ago suggest the Prince Gustav Channel Ice Shelf—which collapsed in this region
in 1995— “was absent and climate was as warm as it has been recently,” when, of course there was much less CO2 in the air. Although it is tempting to cite the twentieth century increase
in atmospheric CO2 concentration as the cause of the recent regional warming, “to do so without offering a mechanism,” say Vaughan et al., “is superficial.” And so it is, as the recent
work of Thompson and Solomon (2002) suggests that much of the warming can be explained by “a systematic bias toward the high-index polarity of the SAM,” or Southern Hemispheric Annular
Mode, such that the ring of westerly winds encircling Antarctica has recently been spending more time in its strong-wind phase. That is also the conclusion of Kwok and Comiso (2002),
who report that over the 17-year period 1982-1998, the SAM index shifted towards more positive values (0.22/decade), noting that a positive polarity of the SAM index “is associated with
cold anomalies over most of Antarctica with the center of action over the East Antarctic plateau.” At the same time, the SO index shifted in a negative direction, indicating “a drift
toward a spatial pattern with warmer temperatures around the Antarctic Peninsula, and cooler temperatures over much of the continent.” Together, the authors say the positive trend in
the coupled mode of variability of these two indices (0.3/decade) represents a “significant bias toward positive polarity” that they describe as “remarkable.” Kwok and Comiso additionally
report that “the tropospheric SH annular mode has been shown to be related to changes in the lower stratosphere (Thompson and Wallace, 2000),” noting that “the high index polarity of
the SH annular mode is associated with the trend toward a cooling and strengthening of the SH stratospheric polar vortex during the stratosphere’s relatively short active season in November,”
which is pretty much the same theory that has been put forth by Thompson and Solomon (2002). In another slant on the issue, Yoon et al. (2002) report that “the maritime record on the
Antarctic Peninsula shelf suggests close chronological
Observations: Temperature Records 133 correlation with Holocene glacial events in the Northern Hemisphere, indicating the possibility of coherent climate variability in the Holocene.”
In the same vein, Khim et al. (2002) say that “two of the most significant climatic events during the late Holocene are the Little Ice Age (LIA) and Medieval Warm Period (MWP), both
of which occurred globally (Lamb, 1965; Grove, 1988),” noting further that “evidence of the LIA has been found in several studies of Antarctic marine sediments (Leventer and Dunbar,
1988; Leventer et al., 1996; Domack et al., 2000).” To this list of scientific journal articles documenting the existence of the LIA in Antarctica can now be added Khim et al.’s own
paper, which also demonstrates the presence of the MWP in Antarctica, as well as earlier cold and warm periods of similar intensity and duration. Further evidence that the Antarctic
as a whole is in the midst of a cooling trend comes from Watkins and Simmonds (2000), who analyzed region-wide wide changes in sea ice. Reporting on trends in a number of Southern Ocean
sea ice parameters over the period 1987 to 1996, they found statistically significant increases in sea ice area and total sea ice extent, as well as an increase in sea ice season length
since the 1990s. Combining these results with those from a previous study revealed these trends to be consistent back to at least 1978. And in another study of Antarctic sea ice extent,
Yuan and Martinson (2000) report that the net trend in the mean Antarctic ice edge over the past 18 years has been an equatorward expansion of 0.011 degree of latitude per year. The
temperature history of Antarctica provides no evidence for the CO2-induced global warming hypothesis. In fact, it argues strongly against it. But what if the Antarctic were to warm as
a result of some natural or anthropogenic-induced change in earth’s climate? What would the consequences be? For one thing, it would likely help to increase both the number and diversity
of penguin species (Sun et al., 2000; Smith et al., 1999), and it would also tend to increase the size and number of populations of the continent’s only two vascular plant species (Xiong
et al., 2000). With respect to the continent’s great ice sheets, there would not be much of a problem either, as not even a warming event as dramatic as 10°C is predicted to result in
a net change in the East Antarctic Ice Sheet (Näslund et al., 2000), which suggests that predictions of catastrophic coastal flooding due to the melting of the world’s polar ice sheets
are way off the mark. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/antarcticatemp.php.
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