Past 1,000 years

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

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).

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.


The Hockey Stick Trend

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 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. 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 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. This website is maintained by rich, paranoid Republicans that rely on opinions rather than facts.

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 peer-reviewed 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.

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 datapresented 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 real-world data presented here (and hereafter), however, that claim is seen to be false.


Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change.

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 Wegman Report.pdf.

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