The hockey stick controversy is a dispute over the reconstructed estimates of Northern Hemisphere mean temperature changes over the past millennium,
especially the particular reconstruction of Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes, frequently referred to as the MBH98 reconstruction. The term "hockey stick" was coined by the head of National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory, Jerry Mahlman, to describe the pattern.
Nature of the dispute
A quasi-global instrumental temperature record exists from approximately 1850; but to construct a millennial-scale record proxies for temperature are required; issues arise over the faithfulness with which these proxies reflect actual temperature change, their geographical coverage, and the statistical methods used to combine them.
The political significance of the scientific controversy over the graph centers on its use as part of the evidence for anthropogenic global warming. The MBH98 reconstruction was prominently featured in the 2001 United Nations Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) and as a result has been widely published in the media.
This dispute centered on technical aspects of the methodology and data sets used in creating the MBH98 reconstruction. The issue was originally raised by former mining executive Stephen McIntyre and economist Ross McKitrick. Their criticisms were that Mann et al.'s reconstructed millennial temperature graph (the hockey stick) was an artifact of flawed calculations and serious data defects; in turn, MBH replied that these criticisms were spurious.
The dispute eventually led to an investigation at the behest of U.S. Congress by a panel of scientists convened by the National Research Council (NRC) of the United States National Academy of Sciences to consider reconstructions of the last 2000 years in general; in addition, an investigation was performed at the behest of Congressman Joe Barton by a panel of three statisticians, chaired by Edward Wegman specifically addressing the MBH work. Both the NRC and Wegman teams issued reports in 2006.
The second graph on the right shows the data from MBH98 and from several other climate reconstructions, subsequent to the 1998 reconstruction. Two of the other temperature reconstructions included on the graph are by Mann and co-authors.
There is an ongoing debate about the details of the temperature record and the means of its reconstruction. The debate centers around several discussion points:
- How well can past temperatures be reconstructed from the data we have?
- Was the late 20th century the warmest period during the last 1,000 years?
- Was the Medieval Warm Period observed in the North Atlantic region part of a broader global or hemispheric warming?
- Are bristlecone and foxtail pine tree rings valid temperature proxies?
- Without using the bristlecone and foxtail proxies in the reconstruction, does a hockey stick even exist?
Discussion of the MBH reconstruction
The hockey stick controversy has to a large extent been focussed on Mann and on the MBH98 reconstruction on which he was the lead author. Scientific American magazine described him as the "Man behind the Hockey Stick," referring to this reconstruction of temperatures. The BBC described
the "hockey stick" as a term coined for the chart of temperature variation over the last 1,000 years. The chart is relatively flat from the period A.D. 1000 to 1900, indicating that temperatures were relatively stable for this period of time. The flat part forms the stick's "shaft." After 1900, however, temperatures appear to shoot up, forming the hockey stick's "blade." The combination of the two in the chart suggests a recent sharp rise in temperature caused by human activities. The BBC further stated
- "The high-profile publication of the data led to the "hockey stick" being used as a key piece of supporting evidence in the Third Assessment Report by the United Nations' Intergovernmental Panel on Climate Change (IPCC) in 2001."
In 2003, Stephen McIntyre and Ross McKitrick published "Corrections to the Mann et al (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series" in the (JCR-unlisted) journal Energy and Environment 14(6) 751-772, raising concerns about their ability to reproduce the results of MBH. The IPCC AR4 reports that "Wahl and Ammann (2007) showed that this was a consequence of differences in the way McIntyre and McKitrick (2003) had implemented the method of Mann et al. (1998) and that the original reconstruction could be closely duplicated using the original proxy data." . In 2004 Mann, Bradley, and Hughes published a corrigendum to their 1998 article, correcting a number of mistakes in the online supplementary information that accompanied their article but leaving the actual results unchanged.
Hans von Storch and colleagues claimed that the method used by Mann et al. probably underestimates the temperature fluctuations in the past by a factor of two or more. however, this conclusion rests at least in part on the reasonableness of the global climate model (GCM) simulation used, which has been questioned; Wahl et al. assert errors in the reconstruction technique that von Storch used.. Von Storch's claim implied that MBH98 was less accurate because if there was more variability than originally shown, then Mann's "hockey stick" would look less like a hockey stick and therefore be weaker argument for recent dramatic climate change.
The IPCC AR4 reports that the extent of any such biases in specific reconstructions... is uncertain ... It is very unlikely, however, that any bias would be as large as the factor of two suggested.
Anders Moberg and his Swedish and Russian collaborators have also generated reconstructions with significantly more variability than the reconstructions of Mann et al.
After testing the work of Mann et al. (1998), McKitrick commented
- "The Mann multiproxy data, when correctly handled, shows the 20th century climate to be unexceptional compared to earlier centuries. This result is fully in line with the borehole evidence. (As an aside, it also turns out to be in line with other studies that are sometimes trotted out in support of the hockey stick, but which, on close inspection, actually imply a MWP as well.)"
In turn, Mann (supported by Tim Osborn, Keith Briffa and Phil Jones of the Climatic Research Unit) has disputed the claims made by McIntyre and McKitrick, saying the
- "...so-called 'correction' was nothing more than a botched application of the MBH98 procedure, where the authors (MM) removed 80% of the proxy data actually used by MBH98 during the 15th century period... Indeed, the bizarre resulting claim by MM of anomalous 15th century warmth (which falls within the heart of the "Little Ice Age") is at odds with not only the MBH98 reconstruction, but, in fact the roughly dozen other estimates now published that agree with MBH98 within estimated uncertainties...".
On February 12, 2005, Stephen McIntyre and Ross McKitrick published a paper in Geophysical Research Letters that claimed various errors in the methodology of Mann et al. (1998). The paper claimed that the "Hockey Stick" shape was the result of an invalid principal component method. They claimed that using the same steps as Mann et al., they were able to obtain a hockey stick shape as the first principal component in 99 percent of cases even if trendless red noise was used as input.
This paper was nominated as a journal highlight by the American Geophysical Union, which publishes GRL, and attracted international attention for its claims to expose flaws in the reconstructions of past climate.. The IPCC AR4 says this paper may have some theoretical foundation, but Wahl and Amman (2006) also show that the impact on the amplitude of the final reconstruction is very small.
Mann has been personally involved in the debate over climate change. In testimony before the U.S. Senate in 2003, he stated:
- "It is the consensus of the climate research community that the anomalous warmth of the late 20th century cannot be explained by natural factors, but instead indicates significant anthropogenic, that is human influences... More than a dozen independent research groups have now reconstructed the average temperature of the northern hemisphere in past centuries... The proxy reconstructions, taking into account these uncertainties, indicate that the warming of the northern hemisphere during the late 20th century... is unprecedented over at least the past millennium and it now appears based on peer-reviewed research, probably the past two millennia."
More recently, the National Academy of Sciences considered the matter. On June 22, 2006, the Academy released a pre-publication version of its report Report-Surface Temperature Reconstructions for the Last 2,000 Years, supporting Mann's more general assertion regarding the last decades of the Twentieth Century, but showing less confidence in his assertions regarding individual decades or years, due to the greater uncertainty at that level of precision.
- "The basic conclusion of Mann et al. (1998, 1999) 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 ...
Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. 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." 
One point of contention relates to McIntyre's requests for Mann to provide him with the data, methods and source code McIntyre needed to "audit" MBH98. Mann provided some data and then stopped. After a long process - in which the National Science Foundation supported Mann - the code was made publicly available . It happened because Congress investigated after an article in the Wall Street Journal detailed criticisms raised by McIntyre. Congress was especially concerned about Mann’s reported refusal to provide data. In June 2005, Congress asked Mann to testify before a special subcommittee. The chairman of the committee (Joe Barton, a prominent global warming skeptic) wrote a letter to Mann requesting he provide his data, including his source code, archives of all data for all of Mann's scientific publications, identities of his present and past scientific collaborators, and details of all funding for any of Mann's ongoing or prior research, including all of the supporting forms and agreements. The American Association for the Advancement of Science viewed this as "a search for some basis on which to discredit these particular scientists and findings, rather than a search for understanding. When Mann complied, all of the data was available for McIntyre. Congress also requested that third party science panels review the criticisms by McIntyre and McKitrick. The Wegman Panel and the National Academy of Sciences both published reports. McIntyre and McKitrick (2005) claim that 7 of their 10 findings in 2003 have been largely confirmed by these reviews. Nature reported it as "Academy affirms hockey-stick graph - But it criticizes the way the controversial climate result was used."
National Research Council Report
At the request of the U.S. Congress, a special "Committee on Surface Temperature Reconstructions for the Past 2,000 Years" was assembled by the National Research Council's
Board on Atmospheric Sciences and Climate. The Committee consisted of 12 scientists from different disciplines and was tasked with explaining the current scientific information on the temperature record for the past two millennia, and identifying the main areas of uncertainty, the principal methodologies used, any problems with these approaches, and how central the debate is to the state of scientific knowledge on global climate change.
The panel published its report in 2006. The report agreed that there were statistical shortcomings in the MBH analysis, but concluded that they were small in effect. The report summarizes its main findings as follows:
- The instrumentally measured warming of about 0.6 °C (1.1 °F) during the 20th century is also reflected in borehole temperature measurements, the retreat of glaciers, and other observational evidence, and can be simulated with climate models.
- Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the “Medieval Warm Period”) and a relatively cold period (or “Little Ice Age”) centered around 1700. The existence and extent of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
- It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies.
- Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.
- Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods.
In response, a group-authored post on RealClimate, of which Mann is one of the contributors, stated, "the panel has found reason to support the key mainstream findings of past research, including points that we have highlighted previously. Similarly, according to Roger A. Pielke, Jr., the National Research Council publication constituted a "near-complete vindication for the work of Mann et al."; Nature (journal) reported it as "Academy affirms hockey-stick graph.
According to Hans von Storch, Eduardo Zorita and Jesus Rouco, reviewing the NAS report on McIntyre's blog ClimateAudit, "With respect to methods, the committee is showing reservations concerning the methodology of Mann et al. The committee notes explicitly on pages 91 and 111 that the method has no validation (CE) skill significantly different from zero. In the past, however, it has always been claimed that the method has a significant nonzero validation skill. Methods without a validation skill are usually considered useless. It was noted by their critics, however, that no such statement, explicit or implicit, is present on the two pages cited; the closest the report comes being a statement that "Some recent results reported in Table 1S of Wahl and Ammann (in press) indicate that their reconstruction, which uses the same procedure and full set of proxies used by Mann et al. (1999), gives CE values ranging from 0.103 to -0.215, depending on how far back in time the reconstruction is carried.
However, CE is not the only measure of skill; Mann et al. (1998) used the more traditional "RE" score, which, unlike CE, accounts for the fact that time series change their mean value over time. The statistically significant reconstruction skill in the Mann et al. reconstruction is independently supported in the peer-reviewed literature.
Committee on Energy and Commerce Report (Wegman report)
A team of statisticians led by Edward Wegman
, chair of the National Academy of Sciences’ (NAS) Committee on Applied and Theoretical Statistics, was assembled at the request of U.S. Rep. Joe Barton
and U.S. Rep. Ed Whitfield
. The report primarily focused on the statistical analysis used in the MBH paper, and also considered the personal and professional relationships between Mann et al
and other members of the paleoclimate community. Findings presented in this report (commonly known as the "Wegman Report") at a hearing of the subcommittee on oversight and investigations, chaired by Whitfield, included the following:
- MBH98 and MBH99 were found to be "somewhat obscure and incomplete" and the criticisms by McIntyre and McKitrick were found to be "valid and compelling".
- The report found that MBH method creates a PC1 statistic dominated by bristlecone and foxtail pine tree ring series (closely related species). However there is evidence in the literature, that the use of the bristlecone pine series as a temperature proxy may not be valid (suppressing "warm period" in the hockey stick handle); and that bristlecones do exhibit CO2-fertilized growth over the last 150 years (enhancing warming in the hockey stick blade).
- It is noted that there is no evidence that Mann or any of the other authors in paleoclimatology studies have had significant interactions with mainstream statisticians.
- A social network of authorships in temperature reconstruction of at least 43 authors having direct ties to Mann by virtue of coauthored papers with him is described. The findings from this analysis suggest that authors in the area of paleoclimate studies are closely connected and thus ‘independent studies’ may not be as independent as they might appear on the surface. Dr. Wegman stated this was a "hypothesis", and "should be taken with a grain of salt".
- It is important to note the isolation of the paleoclimate community; even though they rely heavily on statistical methods they do not seem to interact with the statistical community. Additionally, the Wegman team judged that the sharing of research materials, data and results was haphazardly and grudgingly done.
- Overall, the 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.
The Wegman report has itself been criticized on several contentious grounds:
- The report was not subject to formal peer review At the hearing, Wegman lists 6 people that participated in his own informal peer review process via email after the report was finalized and said they had no objection to the subcommittee submitting it.
- Dr. Thomas Crowley, Professor of Earth Science System, Duke University, testified at the committee hearing, "The conclusions and recommendations of the Wegman Report have some serious flaws."
- The result of fixing the alleged errors in the overall reconstruction does not change the general shape of the reconstruction.
- Similarly, studies that use completely different methodologies also yield very similar reconstructions.
- The social network analysis is not based on meaningful criteria, does not prove a conflict of interest and did not apply at the time of the 1998 and 1999 publications. Such a network of co-authorship is not unusual in narrowly defined areas of science. During the hearing, Wegman defined the social network as peer reviewers that had "actively collaborated with him in writing research papers" and answered that none of his peer reviewers had.
- Gerald North, chairman of the National Research Council panel that studied the hockey-stick issue and produced the report Surface Temperature Reconstructions for the Last 2,000 Years, stated the politicians at the hearing at which the Wegman report was presented "were twisting the scientific information for their own propaganda purposes. The hearing was not an information gathering operation, but rather a spin machine." In testimony when asked if he disputed the methodology conclusions of Wegman's report, he stated that "No, we don’t. We don’t disagree with their criticism. In fact, pretty much the same thing is said in our report. But again, just because the claims are made, doesn’t mean they are false."
- Mann has himself said that the report "uncritically parrots claims by two Canadians (an economist and a mineral-exploration consultant) that have already been refuted by several papers in the peer-reviewed literature inexplicably neglected by Barton's 'panel'. These claims were specifically dismissed by the National Academy in their report just weeks ago.
In his opening remarks, Chairman Barton (at the hearing ex officio) commented on the politically charged nature of the entire process, and the level of disagreement on a great many of these issues, in fact:
So I want to thank Dr. Wegman and his colleagues for giving us an unvarnished, flat out non-political report. Now, admittedly, that report is going to be used probably for political purposes but that is not what he did, and I want to thank Dr. North for the work that he did in this document. Now, it is a lot thicker than Dr. Wegman's document, and Dr. North and his colleagues have kind of
looked at the same subject and they have come to a somewhat little--they are little bit more, I don't want to use the technical term wishy-washy but they are kind of on both sides of it, but even Dr. North's report says that the absolute basic conclusion in Dr. Mann's work cannot be guaranteed. This report says it is
plausible. Lots of things are plausible. Dr. Wegman's report says it is wrong.
Now, what we are going to do after today's hearing, we are going to take Dr. Wegman's report, and if my friends on the Minority want to shop it to their experts, so be it. We are going to put it up there, let everybody who wants to, take a shot at it. Now, my guess is that since Dr. Wegman came into this with no political axe to grind, that it is going to stand up pretty well. If Dr. Mann and
his colleagues are right, their conclusion may be right--Dr. Mann's conclusion may be right but you can't verify it from his statistics in his model so if Dr. Mann's conclusion is right, it is incumbent upon him and his colleagues to go back, get the math right, get the data points right, get the modeling right. That is what science is about.
This contention is further illustrated by the first sentence of the subcommittee's Ranking Member Bart Stupak's remarks:
"Thank you, Mr. Chairman. It is a little bewildering to me why the committee is holding its very first hearing on global warming to referee a dispute over a 1999 hockey stick graph of global temperatures for the past millennium."
In a letter to Nature
on August 10
, Bradley, Hughes and Mann pointed at the original title of their 1998 article: "Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations
and pointed out "more widespread high-resolution data are needed before more confident conclusions can be reached and that the uncertainties were
the point of the article." Mann and his colleagues said that it was "hard to imagine how much more explicit" they could have been about the uncertainties surrounding their work and blaming "poor communication by others" for the "subsequent confusion." He has further suggested that the criticisms directed at his statistical methodology are purely political and add nothing new to the scientific debate.
Paleoclimate findings by the IPCC before and after the Hockey Stick Controversy:
Before: 2001 (page 2)
- " proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year."
Current SPM statement from 2007 (page 10)
- "“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 last 500 years and likely the highest in at least the past 1300 years. Some recent studies indicate greater variability in Northern Hemisphere temperatures than suggested in the TAR, particularly finding that cooler periods existed in the 12 to 14th, 17th, and 19th centuries. Warmer periods prior to the 20th century are within the uncertainty range given in the TAR.”
In May 2007, Hans von Storch reviewed the changes in thought caused by the hockey stick controversy writing:
- In October 2004 we were lucky to publish in Science our critique of the ‘hockey-stick’ reconstruction of the temperature of the last 1000 years. Now, two and half years later, it may be worth reviewing what has happened since then.
- At the EGU General Assembly a few weeks ago there were no less than three papers from groups in Copenhagen and Bern assessing critically the merits of methods used to reconstruct historical climate variable from proxies; Bürger’s papers in 2005; Moberg’s paper in Nature in 2005; various papers on borehole temperature; The National Academy of Science Report from 2006 – all of which have helped to clarify that the hockey-stick methodologies lead indeed to questionable historical reconstructions. The 4th Assessment Report of the IPCC now presents a whole range of historical reconstructions instead of favoring prematurely just one hypothesis as reliable.
McIntyre was critical of this Nature blog entry because von Storch did not acknowledge the role of McIntyre and McKitrick; however von Storch replied that:
- This was on purpose, as we do not think that McIntyre has substantially contributed in the published peer-reviewed literature to the debate about the statistical merits of the MBH and related method. They have published one peer-reviewed article on a statistical aspect, and we have published a response – acknowledging that they would have a valid point in principle, but the critique would not matter in the case of the hockey-stick ... we see in principle two scientific inputs of McIntyre into the general debate – one valid point, which is however probably not relevant in this context, and another which has not been properly documented.
As a lot of claims regarding the hockey stick revolve around statistical aspects, the American Statistical Association held a session at the 2006 Joint Statistical Meetings, on climate change with Edward Wegman, John Michael Wallace, and Richard L. Smith. E. Wegman presented the discussion of the methodological aspects of PC analysis by MBH98, and his view that Method Wrong + Answer Correct = Bad Science. J. M. Wallace outlined the NRC report and its cautious conclusion that the claims of unprecedented temperatures in the last decades can be considered as plausible (2:1 odds in favor). R. L. Smith (U. of North Carolina, Statistics) analyzed statistical methodology behind the CCSP "Report on Temperature Trends in the Lower Atmosphere” and shared his vision of the role of statisticians in the process. The session was summarized by R. L. Smith in ASA Section on Statistics and the Environment newsletter.
In a paper on 9 September 2008, Mann and colleagues published an updated reconstruction of Earth surface temperature for the past two millennia. This reconstruction used a more diverse dataset that was significantly larger than the original tree-ring study. Similarly to the original study, this work found that recent increases in northern hemisphere surface temperature are anomalous relative to at least the past 1300 years, and that this result is robust to the inclusion or exclusion of the tree-ring dataset.
- Kyoto protocol based on flawed statistics by Marcel Crok, translation by Angela den Tex, Natuurwetenschap & Techniek, February, 2005.
- Surface Temperature Reconstructions for the Last 2,000 Years NAS Report for the Committee on Surface Temperature Reconstructions (PDF download free)
- Climate Reconstructions A collection of reconstructions of global and regional temperature for centuries to millennia, from the World Data Center for Paleoclimatology
- Proxy climate data from the World Data Center for Paleoclimatology
- What is the ‘Hockey Stick’ Debate About?, by Ross McKitrick
- Supplementary information for Mann, M. E. et al. corrigendum: Global-scale temperature patterns and climate forcing over the past six centuries Nature 430, 105(2004) Letters to Nature. http://www.nature.com/nature/journal/v430/n6995/suppinfo/nature02478.html
- Corrections to the Mann et al (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series Steven McIntyre, Ross McKitrick. Energy and Environment 14(6) 751-772. http://www.uoguelph.ca/~rmckitri/research/trc.html
- "A Global Warming Bombshell" by Richard A. Muller, Technology Review , Oct. 2004; calls into question famous graph by Michael E. Mann
- Backgrounder for McIntyre and McKitrick “Hockey Stick Project” by McIntyre and McKitrick, January 27 2005
- Myth vs. Fact Regarding the "Hockey Stick" — Real Climate blog
- Was the climate of the 20th century unusual? - essay by global warming skeptics Willie Soon and Sallie Baliunas
- Ad hoc committee report on the ‘Hockey Stick’ Global Climate Reconstruction
- Discussion article about the relevance of the debate to climate policy and climate science policy with contributions from both 'sides'
- The IPCC, the "Hockey Stick" Curve, and the Illusion of Experience Washington Roundtable on Science & Public Policy hosted by the George C. Marshall Institute
- The Decay of the Hockey Stick by Von Storch web published May 3, 2007
- Bias and Concealment in the IPCC Process: The "Hockey-Stick" Affair and Its Implications by David Holland, Energy and Environment, 2007.
- The Hockey Stick Debate as a Matter of Science Policy by Roger Pielke Jr, discusses the Bishop Hill blog on the Wahl and Ammann publications. Web published August 13, 2008.
Ongoing updates related to the MBH work are accessible in two weblogs: