Horse Hockey Climate Scientology: “Getting Rid” of the Medieval Warming Period

Hockey Stick Climate Scientologists Contemplating Their Work




U.S. Senate Committee on Environment & Public Works
Hearing Statements



Statement of Dr. David Deming


University of Oklahoma
College of Earth and Energy
Climate Change and the Media


Mr. Chairman, members of the Committee, and distinguished guests, thank you for inviting me to testify today. I am a geologist and geophysicist. I have a bachelor’s degree in geology from Indiana University, and a Ph.D in geophysics from the University of Utah. My field of specialization in geophysics is temperature and heat flow. In recent years, I have turned my studies to the history and philosophy of science. In 1995, I published a short paper in the academic journal Science. In that study, I reviewed how borehole temperature data recorded a warming of about one degree Celsius in North America over the last 100 to 150 years. The week the article appeared, I was contacted by a reporter for National Public Radio. He offered to interview me, but only if I would state that the warming was due to human activity. When I refused to do so, he hung up on me.


I had another interesting experience around the time my paper in Science was published. I received an astonishing email from a major researcher in the area of climate change. He said, “We have to get rid of the Medieval Warm Period.”



The Medieval Warm Period (MWP)—a well-established phenomenon (see for example, Hubert H. Lamb’s Climate History and the Modern World, London, 1995, chapter 10, pp. 159-163)—dated from approximately 1000 to 1300 C.E., and comprised an interval during which global temperature conditions were warmer than those at present. The International Panel on Climate Change (IPCC), produced the graph below (Figure 1) in its Second Assessment Report from 1995/1996: the second millennial climate history includes the aforementioned MWP, as well as a subsequent Little Ice Age. As Professor Ross McKitrick notes (in “What is the Hockey-Stick Debate About?”),


The late 20th century appears to be nothing special by comparison. It is easy to see why this graph was a problem for those pushing the global warming alarm. If the world could warm so much on such a short time scale as a result of natural causes, surely the 20th century climate change could simply be a natural effect as well. And the present climate change could hardly be considered unusually hazardous if even larger climate changes happened in the recent past, and we are simply fluctuating in the middle of what nature regularly dishes out?


David Holland, in his thorough 2007 analysis of the IPCC process, observes more pointedly,


The extreme but frequently articulated view that, because of “positive feedback”, a little further warming will lead to a “tipping point”and “runaway” global warming was clearly unfounded in comparison with historic higher temperatures from which the earth has previously “recovered”. Since it is argued that present carbon dioxide levels are higher now than for several hundred thousand years, any previous higher temperatures in that period must mean that factors other than human-emitted carbon dioxide were responsible.


But facts are (supposed to be—see further discussion below) stubborn things. Huang et al (Huang, Shaopeng, Henry N. Pollack and Po Yu Shen (1997). “Late Quaternary Temperature Changes Seen in Worldwide Continental Heat Flow Measurements.” Geophysical Research Letters 24: 1947—1950.) published a 1997 analysis of 6000 borehole records (boreholes drilled into the ground provide a vertical temperature profile that can be inverted to yield an estimate of the historical surface temperature sequence) from each continent, dating back 20,000 years. McKitrick, using data supplied by Huang, reproduced the portion of the graphic (see Figure 2) which highlights the interval from 1000 to 1990 C.E. He concludes,


The similarity to the IPCC’s 1995 graph is obvious. The world experienced a “warm” interval in the medieval era that dwarfs 20th century changes. The present-day climate appears to be simply a recovery from the cold years of the “Little Ice Age.”


Figure 1


Figure 2


Nevertheless, despite the publication of Huang et al’s findings in 1997, as summarized by McKitrick,


The next year, Nature published the first Mann hockey stick paper, commonly called “MBH98.” (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 et al. followed up in 1999 with a paper (Mann, M.E., Bradley, R.S. and Hughes, M.K., Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations, Geophysical Research Letters, 26, 759-762, 1999.) in GRL (“MBH99”) extending their results from AD1400 back to AD1000.8 In early 2000 the IPCC released the first draft of the TAR. The hockey stick was the only paleoclimate reconstruction shown in the Summary, and was the only one in the whole report to be singled out for repeated presentation. The borehole data received a brief mention in Chapter 2 but the Huang et al. graph was not shown. A small graph of borehole data taken from another study and based on a smaller sample was shown, but it only showed a post-1500 segment, which, conveniently, trended upwards.


As soon as the IPCC Report came out, the hockey stick version of climate history became canonical. Suddenly it was the “consensus” view, and for the next few years it seemed that anyone publicly questioning the result was in for a ferocious reception.


Stephen McIntyre and Ross McKitrick (in McIntyre, Steven and Ross McKitrick, (2003). “Corrections to the Mann et. al. (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series.” Environment and Energy 14(6) pp. 751-771.) detailed meticulously the “errors and defects” in the original Mann, Bradley, and Hughes paper (“MBH98”). Below, I have reprinted McIntyre and McKitrick’s conclusions followed by the hockey-stick graph of Mann et al (Figure 3) and the McIntyre/ McKitrick reconstruction (Figure 4) of the “MBH98” graphic simply applying “MBH98” statistical modeling procedures to an updated, correctly collated assembly of “MBH98” proxy data.


The MBH98 hockey stick-shaped Northern Hemispheric temperature index discussed here has been extremely influential in discussions of 20th century global warming. Together with a pre-1400 extension derived in Mann et. al. (1999) and a spliced instrumental temperature series, this index figured prominently in the IPCC Third Assessment Report (IPCC 2001) and numerous other publications. However, the dataset used to make this construction contained collation errors, unjustified truncation or extrapolation of source data, obsolete data, incorrect principal component calculations, geographical mislocations and other serious defects. These errors and defects substantially affect the temperature index.


Although not all of the dataset could be audited, it was possible to prepare a data base with substantially improved quality control, by using the most recent data and collating it correctly, by avoiding arbitrary filling in or truncation of data and by computing principal components using standard algorithms. Without endorsing the MBH98 methodology or choice of source data, we were able to apply the MBH98 methodology to a database with improved quality control and found that their own method, carefully applied to their own intended source data, yielded a Northern Hemisphere temperature index in which the late 20th century is unexceptional compared to the preceding centuries, displaying neither unusually high mean values nor variability. More generally, the extent of errors and defects in the MBH98 data means that the indexes computed from it are unreliable and cannot be used for comparisons between the current climate and that of past centuries, including claims like “temperatures in the latter half of the 20th century were unprecedented,” and “even the warmer intervals in the reconstruction pale in comparison with mid-to-late 20th-century temperatures” (see press release accompanying Mann et al 1999) or that the 1990s was “likely the warmest decade” and 1998 the “warmest year” of the millennium (IPCC 2001).


Figure 3


Figure 4


During June, 2006 a National Research Council (NRC) panel chaired by Gerald North, of Texas A&M University, endorsed specific criticisms of Mann’s methodology and concluded that no statistical confidence could be placed in Mann’s insistence that temperatures in the 1990s exceeded those in the medieval warm period. From Professor North’s summary statement:


…the substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that “the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium” because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales. We also question some of the statistical choices made in the original papers by Dr. Mann and his colleague.


McIntyre and McKitrick’s  most devastating criticisms of the analyses by Mann et al—invalidating the latter’s work—were upheld without equivocation (or deference to Mann et al) in a subsequent 2006  report chaired by the eminent statistician Edward Wegman (chairman of the U.S. National Academy of Sciences committee on theoretical and applied statistics), and submitted to the US House of Representatives. Here are key excerpts (a few extended) from the report by Wegman et al:



Overall, our committee believes that Mann’s assessments that the decade of the 1990s was the hottest decade of the millennium and that 1998 was the hottest year of the millennium cannot be supported by his analysis.


Where we have commonality, I believe our report and the NRC panel essentially agree. …We believe that our discussion together with the discussion from the NRC report should take the “centering’ issue off the table. [Mann’s] decentered methodology is simply incorrect mathematics … I am baffled by the claim that the incorrect method doesn’t matter because the answer is correct anyway. Method Wrong Answer Correct = Bad Science.


The papers of Mann et al. in themselves are written in a confusing manner, making it difficult for the reader to discern the actual methodology and what uncertainty is actually associated with these reconstructions. It is not clear that Dr. Mann and his associates even realized that their methodology was faulty at the time of writing the MBH paper.


We have been to Michael Mann’s University of Virginia Web site and downloaded

the materials there. Unfortunately, we did not find adequate material to reproduce the MBH98 materials. We have been able to reproduce the results of McIntyre and McKitrick…In general, we find the criticisms by MM03 [McIntyre, S., and R. McKitrick, 2003: Corrections to the Mann et al. (1998) proxy database and northern hemispheric average temperature series. Energy and Environment, 14, 751 771., MM05a [McIntyre, S. and R. McKitrick, (2005a), Hockey Sticks, Principal Components and Spurious Significance, Geophys. Res. Let., 32] and MM05b [McIntyre, S., and R. McKitrick, 2005b: The M&M critique of the MBH98 Northern Hemisphere climate index: Update and implications. Energy and Environment, 16, 69 99.] to be valid and their arguments to be compelling. We were able to reproduce their results and offer both theoretical explanations (Appendix A) and simulations to verify that their observations were correct.



M&M also evaluated the MBH98 usage of the Reduction of Error (RE) statistic in place of the more reliable and widely used Monte Carlo Model to establish significant benchmarks. By using the Monte Carlo Model, M&M found that a more accurate significance level for the MBH98 procedures is .59, as opposed to the level of 0.0 reported in the original study. A guard against spurious RE significance is to examine other statistics, such as the R2 and CE statistics. However, MBH98 did not report any additional statistics for the controversial 15th century period. The M&M calculations indicate that these values for the 15th century section of the temperature reconstruction are not significant, thereby refuting the conclusions made by MBH98.”


Based on the literature we have reviewed, there is no overarching consensus on MBH98/ 99. As analyzed in our social network, there is a tightly knit group of individuals who passionately believe in their thesis. However, our perception is that this group has a self-reinforcing feedback mechanism and, moreover, the work has been sufficiently politicized that they can hardly reassess their public positions without losing credibility…The social network analysis of authors’ relations suggests that the “independent reconstructions” are not as independent as one might guess. Indeed, the matrix outlined in Figure 5.8 illustrates the proxies that are used more than one time in twelve major temperature reconstruction papers. The black boxes indicate that the proxy was used in a given paper. It is clear that many of the proxies are re-used in most of the papers. It is not surprising that the papers would obtain similar results and so cannot really claim to be independent verifications.”



It is important to note the isolation of the paleoclimate community; even though they rely heavily on statistical methods they do not seem to be interacting with the statistical community. Additionally, we judge that the sharing of research materials, data and results was haphazardly and grudgingly done. In this case we judge that there was too much reliance on peer review, which was not necessarily independent.


It is clear that many of the proxies are re-used in most of the papers. It is not surprising that the papers would obtain similar results and so cannot really claim to be independent verifications.


We note that the American Meteorological Society has a Committee on Probability and Statistics. I believe it is amazing for a committee whose focus is on statistics and probability that of the nine members only two are also members of the American Statistical Association, the premier statistical association in the United States, and one of those is a recent Ph.D. with an assistant professor appointment in a medical school. The American Meteorological Association recently held the 18th Conference on Probability and Statistics in the Atmospheric Sciences.. Of the 62 presenters at a conference with a focus on statistics and probability, only 8 … are members of the American Statistical Association. I believe that these two communities should be more engaged and if nothing else our report should highlight to both communities a need for additional cross-disciplinary ties.


[The] fact that their paper fit some policy agendas has greatly enhanced their paper’s visibility. … The “hockey stick” reconstruction of temperature graphic dramatically illustrated the global warming issue and was adopted by the IPCC and many governments as the poster graphic. The graphics’ prominence together with the fact that it is based on incorrect use of [principal components analysis] puts Dr. Mann and his co-authors in a difficult face-saving position.


Especially when massive amounts of public monies and human lives are at stake, academic work should have a more intense level of scrutiny and review. It is especially the case that authors of policy-related documents like the IPCC report, Climate Change 2001: The Scientific Basis, should not be the same people as those that constructed the academic papers.



The question arises: how could Mann et al complete and publish such spurious analyses? David Holland has provided salient examples of how Mann et al and their abettors cherry-picked data for their key tree ring temperature proxy reconstructions. The underlying principle, in brief, is that plant growth is dependent, in part, upon temperature, and good correlations between tree-ring growth and local instrumental temperature records may be found. In other series, however, no correlations are found. Moreover, Mann et al focused excessively on Bristlecone and Foxtail tree rings documented in a specific 1993 paper by Graybill and Idso. [Graybill, D.A., and S.B. Idso. 1993. “Detecting the Aerial Fertilization Effect of Atmospheric CO2 Enrichment in Tree Ring Chronologies.” Global Geochemical Cycles 7(1): 81-95.]. Graybill and Idso were looking specifically for “CO2 fertilization (growth promoting) effects,” and stated specifically (on p. 82) that this phenomenon was unrelated to local temperature changes. Finally Holland demonstrates with two graphic displays (Figures 5 and 6, below) the selective, willful misrepresentation of overall climate data incorporated into the influential IPCC 2001 Third Assessment Report.



The ‘strip-bark’ data Mann et al. used had been collected by Graybill and Idso and reported in their 1993 paper. These are particular examples of Bristlecone and Foxtail tree-rings which generally had a pronounced growth spurt in the first half of the 20th century, and which on page 82 the authors say could not be shown to be related to local temperature changes…The most damaging aspect of the ‘strip-bark’ matter is that when the data for MBH98 study was finally released, a directory on Michael Mann’s ftp server was found with the highly suggestive name “BACKTO 1400-CENSORED”. It contained all but 20 of the 212 series used in the published paper. Nineteen were ‘strip-bark’ and the twentieth was inappropriate for reasons made plain in McIntyre, S., and R. McKitrick, 2005b: The M&M critique of the MBH98 Northern Hemisphere climate index: Update and implications. Energy and Environment, 16, 69-9. The implication is clear that Michael Mann knew these proxies were of doubtful validity, and he had tested the reconstruction without them. He presumably knew but did not report that his reconstruction was significantly affected by the removal of these proxies.


In 1998 Keith Briffa with co-authors including Philip Jones published a paper [Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., Harris, I.C., Shiyatov, S.G., Vaganov, E.A. and Grudd, H., 1998 “Trees tell of past climates: but are they speaking less clearly today?”Philosophical Transactions Royal Society London B 353, 65 73 (R)] with the title “Trees tell of past climates: but are they speaking less clearly today?” The answer they found is that, in many cases, tree rings series which correlated with the temperature rise from 1900 to 1960 “diverge” thereafter, indicating falling temperatures after 1960 while instrumental temperature measurements are generally believed to have risen. [See Figure 5]  Thus at the time Mann, Bradley, Hughes, Briffa and Jones were writing the critical IPCC, 2001 Third Assessment Report (TAR) Chapter 2, some of them certainly knew – and all should have known -that the data that they were using to assert that current warming is exceptional are inadequate to sustain that conclusion.


(Figure 2.21 on page 134 of Chapter 2 of IPCC, 2001 WGI,) shows a number of

studies that are purported to corroborate the “hockey stick”. Close examination reveals that none of the reconstructed temperature curves extend beyond 1980. That is to say that most of the period, which the authors claim to be exceptionally warm, is not replicated in reconstructions that their claim relies upon. Further examination shows that the IPCC truncated two of the datasets for their graphs. Briffa’s reconstruction stops at 1960 even though the original study included data for later. When these post-1960 data are included, the divergence problem is immediately apparent and the reconstructions become far less convincing. Equally misleading is the fact that from 1850 to 1902 instrumental temperature, which elsewhere the IPCC treat as reliable, is also omitted from Figure 2.21. Had the IPCC figure been plotted with all the data shown [see Figure 6 below], it would have been clear that we were being invited to accept the temperature estimates of a thousand years ago from these reconstructions when they cannot even replicate current temperatures for half the known instrumental record. Such manipulation of data and graphical presentation might charitably be viewed as “graphsmanship”; alternatively, it is fraudulent.


Figure 5


Figure 6



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