Hockey stick controversy

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The so-called Hockey stick graph as shown in the 2001 IPCC report. This chart shows the data from Mann et al. 1999. The colored lines are the reconstructed temperatures, and the gray shaded region represents estimated error bars.

Reconstructions of Northern Hemisphere temperatures for the last 1,000 years according to various older articles (bluish lines), newer articles (reddish lines), and instrumental record (black line).

The Hockey stick controversy is a dispute over the reconstructed estimates of Northern Hemisphere mean temperature changes over the past millennium,^[1] especially the particular reconstruction of Mann, Bradley and Hughes,^[2] 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.

[edit] Nature of the dispute

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. The issue was originally highlighted by Stephen McIntyre and Ross McKitrick. The major criticism was the claim that Mann et al.'s choice of data sets and their processing largely suppressed two notable climatic variations: the Medieval Warm Period (MWP) around the beginning of the second millennium and the Little Ice Age centered around 1600-1700 AD.

The dispute eventually led the U.S. Congress to request two reevaluations of the work, one by a panel of scientists convened by the National Research Council (NRC) of the United States National Academy of Sciences, and the other by a panel of three statisticians, chaired by Edward Wegman. Both the NRC and Wegman teams issued reports in 2006.

There was also controversy regarding the prominence given by the IPCC to the MBH reconstruction, and the extent to which qualifications and uncertainty may have been understated.

The second graph on the right shows the data from MBH and from several other climate reconstructions, most subsequent (two of the other graphs included are also 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?
• Did the Medieval Warm Period exist? If it did exist, how extensive was it?
• Are Bristlecone treerings valid temperature proxies?
• How were the error bars in the Hockeystick curve calculated ?

[edit] Criticisms of the MBH method

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;^[3] however, this conclusion rests at least in part on the reasonableness of the global climate model (GCM) simulation used, which has been questioned;^[4][5] Wahl et al. assert errors in the reconstruction technique that von Storch used.^[6]

Anders Moberg and his Swedish and Russian collaborators have also generated reconstructions with significantly more variability than the reconstructions of Mann et al.^[7][8]

In 2003, Stephen McIntyre and Ross McKitrick published "Corrections to the Mann et al (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series" Energy and Environment 14(6) 751-772. 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.

On February 12, 2005, the Geophysical Research Letters paper by Stephen McIntyre and Ross McKitrick claimed various errors in the methodology of Mann et al. (1998), claiming that the "Hockey Stick" shape was a result an invalid principal component method.^[9] They claimed that using the same steps as Mann et al., they were able to obtain a hockey stick shape as the first PC in 99 percent of cases even if red noise was used as input.^[10] This paper was nominated as a journal highlight by the American Geophysical Union,^[11] which publishes GRL, and attracted international attention for its claims to expose flaws in the reconstructions of past climate.^[12]

After auditing 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.)"^[13]

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,^[14][15] saying, "...MM have made critical errors in their analysis that have the effect of grossly distorting the reconstruction of MBH98..." The authors of the reconstructions have acknowledged some of the flaws, but claim that their conclusions are not significantly affected.^[16]

In a letter to Nature on August 10, 2006, Bradley, Hughes and Mann pointed at the original title of their 1998 article: "Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations" (Geophys. Res. Lett. 26, 759–762; 1999) 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 has suggested that the criticisms directed at his statistical methodology are purely political and add nothing new to the debate.^[17]

[edit] 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, consisting of 12 scientists from different disciplines, published its report in 2006.^[18] 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:^[19]

• 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.

[edit] Committee on Energy and Commerce Report (Wegman report)

McIntyre and McKitrick's paper has been reinforced by a team of statisticians led by Edward Wegman, chair of the National Academy of Sciences’ (NAS) Committee on Applied and Theoretical Statistics. The Wegman team was assembled at the request of U.S. Rep. Joe Barton, an outspoken global warming skeptic.^[20] 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"^[21][22]) include 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.
• 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.
• 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 for a number of things:

• While Wegman says that there was peer review of the report, Mann states that the report was not subject to formal peer review,^[23], a position supported by Gerald North of the National Research Council.^[24] Wegman admitted in Congressional testimony that he did not receive written review feedback until after the report was finalized and delivered.^[25]
• The result of fixing the alleged errors in the overall reconstruction does not change the general shape of the reconstruction. [1]
• Similarly, studies that use completely different methodologies also yield very similar reconstructions.^[26][27]
• The social network analysis is not based on meaningful criteria and does not prove a conflict of interest. Such a network of co-authorship is not unusual in narrowly defined areas of science.^{[28][not in citation given][29][not in citation given]}
• 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."^[30]

[edit] Updates

Ongoing updates and future events related to the MBH work are accessible in two weblogs:

• RealClimate - Climate scientists blog, including Mann
• ClimateAudit - McIntyre blog

[edit] Mann

Scientific American magazine described him as the "Man behind the Hockey Stick," referring to his reconstruction of temperatures, originally published in 1998. The BBC wrote:

• The hockey stick was a term coined for a chart of temperature variation over the last 1,000 years, which suggested a recent sharp rise in temperature caused by human activities.
• 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."
• But after 1900, temperatures appear to shoot up, forming the hockey stick's "blade."
• 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.^[31]

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.

The battle over his work has been unusually personal, with Mann saying the attacks were "intellectually pathetic" and "deceptive." Representative Joe Barton (R-TX-06) requested information from Mann and co-authors about his work;^[32] this has been widely seen as "a search for some basis on which to discredit these particular scientists and findings, rather than a search for understanding."^[33] Rep. Barton´s inquiry cited an article in the Wall Street Journal which detailed criticisms raised by Stephen McIntyre.^[34] McIntyre claims that many statistical methods used to generate Mann's hockey stick were applied incorrectly and that Mann used data known to be faulty. McIntyre created his own plot of past temperatures which shows a smaller temperature rise in the 20th century than in the 14th century.^[35]

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.^[36]

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."^[37]

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."^[38] Similarly, according to Roger A. Pielke, Jr., the National Research Council publication constituted a "near-complete vindication for the work of Mann et al.";^[39] Nature reported it as "Academy affirms hockey-stick graph."^[40]

According to Von Storch, Zorita and Gonzalez-Rouco, reviewing the NAS report on 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."[2] It was noted by their critics, however, that no such statement, explicit or implicit, is present on the two pages cited[3] and[4]; 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."[5]

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 by Huybers (2005) and Wahl and Ammann (2006).

A report on July 19, 2006, by Edward Wegman, chair of the National Academy of Sciences' Committee on Applied and Theoretical Statistics, for the Committee on Energy and Commerce of the U.S. Congress largely repeated the criticisms of M&M;[6] Mann has said that the report "uncritically parrots claims by two Canadians (an economist and an 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."[7]

In a letter to Nature (442, 627, 2006), 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."

[edit] McIntyre

McIntyre commented on how he sees his own role in the global warming debate as follows:[8]

[edit] References

[edit] External links

• Surface Temperature Reconstructions for the Last 2,000 Years NAS Report for the Committee on Surface Temperature Reconstructions (PDF download free)
• Mann's home page
• A collection of various reconstructions of global and local temperature from centuries on up:
• An NOAA collection of individual data records
• 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
• Climate Audit Webpage of Stephen McIntyre
• "A Global Warming Bombshell" by Richard A. Muller, Technology Review , Oct. 2004; calls into question famous graph by Michael Mann
• What is the ‘Hockey Stick’ Debate About? McKitrick reviews the criticisms of the Mann hockey stick for a general scientific audience, April, 2005.
• Backgrounder for McIntyre and McKitrick “Hockey Stick Project” by McIntyre and McKitrick, January 27 2005
• Myth vs. Fact Regarding the "Hockey Stick" — Real Climate
• Was the climate of the 20th century unusual?
• 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 `Hockey Stick': A New Low in Climate Science by Tasmanian author John Daly. Criticism of the Mann graph