Congressional Testimony of Jay Gulledge – Examining the “Hockey Stick” Controversy



July 27, 2006

At the U.S. House of Representatives Committee on Energy and Commerce, Subcommittee on Oversight and Investigations Hearing: Questions Surrounding the ‘Hockey Stick’ Temperature Studies: Implications for Climate Change Assessments

Examining the “Hockey Stick” Controversy

Mr. Chairman, Ranking Member, and Members of the Committee:

Thank you for the opportunity to speak today. I am Jay Gulledge, Ph.D., Senior Research Fellow for Science and Impacts at the Pew Center on Global Climate Change. I am also an Adjunct Assistant Professor at the University of Louisville, which houses my academic research program on carbon cycling.

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If you take nothing else from my testimony, please take these three points:

1. The scientific evidence of significant human influence on climate is strong and would in no way be weakened if there were no Mann hockey stick.

2. The scientific debate over the Medieval Warm Period (MWP) has been gradually evolving for at least 20 years. The results of the Mann hockey stick simply reflect the gradual development of thought on the issue over time.

3. The impact of the McIntyre and McKitrick critique on the original Mann paper, after being scrutinized by the National Academy of Science, the Wegman panel and a number of meticulous individual research groups, is essentially nil with regard to the conclusions of the Mann paper and the 2001 IPCC assessment.

The science of climate change is an extraordinary example of a theory-driven, data-rich scientific paradigm, the likes of which, arguably, has not occurred since the development of quantum mechanics in the first half of the twentieth century. The product of this strong scientific framework is a body of strong, multifaceted evidence that man-made greenhouse gases are causing contemporary global warming, and that this warming trend is inducing large-scale changes in global climate. The primary evidence is based on physical principles and observational and experimental analysis of contemporary climate dynamics, as opposed to analyses of past climates, which are the subject of this hearing. We can now say with confidence that the evidence of human influence on climate is strong, as described by Dr. Cicerone.

Although paleoclimatology – the study of ancient climates – is an important part of the climate science framework, reconstructions of temperature over the past millennium play a secondary, expendable role in the larger body of evidence, as stated in the recent NAS report titled, Surface Temperature Reconstructions for the Last 2,000 Years: “Surface temperature reconstructions are consistent with other evidence of global climate change and can be considered as additional supporting evidence” (National Research Council 2006, p. 23; hereafter referred to as the NAS report). Dispensing with such reconstructions entirely or proving them fundamentally flawed would have little, if any, impact on our understanding of contemporary climate change. This statement does not imply that millennial climate reconstructions are unimportant, but their main influence will be in the future, when their potential to reveal how climate varied across the earth’s surface from year-to-year in the past (i.e. an annual record of spatially explicit climate dynamics) is fully realized. At that point, such reconstructions will be used in a manner parallel to thermometer records today. This capability would contribute significantly to resolving the current genuine debate in climate science, which is not about whether humans are changing the climate—a point over which there is no scientific controversy—but is about how much human influences will change the climate in the future as a result of greenhouse gas accumulation and other forcings we apply to the climate system. In other words, the goal of spatially explicit paleoclimate reconstructions is to help climatologists determine how physical forcings, such as solar radiation, volcanic eruptions, land-use changes, and changes in atmospheric greenhouse gases, have affected the planet in the past, so that we can improve estimates of how they will do so in the future.

The early MBH reconstructions (Mann et al. 1998; Mann et al. 1999; hereafter referred to as MBH98 or MBH99 or, collectively, MBH) were the first to offer spatially explicit climate reconstructions and therefore represented a breakthrough in climate change science that continues to develop and promises to further our understanding of climate physics in the future. The Wegman report’s conclusion that paleoclimatology “does not provide insight and understanding of the physical mechanisms of climate change” (p. 52), fails to appreciate that the purpose of Dr. Mann’s research is to improve our knowledge of physical mechanisms of climate change by examining how they operated in the past.

Turning our attention to the methodological issues this hearing seeks to investigate, in my opinion, the Wegman report failed to accomplish its primary objective, which was “to reproduce the results of [McIntyre & McKitrick] in order to determine whether their criticisms are valid and have merit” (p. 7). Although the panel reproduced MM’s work—verbatim—it only partially assessed the validity, and did not at all assess the merits, of the criticisms directed toward the MBH reconstructions. For instance, MM (McIntyre and McKitrick 2003; McIntyre and McKitrick 2005; heafter referred to collectively as MM) allege that the so-called MBH “hockey stick” result is biased by methodological errors that undermine the conclusion that the late 20th century was uniquely warm relative to the past 1,000 years. This critique only has merit if, after correcting for the errors pointed out by MM, the resulting reconstruction yields results significantly different from the original result that can no longer support the claim of unusual late 20th century warmth. However, the Wegman Report takes no steps to make such a determination.

Fortunately, a different group, one well qualified both statistically and climatologically to tackle this question of merit, had already performed the task several months before the Wegman Report was released. The study by Wahl & Ammann (In press; hereafter referred to as WA06), was peer-reviewed and accepted for publication in the journal Climatic Change early last spring, and has been publicly available in accepted form since last March ( WahlAmmann_ClimChange2006.html). This study, titled, 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, carefully reproduced the MBH98 reconstruction and then used their faithful reproduction to test MM’s suggested corrections. They tested each of the criticisms raised by MM in all of their published papers, including both the peer-reviewed and non-peer-reviewed papers. Given that this report specifically examined MM’s criticisms, including the decentering issue that was the main focus of the Wegman report, it is unfortunate that the Wegman report dismissed it in a footnote (p. 48) as “not to the point.”

WA06 have performed a meticulous and thorough evaluation of MBH98, and the answers that this committee seeks about the MBH reconstructions are to be found within this report. After examining each of MM’s three methodological criticisms, WA06 accepted two of them as valid, and have used them to correct the MBH98 reconstruction. I will now show you what effect these corrections have on the MBH98 reconstruction, and then reconsider the uniqueness of the late 20th-century warming trend in the light of these corrections.

The original MBH98 “hockey stick” is shown as a gray line (Fig. 1). The WA06 reproduction of MBH98 is shown in red (Fig. 1). Except for a couple of minor simplifications, WA06 remained faithful to the original MBH method and retained all of the original MBH data, including the original instrumental temperature series from 1992. They wrote their own computer code to perform the calculations, using the R programming language, as recommended by the MM and the Wegman report, rather than the original Fortran language used by Dr. Mann. As you can see, the two reconstructions are materially the same. This result demonstrates that MBH98 can be reproduced based on information available in the original MBH papers and supplemental information and data available on the Internet.

With this successful reproduction in hand, WA06 were able to test the effects of each of MM’s criticisms on the outcome of the MBH98 reconstruction. After carefully considering the validity of MM’s three criticisms of MBH’s reconstruction methodology, WA06 agreed that 1) decentering the proxy data prior to Principle Component analysis and 2) including the poorly replicated North American Gaspé tree-ring series from 1400-1449 both affected the MBH results. After correcting for these effects, WA06 obtained the results shown in blue (Fig. 2, left frame). The result is a slightly warmer (0.1 °C) early 15th century, with no other time period affected. MM’s third methodological criticism surrounding the inclusion of the bristlecone/foxtail pine series was rejected for several reasons. The right frame in Fig. 2 illustrates that excluding these series has little effect on the MBH98 reconstruction, except to force it to begin in 1450 instead of 1400, because of lack of a data. Since the exclusion had little effect, and losing these data series would hinder reconstructions of earlier climate, WA06 rejected this criticism.

The additional 15th-century warmth revealed by making the valid MM corrections still does not approach the warmth of the late 20th century, so MM’s critique cannot yet be said to have merit. However, the corrected result creates the impression of an upward temperature trend backward in time before 1400, begging the question of what would happen to the Middle Ages in the 1,000-year MBH99 reconstruction if it were also corrected? Answering that question is requisite for determining the merit of MM’s critique of MBH. The original 1,000-year MBH99 reconstruction is shown in blue and the corrected version is shown in red (Fig. 3; Ammann & Wahl, submitted). Carrying the correction back to the full millennium reveals that the largest effects remain in the early 15th century, and both earlier and later periods were less affected. Therefore, there is very little difference between the corrected MBH98 and MBH99 reconstructions and the originals, and the original observation that the late 20th century is uniquely warm in the context of the past 1,000 years is not affected. Hence, the valid methodological caveats that MM pointed out do not undermine the main conclusions of the original MBH papers or the conclusion of the 2001 IPCC assessment.

The scientific debate over the Medieval Warm Period (MWP) has been on the same trajectory for at least 20 years, with early indications that the MWP was not a globally coherent event becoming more solid over time. The MBH99 reconstruction represented an evolutionary step—not a revolutionary change—in this established trajectory. The 1990 IPCC figure that Mr. McIntyre, the Wall Street Journal editorial page, and Dr. Wegman have used in their own assessment of past climate is a cartoon, as stated by Dr. Wegman in his testimony last week. I have confirmed this with a number of individuals who were involved with the 1990 IPCC report or with versions of the schematic that pre-dated the 1990 IPCC report. The schematic is not a plot of data and is inappropriate as a comparison to MBH. The text of the 1990 IPCC report clearly states that the figure is a “schematic diagram” and that “it is still not clear whether all the fluctuations indicated were truly global” (p. 202). Furthermore, only three sources of information were cited and those sources conflicted on whether the Northern Hemisphere was warm or cold: “The late tenth to early thirteenth centuries… appear to have been exceptionally warm in parts of western Europe, Iceland and Greenland… China was, however, cold at this time, but South Japan was warm…” Clearly, this report certainly did not paint a picture of any consensus regarding a Medieval Warm Period as a hemisphere-wide phenomenon and characterizing it as such reveals a fundamental misunderstanding of climate science.

The 1992 and 1995 IPCC reports continued this same trajectory of thought. Four years before MBH99, citing 6 papers—still a very limited number, but twice as many as were cited in 1990—the 1995 report stated:

Remember that this was written by a team of climatologists as a consensus statement. The consensus at this time, as in 1990 and 1995, was that there was no strong evidence of a hemisphere-wide MWP.

Continuing the same trajectory, the 2001 IPCC Third Assessment Report examined evidence from 10 cited sources for the MWP. The consensus at this point seemed to be turning to the conclusion that there actually was a generally warm Northern Hemisphere during the Middle Ages, but that it was not a strong, coherent pattern of warming:

It is likely that temperatures were relatively warm in the Northern Hemisphere as a whole during the earlier centuries of the millennium, but it is much less likely that a globally-synchronous, well defined interval of “Medieval warmth” existed, comparable to the near global warmth of the late 20th century… Marked warmth seems to have been confined to Europe and regions neighboring the North Atlantic.

Since the MBH reconstructions were hemisphere-wide, and the MWP probably was not, it should not surprise us that the reconstructions lack a strong MWP (MBH99 does show slightly warmer temperatures in the 9th to 14th centuries than in the 15th to 19th centuries).

All available evidence indicates that the situation during the Middle Ages was fundamentally different that what is happening with climate today, which is a well-documented, globally coherent warming trend that is happening North, South, East, and West; at low latitudes and high latitudes; over land and over—and into—the sea. There are new data, published earlier this year, indicating that the atmosphere above Antarctica has warmed dramatically in recent decades (Turner et al. 2006). There is no large region on Earth where large-scale 20th century warming has not been detected, which simply cannot be said of the MWP.

Wahl and Ammann (2006) have demonstrated that the results of MBH are robust “down in the weeds”:

Our examination does suggest that a slight modification to the original Mann et al. reconstruction is justifiable for the first half of the 15th century (~ +0.05°), which leaves entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous over at least the last 600 years.

The NAS has affirmed the MBH results are also robust in the bigger picture, as well:

The basic conclusion of MBH99 was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on icecaps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2,000 years. Not all individual proxy records indicate that the recent warmth is unprecedented, although a larger fraction of geographically diverse sites experienced exceptional warmth during the late 20th century than during any other extended period from A.D. 900 onward. (p. 3)

Examination of the IPCC reports through time, as well as the primary scientific literature, reveals why the MBH results are so robust—MBH simply assimilated all the available evidence into a quantitative reconstruction—evidence that had already been evaluated qualitatively as lacking a coherent MWP.

This committee is seeking to know the significance of the criticisms leveled at the MBH reconstruction for climate change assessments. The significance is that these criticisms have resulted in the most thoroughly vetted single climate study in the history of climate change research. Dr. Tom Karl summarized the impact most succinctly in his testimony to this committee last week when he said that he would stand by the IPCC’s original assessment: “If you ask me to give qualifications about the findings in the 2001 report with the same caveat in terms of defining likelihood, I personally would not change anything.” Hence, the impact of the MM critique, after being scrutinized by the NAS, the Wegman panel, and a number of meticulous individual research groups, is essentially nil with regard to the conclusions of MBH and the 2001 IPCC assessment.

Also relevant to this committee’s questions about climate change assessments is the revelation that climate scientists do know their business, and that a lack of knowledge of geophysics is a genuine handicap to those who would seek to provide what they deem “independent review.” If the assessment of climate science presented in Mr. McIntyre’s presentation to the NAS committee, the Wegman Report, and the WSJ is an example of what can be expected from those who have not conducted climate research, then the investigation launched by this committee has demonstrated clearly that “independent review” by non-climate scientists is an exceedingly ineffective way to make climate change assessments.


Mann, M E, R S Bradley and M K Hughes (1998). “Global-scale temperature patterns and climate forcing over the past six centuries.” Nature 392(6678): 779-787.

Mann, M E, R S Bradley and M K Hughes (1999). “Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.” Geophysical Research Letters 26(6): 759-762.

McIntyre, S and R McKitrick (2003). “Corrections to the Mann et al. (1998) proxy data base and northern hemisphere average temperature series.” Energy & Environment 14(6): 751-771.

McIntyre, S and R McKitrick (2005). “Hockey sticks, principal components, and spurious significance.” Geophysical Research Letters 32(3).

National Research Council, C O S T R F T L, 000 Years. (2006). “Surface temperature reconstructions for the last 2,000 years.” from 11676.html.

Turner, J, T a Lachlan-Cope, S Colwell, et al. (2006). “Significant warming of the Antarctic winter troposphere.” Science 311: 1914-1917.

Wahl, E and C Ammann (In press). “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.” Climatic Change (accepted).