The hockey stick controversy refers to debates over the technical correctness and implications for global warming of graphs showing reconstructed estimates of the temperature record of the past 1000 years. At a political level, the debate is about the use of these graphs to convey complex science to the public, and the question of the robustness of the assessment presented by the Intergovernmental Panel on Climate Change (IPCC).
By the late 1990s a number of competing teams were using proxy indicators to estimate the temperature record of past centuries, and finding suggestions that recent warming was exceptional.[1] In 1998 Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes produced the first quantitative hemispheric-scale reconstruction, from an analysis of a variety of measures, which they summarised in a graph going back to 1400 showing recent measured temperatures increasing sharply. Their 1999 paper extended this study back to 1000, and included a graph which was featured prominently in the 2001 United Nations Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) as supporting the mainstream view of climate scientists that there had been a relatively sharp rise in temperatures during the second half of the 20th century. It became a focus of dispute for those opposed to this strengthening scientific consensus.[2] The term hockey stick was coined by the climatologist Jerry Mahlman, to describe the pattern, envisaging a graph that is relatively flat to 1900 as forming the hockey stick's "shaft", followed by a sharp increase corresponding to the "blade".[3]
In 2003, Willie Soon and Sallie Baliunas argued against this pattern in a paper which was dismissed by scientists as faulty in the Soon and Baliunas controversy.[1] In the United States there was already a hot political dispute over action on global warming following lobbying regarding the 1997 Kyoto Protocol, and on July 28, Republican Jim Inhofe made a Senate speech citing Soon and Baliunas to support his view "that man-made global warming is the greatest hoax ever perpetrated on the American people".[4] Also in 2003, Stephen McIntyre and Ross McKitrick published a paper questioning the statistical methods used in the Mann et al. paper, and there was continued debate on these issues. Hans von Storch regards that paper as of little consequence, and believes his paper of 2004 to be the first significant criticism.[5] At the request of Congress, a panel of scientists convened by the National Research Council was set up, which reported in 2006 supporting Mann's findings with some qualifications, including agreeing that there were some statistical failings but these had little effect on the result.[6] U.S. Rep. Joe Barton and U.S. Rep. Ed Whitfield requested Edward Wegman to set up a team of statisticians to investigate, and they supported the view that there were statistical failings, although their report has itself been criticized on several grounds.
More than twelve subsequent scientific papers, using various statistical methods and combinations of proxy records, produced reconstructions broadly similar to the original MBH hockey-stick graph, with variations in how flat the pre-20th century "shaft" appears. Almost all of them supported the IPCC conclusion that the warmest decade in 1000 years was probably that at the end of the 20th century.[6]
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Paleoclimatology influenced the 19th century physicists John Tyndall and Svante Arrhenius who found the greenhouse gas effect of carbon dioxide (CO2) in the atmosphere to explain how past ice ages had ended.[7] In 1965 Hubert Lamb, a pioneer of historical climatology, generalised from temperature records of central England by using historical, botanical and archeological evidence to popularise the idea of a Medieval Warm Period from around 900 to 1300, followed by a cold epoch culminating between 1550 and 1700.[8][9] In 1972 he became the founding director of the Climatic Research Unit (CRU) in the University of East Anglia (UEA), which aimed to improve knowledge of climate history in both the recent and far distant past, monitor current changes in global climate, identify processes causing changes at different timescales, and review the possibility of advising about future trends in climate.[10] During the cold years of the 1960s Lamb had anticipated that natural cycles were likely to lead over thousands of years to a future ice age, but after 1976 he supported the emerging view that greenhouse gas emissions caused by humanity would cause detectable global warming "by about A.D. 2000".[8]
There was increasing public and political interest, and in 1988 the Reagan administration, concerned in part about political influence of scientists, successfully lobbied for the 1988 formation of the Intergovernmental Panel on Climate Change, its reports subject to detailed approval by government delegates.[11] The IPCC was organized by United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) December 6 as an effort by the United Nations to provide the governments of the world with a clear scientific view of what is happening to the world's climate.[12] The IPCC First Assessment Report in 1990 was the first global assessment of Climate science and charted a steady rise in global temperatures which prompted calls for international emissions caps.[13]
The agreed report noted evidence that Holocene climatic optimum around 5,000-6,000 years ago had been warmer than the present (at least in summer) and that in some areas there had been exceptional warmth during "a shorter Medieval Warm Period (which may not have been global)" about AD 950-1250, followed by a cooler period of the Little Ice Age which ended only in the middle to late nineteenth century. A "schematic diagram" of global temperature variations over the last thousand years[14] has been traced to a graph based loosely on Lamb's 1965 paper, nominally representing central England.[9] Mike Hulme describes this schematic diagram as "Lamb's sketch on the back of an envelope", a "rather dodgy bit of hand-waving".[15]
Archives of climate proxies were developed and the first hemispheric reconstructions were published: in 1993 Raymond S. Bradley and Phil Jones used historical records, tree-rings and ice cores for the Northern Hemisphere from 1400 up to the 1970s,[9] to come up with findings similar to later "hockey stick" studies.[16] The IPCC Second Assessment Report (SAR) of 1996 featured a graph of this reconstruction which included a separate curve plotting instrumental thermometer data from the 1850s onwards. The section proposed that "The data from the last 1000 years are the most useful for determining the scales of natural climate variability", and noted the 1994 reconstruction by Hughes and Diaz questioning how widespread the Medieval Warm Period been at any one time. It concluded, "it appears that the 20th century has been at least as warm as any century since at least 1400 AD. In at least some areas, the recent period appears to be warmer than has been the case for a thousand or more years".[17]
Among other large-scale palaeoclimate reconstructions, a November 1995 study by Mann, Park and Bradley also went back to 1400.[18] Tim Barnett was working towards the next IPCC assessment with Jones, and in 1996 told journalist Fred Pearce "What we hope is that the current patterns of temperature change prove distinctive, quite different from the patterns of natural variability in the past".[19] Tree ring specialist Keith Briffa's February 1998 study reporting a divergence problem affecting some tree ring proxies after 1960 warned that this problem had to be taken into account to avoid overestimating past temperatures.[20]
Meanwhile as the details of the science were being worked out, the international community had been steadily working towards a global framework for a cap on green-house-gas emissions.The UN Conference on Environment and Development was held in Rio de Janeiro in 1992, and produced a treaty framework that called for voluntary capping of emissions at 1990 levels. Government representatives gathered in Kyoto later during 1998 and turned this framework into a binding commitment know as the Kyoto protocol.[7][21] Due to the nature of the treaty, it essentially required the United States and other highly industrial nations commitment for proper implementation. By 1998 the Clinton Administration had signed the treaty, but vigorous lobbying meant ratification of the treaty was successfully opposed in the Senate by a bipartisan coalition of economic and energy interests. Lobbyists such as the Western Fuels Association funded scientists whose work might undermine the scientific basis of the treaty, and in 1998 it was revealed that the American Petroleum Institute had hosted informal discussions between individuals from oil companies, trade associations and conservative policy research organizations who opposed the treaty, and who had tentatively proposed an extensive plan to recruit and train scientists.[22][23][24]
On 23 April 1998, the journal Nature published the Mann, Bradley and Hughes multiproxy study on "Global-scale temperature patterns and climate forcing over the past six centuries" (MBH98) which used a new statistical approach to find patterns of climate change in both time and global distribution, building on previous multiproxy reconstructions. They concluded that "Northern Hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) AD1400", and estimated empirically that greenhouse gases had become the dominant climate forcing during the 20th century.[25]
The New York Times highlighted their finding that the 20th century had been the warmest century in 600 years, quoting Mann saying that "Our conclusion was that the warming of the past few decades appears to be closely tied to emission of greenhouse gases by humans and not any of the natural factors". Most proxy data are inherently imprecise, and Mann said "We do have error bars. They are somewhat sizable as one gets farther back in time, and there is reasonable uncertainty in any given year. There is quite a bit of work to be done in reducing these uncertainties." Climatologist Tom Wigley had high regard for the progress the study made, but doubted if proxy data could ever be wholly convincing in detecting the human contribution to changing climate. Phil Jones of the UEA Climatic Research Unit was doubtful about adding the 150-year thermometer record to extend the proxy reconstruction, and compared this with putting together apples and oranges; Mann et al. said they used a comparison with the thermometer record to check that recent proxy data were valid. Jones thought the study would provide important comparisons with the findings of climate modeling, which showed a "pretty reasonable" fit to proxy evidence.[26]
In May 1998, Jones, Keith Briffa, Tim P. Barnett and Simon Tett published their own analysis, comparing tree ring, coral layer, and glacial proxy records, extending their reconstruction back for a thousand years but not specifically estimating uncertainties. As Bradley recalls, Mann's initial view was that there was too little information and too much uncertainty to go back so far, but Bradley said "Why don't we try to use the same approach we used in Nature, and see if we could push it back a bit further?" Within a few weeks, Mann responded that to his surprise, "There is a certain amount of skill. We can actually say something, although there are large uncertainties."[27][28]
Despite the uncertainties, Mann, Bradley and Hughes found they were able to extend their approach to 1,000 years ago. Their paper (MBH99) was published in March 1999 with the cautious title Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations to emphasise the increasing uncertainty involved in reconstructions of the period before 1400 when fewer proxies were available.[28] A University of Massachusetts news release dated 3 March 1999 announced publication in the 15 March issue of Geophysical Research Letters, "strongly suggesting that the 1990s were the warmest decade of the millennium, with 1998 the warmest year so far." Bradley was quoted as saying "Temperatures in the latter half of the 20th century were unprecedented", while Mann said "As you go back farther in time, the data becomes sketchier. One can’t quite pin things down as well, but, our results do reveal that significant changes have occurred, and temperatures in the latter 20th century have been exceptionally warm compared to the preceding 900 years. Though substantial uncertainties exist in the estimates, these are nonetheless startling revelations." While the reconstruction supported theories of a relatively warm medieval period, Hughes said "even the warmer intervals in the reconstruction pale in comparison with mid-to-late 20th-century temperatures."[29] The New York Times report had a colored version of the graph, distinguishing the instrumental record from the proxy evidence and emphasising the increasing range of possible error in earlier times, which MBH said would "preclude, as yet, any definitive conclusions" about climate before 1400.[30]
Contrary to a long-term cooling trend expected from orbital forcing, 20th century warming stood out from the whole period, with the 1990s "the warmest decade, and 1998 the warmest year, at moderately high levels of confidence." The time series line graph Figure 2(a) showed their reconstruction from AD 1000 to 1980 as a thin line, wavering around a thicker dark 40-year smoothed line. This curve followed a downward trend (shown as a thin dot-dashed line) from a Medieval Warm Period (about as warm as the 1950s) down to a cooler Little Ice Age before rising sharply in the 20th century. Thermometer data shown with a dotted line overlapped the reconstruction for a calibration period from 1902 to 1980, then continued sharply up to 1998. A shaded area showed uncertainties to two standard error limits, in medieval times rising almost as high as recent temperatures.[31][32][33] When Mann gave a talk about the study to the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, Jerry Mahlman nicknamed the graph the "hockey stick",[28] with the slow cooling trend the "stick", and the anomalous 20th century warming the "blade".[16]
Briffa and Tim Osborn critically examined MBH99 in a May 1999 detailed study of the uncertainties of various proxies. They raised questions later adopted by critics of Mann's work, including the point that bristlecone pines from the Western U.S. could have been affected by pollution such as rising CO2 levels as well as temperature. The temperature curve was supported by other studies, but most of these shared the limited well dated proxy evidence then available, and so few were truly independent. The uncertainties in earlier times rose as high as those in the reconstruction at 1980, but did not reach the temperatures of later thermometer data. They concluded that although the 20th century was almost certainly the warmest of the millennium, the amount of anthropogenic warming remains uncertain."[34]
With work progressing on the next IPCC report, Chris Folland told researchers on 22 September 1999 that a figure showing temperature changes over the millennium "is a clear favourite for the policy makers' summary". Two graphs competed: Jones et al. (1998) and MBH99. In November, Jones produced a simplified figure for the cover of the short annual World Meteorological Organization report, which lacks the status of the more important IPCC reports. Two fifty-year smoothed curves going back to 1000 were shown, from MBH99 and Jones et al. (1998), with a third curve to 1400 from Briffa's new paper, combined with modern temperature data bringing the lines up to 1999: the lack of a clarity about this change of data has been criticised as misleading. [35] Briffa's paper as published in the January 2000 issue of Quaternary Science Reviews showed the unusual warmth of the last century, but cautioned that the impact of human activities on tree growth made it subtly difficult to isolate a clear climate message.[36] In February Thomas J. Crowley and Thomas S. Lowery's paper said that peak Medieval warmth only occurred during two or three short periods of 20 to 30 years, with temperatures around 1950s levels, refuting claims that 20th century warming was not unusual.[37]
Reviewing twenty years of progress in palaeoclimatology, Jones noted the reconstructions by Jones (1998), MBH99, Briffa (2000) and Crowley (2000) showing good agreement using different methods, but cautioned that use of many of the same proxy series meant that they were not independent, and more work was needed.[38]
The Working Group 1 (WG1) section of the IPCC Third Assessment Report (TAR) included a section on multi-proxy synthesis of recent temperature change, which emphasised the contribution of earlier studies and the uncertainties highlighted in MBH99. It noted that Jones et al. (1998) using different data and methods as well as Crowley and Lowery (2000) gave support to the MBH conclusion that the 1990s were likely to have been the warmest decade, and 1998 the warmest year, of the past millennium in the Northern Hemisphere. It defined "likely" as "66-90% chance". Its Figure 2.21 showed smoothed curves from the MBH99, Jones et al. and Briffa reconstructions, together with modern thermometer data as a red line and the grey shaded 95% confidence range from MBH99. Above it, figure 2.20 was adapted from MBH99,[39] Figure 5 in WG1 Technical Summary B (as shown to the right) repeated this figure without the linear trend line declining from AD 1000 to 1850.[40] This iconic graph was featured prominently in the WG1 Summary for Policymakers under a graph of the instrumental temperature record for the past 140 years.[41] Versions of these graphs also featured less prominently in the short Synthesis Report Summary for Policymakers, which included a sentence stating that "The increase in surface temperature over the 20th century for the Northern Hemisphere is likely to have been greater than that for any other century in the last thousand years", and the Synthesis Report - Questions.[42]
A large poster of the IPCC illustration based on the MBH99 graph formed the backdrop when Sir John T. Houghton, Co-Chair of Working Group 1, presented the scientific basis report in an announcement shown on television, leading to wide publicity.[28]
A suggestion that the graph underestimated the Medieval Warm Period appeared in a March 2002 paper by Jan Esper et al.,[43] but in Mann's view this did not contradict MBH as it dealt only with extratropical land areas, and stopped before the late 20th century. Edward R. Cook, a co-author on the paper, agreed with Mann,[44] and later reconsidered the paper's conclusions.[45]
Lonnie Thompson published a paper on "Tropical Glacier and Ice Core Evidence of Climate Change" in January 2003, featuring Figure 7 showing graphs based on ice cores closely resembling a graph based on the MBH99 reconstruction combined with thermometer readings from Jones et al. 1999.[46]
Rather than displaying all of the long term temperature reconstructions, the opening figure of the Working Group 1 Summary for Policymakers in the IPCC Third Assessment Report highlighted an IPCC illustration based only on the MBH99 paper,[41] and a poster of the hockey stick graph was the backdrop when the report was announced on television. The graph was seen by mass media and the public as central to the IPCC case for global warming, which had actually been based on other unrelated evidence. Jerry Mahlman, who had coined the "hockey stick" nickname, described this emphasis on the graph as "a colossal mistake, just as it was a mistake for the climate-science-writing press to amplify it." He added that it was "not the smoking gun. That's the data we've had for the past 150 years, which is quite consistent with the expectation that the climate is continuing to warm."[28] From an expert viewpoint the graph was, like all newly published science, preliminary and uncertain, but it was widely used to publicise the issue of global warming.[31] The 1999 study had been a pioneering work in progress, and had emphasised the uncertainties, but publicity often played this down. Mann later said "The label was always a caricature and it became a stick to beat us with".[6]
Controversy over the graph now extended outside the scientific community, with accusations from political opponents of climate science.[6] As science historian Spencer Weart said, "The dedicated minority who denied that there was any global warming problem promptly attacked the calculations."[31] The graph was targeted by those opposing ratification of the Kyoto Protocol on global warming. As Mann said, "Advocates on both sides of the climate-change debate at various times have misrepresented the results for their own purposes". Environmental groups presented the graph flatteringly, and the caution about uncertainty in the original graph tended to be understated or removed: a "hockey stick" graph without error bars featured in a 2001 report by the U.S. National Assessment on Climate Change. Similar graphs were used by those disputing the findings with the claim that the graph was inaccurate. When a later Wall Street Journal editorial used a graph without error bars in this way, Gerald North described this as "very misleading, in fact downright dishonest". Funding was provided by the American Petroleum Institute for research critical of the graph.[28] A paper by Chris de Freitas published by the Canadian Society of Petroleum Geologists in June 2002 argued against the IPCC findings and the Kyoto Protocol, claiming that global warming posed no danger and CO2 was innocuous. A section disputing the "hockey stick" curve concluded it was merely a mathematical construct promoted by the IPCC to support the "notion" that recent temperatures were unprecedented.[47] Towards the end of 2002, the book Taken By Storm : the troubled science, policy, and politics of global warming by Christopher Essex and Ross McKitrick, published with assistance from the Fraser Institute,[48] included a chapter about the graph titled "T-Rex Plays Hockey".[49][50]
Iconic use of the IPCC graph came to symbolise conflict in which mainstream climate scientists were criticised, with some sceptics focussing on the hockey stick graph in the hope that they could damage the credence given to climate scientists.[51]
An early attempt to refute the hockey stick graph appeared in a joint paper by Willie Soon, who had already argued that climate change was primarily due to solar variation, and Sallie Baliunas who had contested whether ozone depletion was due to man-made chemicals.[52][53] The Soon and Baliunas literature review used data from previous papers to argue that the Medieval Warm Period had been warmer than the 20th century, and that recent warming was not unusual. They sent their paper to the editor Chris de Freitas, an opponent of action to curb carbon dioxide emissions who has been characterized by Fred Pearce as a "climate contrarian". Chris de Freitas approved the paper for publication in the relatively obscure journal Climate Research, where it appeared on 31 January 2003. In March Soon and Baliunas published an extended paper in Energy & Environment. Two scientists cited in the papers later said that their work was misrepresented.[54][55][56] The Climate Research paper was criticised by many other scientists, including several of the journal's editors.[57] On 8 July Eos featured a detailed rebuttal of both papers by 13 scientists including Mann and Jones, presenting strong evidence that Soon and Baliunas had used improper statistical methods. Responding to the controversy, the publisher of Climate Research upgraded Hans von Storch from editor to editor in chief as of 1 August 2003. After seeing a preprint of the Eos rebuttal, von Storch decided that the Soon and Baliunas paper was seriously flawed and should not have been published as it was. He proposed a new editorial system, and an editorial saying that the review process had failed.[54][58][59]
When the McCain-Lieberman bill proposing restrictions on greenhouse gases was being debated in the Senate on 28 July 2003, Senator James M. Inhofe made a two-hour speech in opposition. He cited a study by the Center for Energy and Economic Development and the Soon and Baliunas paper in supporting his conclusion: "With all of the hysteria, all of the fear, all of the phony science, could it be that man-made global warming is the greatest hoax ever perpetrated on the American people? It sure sounds like it."[57][60] Inhofe convened a hearing of the United States Senate Committee on Environment and Public Works held on 29 July 2003, examining work by the small group of researchers saying there was no evidence of significant human-caused global warming. Three scientists were invited, Mann giving testimony supporting the consensus position, opposed by long term skeptics Willie Soon and David Legates.[57][61] The Soon and Baliunas paper was discussed. Senator Jeffords read out an email in which von Storch stated his view "that the review of the Soon et al. paper failed to detect significant methodological flaws in the paper. The critique published in the Eos journal by Mann et al. is valid." In reply, Mann testified "I believe it is the mainstream view of just about every scientist in my field that I have talked to that there is little that is valid in that paper. They got just about everything wrong."[54][61] He later recalled that he "left that meeting having demonstrated what the mainstream views on climate science are."[62]
The publisher of Climate Research agreed that the flawed Soon and Baliunas paper should not have been published uncorrected, but von Storch's proposals to improve the editorial process were rejected, and von Storch with three other board members resigned. News of his resignation was discussed at the senate committee hearing.[54][61]
In October 2003, a paper by Stephen McIntyre and Ross McKitrick on "Corrections to the Mann et al. (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series" was published in Energy & Environment (MM03). At a November 2003 event at the George C. Marshall Institute, co-hosted by Myron Ebell of the Cooler Heads Coalition, McIntyre said that he had become interested in the paper in April 2002 when the Kyoto Protocol was a major political issue in Canada, and had contacted Mann for the data set but found problems in replicating the curves of the graph due to missing or wrong data. He first met McKitrick, for "lunch at the exact hour that Hurricane Isabel hit Toronto" (September 19, 2003). They prepared their corrections in a proxy data set using 1999 data, and using publicly disclosed methods produced a reconstruction which differed from MBH98 in showing high peaks of temperature in the 15th century. They were not saying that these temperatures had occurred, but that Mann's results were incorrect. When they published their paper, it attracted attention, with David Appell being the first reporter to take an interest. They said that after Appell's article was published with comments from Mann, they had followed links to Mann's FTP site and on October 29 copied data files which were subsequently deleted from the site.[63] [64] A November 2003 response from Mann, Bradley and Hughes objected to not having been given the opportunity to review or respond to the criticism before publication, and said that MM03 was flawed as it had deleted key proxy information, and used different procedures to MBH.[65] In 2007 the IPCC AR4 noted the MM03 claim that MBH98 could not be replicated, and reported 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."[66]
In a corrigendum published on 1 July 2004, Mann, Bradley and Hughes acknowledged that McIntyre and McKitrick had pointed out errors in proxy data that had been included as supplementary information to MBH98, and supplied a full corrected listing of the data. They included an archive of all the data used in MBH98, and expanded details of their methods. They stated that "None of these errors affect our previously published results."[67]
The statistical methods used in the MBH reconstruction were questioned in a 2004 paper by Hans von Storch with a team including Eduardo Zorita,[43][68] which said that the methodology used to average the data and the wide uncertainties might have hidden abrupt climate changes, possibly as large as the 20th century spike in measured temperatures.[32] Tim Osborn and Keith Briffa responded, highlighting this conclusion.[69] Zorita and von Storch later claimed this was a breakthrough in moving the question from "the reality of the blade of the hockey stick" to focus on "the real problems, namely the ‘wobbliness’ of the shaft of the hockey-stick, and the suppressing of valid scientific questions by gate keeping."[70] The von Storch et al. view that the graph was defective overall was later refuted by Wahl, Ritson and Ammann (2006),[52] who pointed to incorrect implementation of the reconstruction procedure.[71]
Later in 2004, Edward R. Cook, Jan Esper and Rosanne D'arrigo re-examined their 2002 paper, and now supported MBH. They concluded that "annual temperatures up to AD 2000 over extra-tropical NH land areas have probably exceeded by about 0.3 °C the warmest previous interval over the past 1162 years".[45][72] In December Mann and Gavin Schmidt launched the RealClimate website as "a resource where the public can go to see what actual scientists working in the field have to say about the latest issues."[62]
In a Senate speech on 4 January 2005, Inhofe repeated his assertion that "the threat of catastrophic global warming" was the "greatest hoax ever perpetrated on the American people". He singled out the hockey stick graph and Mann for criticism, accusing Mann of having "effectively erased the well-known phenomena of the Medieval Warming Period-when, by the way, it was warmer than it is today-and the Little Ice Age". He quoted von Storch as criticising the graph.[73] In a CBS News opinion piece, Chris Mooney said that Inhofe had extensively cited Michael Crichton's fictional thriller, State of Fear, mistakenly describing Crichton as a "scientist", and had misrepresented three scientists as disputing the "hockey stick" when they had been challenging a completely different paper which Mann had co-authored.[74]
A study by Anders Moberg et al. published on 10 February 2005 used a wavelet transform technique to reconstruct Northern Hemisphere temperatures over the last 2,000 years, combining low-resolution proxy data such as lake and ocean sediments for century-scale or longer changes, with tree ring proxies only used for annual to decadal resolution. They found there had been a peak of temperatures around AD 1000 to 1100 similar to those reached in the years before 1990,[75] and supported the basic conclusion of MBH99 by stating "We find no evidence for any earlier periods in the last two millennia with warmer conditions than the post-1990 period".[72][76]
In 2004 Stephen McIntyre blogged on his website climate2003.com about his efforts to get an extended analysis of the hockey stick into the journal Nature, but he was unsuccessful and it was not until 2005 that he and Ross McKitrick got their paper into Geophysical Research Letters.[77] In their renewed criticism of MBH98, McIntyre and McKitrick's 2005 paper (MMO5), published in Geophysical Research Letters reported a technical statistical error.[52][77] The authors wrote that the "Hockey Stick" shape was the result of an invalid principal component method, and that "the MBH98 method effectively selects only one species (bristlecone pine)" to produce the hockey-stick shape.[78] McIntyre and McKitrick said 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 (counting both upwards and downwards-pointing "blades") even if simulated red noise without any inherent trends was used as input.[79] The paper was nominated as a journal highlight by the American Geophysical Union, which publishes GRL.[80]
In an immediate public relations campaign, the Canadian National Post for 27 January carried a front page article alleging that "A pivotal global warming study central to the Kyoto Protocol contains serious flaws." The Bush administration had already decided to disregard the Kyoto Protocol which was to come into effect later that month, and this enabled them to say that the protocol was discredited.[81] On 14 February a lead article in the Wall Street Journal said that McIntyre's new paper was "circulating inside energy companies and government agencies. Canada's environment ministry has ordered a review", and though McIntyre did not take strong position on whether or not fossil-fuel use was causing global warming, "He just says he has found a flaw in a main leg supporting the global-warming consensus, the consensus that led to an international initiative taking effect this week: Kyoto."[82]
Others later found the issues raised by McIntyre and McKitrick were minor and did not affect the main conclusions of MBH.[52] Technical issues were discussed in RealClimate on 18 February in a blog entry by Gavin Schmidt and Caspar Amman,[83] and in a BBC News interview Schmidt said that by using a different convention but not altering subsequent steps in the analysis accordingly, McIntyre and McKitrick had removed significant data which would have given the same result as the MBH papers.[84]
In a presentation to the Asia-Pacific Economic Cooperation Study Centre on 4 April 2005, McKitrick said that the prominence given to the hockey stick graph in the TAR "was deliberate editorial sleight-of-hand" to trick readers into thinking it was more important than the graph of satellite-measured tropospheric data by Christy and Spencer. He said that "The Government of Canada subsequently sent the hockey stick (but not the satellite data) to schools across the country, and its famous conclusion about the 1990s being the warmest decade of the millennium was the opening line of a pamphlet sent to every household in Canada to promote the Kyoto Protocol." Having outlined the arguments about the MBH methodology as detailed in the McIntyre and McKitrick papers, he proposed that an "audit report" by non-climatologists should assess IPCC reports and be published by the IPCC, and that as part of the IPCC process a "counter-weight panel" should critique the Working Group 1 science report on both economic and scientific aspects.[85]
In May the University Corporation for Atmospheric Research advised media about a detailed analysis by Eugene Wahl and Caspar Ammann, first presented at the American Geophysical Union’s December 2004 meeting in San Francisco, which used their own code to replicate the MBH results, and found the MBH method to be robust even with modifications. Their work contradicted the claims by McIntyre and McKitrick about high 15th century global temperatures and allegations of methodological bias towards a hockey stick outcomes, and they concluded that the criticisms of the hockey stick graph were groundless.[86]
The increasing politicisation of the issue was demonstrated when,[87] on 23 June 2005, Rep. Joe Barton, chairman of the House Committee on Energy and Commerce wrote joint letters with Ed Whitfield, Chairman of the Subcommittee on Oversight and Investigations, referring to issues raised by the 14 February 2005 article in the Wall Street Journal and demanding full records on climate research. The letters were sent to the IPCC Chairman Rajendra Pachauri, National Science Foundation Director Arden Bement, and to the three scientists Mann, Bradley and Hughes. [88] [89] The letters told the scientist to provide not just data and methods, but also personal information about their finances and careers, information about grants provided to the institutions they had worked for, and the exact computer codes used to generate their results.[90]
Sherwood Boehlert, chairman of the House Science Committee, told his fellow Republican Joe Barton it was a "misguided and illegitimate investigation" into something that was the under the jurisdiction of the Science Committee, and "My primary concern about your investigation is that its purpose seems to be to intimidate scientists rather than to learn from them, and to substitute congressional political review for scientific review." Barton's committee spokesman sent a sarcastic response to this and to Democrat Henry A. Waxman's letter asking Barton to withdraw the letters and saying he had "failed to hold a single hearing on the subject of global warming" during eleven years as chairman, and had "vociferously opposed all legislative efforts in the Committee to address global warming .... These letters do not appear to be a serious attempt to understand the science of global warming. Some might interpret them as a transparent effort to bully and harass climate change experts who have reached conclusions with which you disagree." The U.S. National Academy of Sciences (NAS) president Ralph J. Cicerone wrote to Barton that "A congressional investigation, based on the authority of the House Commerce Committee, is probably not the best way to resolve a scientific issue, and a focus on individual scientists can be intimidating", and proposed that the NAS should appoint an independent panel to investigate. Barton dismissed this offer.[91][92]
On 15 July, Mann wrote giving his detailed response to Barton and Whitfield. He emphasised that the full data and necessary methods information was already publicly available in full accordance with National Science Foundation (NSF) requirements, so that other scientists had been able to reproduce their work. NSF policy was that computer codes "are considered the intellectual property of researchers and are not subject to disclosure", as the NSF had advised McIntyre and McKitrick in 2003, but notwithstanding these property rights, the program used to generate the original MBH98 temperature reconstructions had been made available at the Mann et al. public ftp site. [93] Following receipt of responses to the letters, Barton and Whitfield had their committee staff contact statistician Edward J. Wegman for advice on the validity of McIntyre and McKitrick's complaints, and Wegman formed an ad-hoc committee consisting of himself, David W. Scott and Yasmin H. Said.[94][95][96]
Many scientists protested, with 20 prominent climatologists writing to Barton questioning his approach.[91][97] Alan I. Leshner wrote to him on behalf of the American Association for the Advancement of Science expressing deep concern about the letters, which gave "the impression of a search for some basis on which to discredit these particular scientists and findings, rather than a search for understanding," He stated that MBH had given out their full data and descriptions of methods, and were not the only evidence in the IPCC TAR that recent temperatures were likely the warmest in 1,000 years; "a variety of independent lines of evidence, summarized in a number of peer-reviewed publications, were cited in support". Thomas Crowley argued that the aim was intimidation of climate researchers in general, and Bradley thought the letters were intended to damage confidence in the IPCC during preparation of its next report.[98][99] A Washington Post editorial on 23 July which described the investigation as harassment quoted Bradley as saying it was "intrusive, far-reaching and intimidating", and Alan I. Leshner of the AAAS describing it as unprecedented in in the 22 years he had been a government scientist; he thought it could "have a chilling effect on the willingness of people to work in areas that are politically relevant."[90]
Congressman Boehlert said the investigation was as "at best foolhardy" with the tone of the letters showing the committee's "inexperience" in relation to science. Barton was given support by global warming sceptic Myron Ebell of the Competitive Enterprise Institute, who said "We've always wanted to get the science on trial ... we would like to figure out a way to get this into a court of law", and "this could work".[98] In in his Junk Science column on Fox News, Steven Milloy said Barton's inquiry was reasonable.[100]
In comments on MM05 made in October, Peter Huybers showed that McIntyre and McKitrick had omitted a critical step in calculating significance levels, and MBH98 had shown it correctly. [101] In their comment, Hans von Storch and Eduardo Zorita examined McIntyre and McKitrick's claim that normalising data prior to principal component analysis by centering in relation to the calibration period of 1902–1980, instead of the whole period, would nearly always produce hockey stick shaped leading principal components. They found that it caused only very minor deviations which would not have a significant impact on the result.[102] In the same month a paper in the Journal of Climate, co-authored by Scott Rutherford, Mann, Osborn, Briffa, Jones, Bradley and Hughes, examined the sensitivity of proxy based reconstruction to method, and found that a wide range of alternative statistical approaches gave nearly indistinguishable results. In particular, omitting principal component analysis made no significant difference.[72][103]
In November 2005, Science Committee chair Sherwood Boehlert requested the National Academy of Science to arrange a review of the matter.[104] The National Research Council set up a special committee to investigate.
In February 2006 two more reconstructions were published, using different methodologies and supporting the main conclusions of MBH. Rosanne D'Arrigo, Rob Wilson and Gordon Jacoby suggested that medieval temperatures had been almost 0.7°C cooler than the late 20th century but less homogenous,[105] Osborn and Briffa found the spatial extent of recent warmth more significant than that during the medieval warm period.[106][107]
Al Gore's film An Inconvenient Truth, which premiered in May 2006, included a section on Lonnie Thompson's ice core measurements showing a correlation between CO2 levels and temperature going back 650,000 years. Gore showed this "thermometer" graph going back a thousand years for a "couple of reasons", the first being that "so called skeptics" will sometimes say "Oh, this whole thing is cyclical phenomenon. There was a medieval warming period after all", but as shown on the graph, "compared to what is going on now, there is just no comparison". [108] The book issued at the same time includes a similar statement beside a graph like Figure 7 from Thompson's 2004 paper, and adds that the skeptics "launched a fierce attack against another measurement of the 1,000 year correlation between CO2 and temperature known as 'the hockey stick,' a graphic image representing the research of Michael Mann and his colleagues. But in fact, scientists have confirmed the same basic conclusions in multiple ways–with Thompson's ice core record as one of the most definitive."[46][109]
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, chaired by Gerald North, 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.[110]
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:[111]
A group-authored post on RealClimate, of which Mann is one of the contributors, said "the panel has found reason to support the key mainstream findings of past research, including points that we have highlighted previously."[112] Similarly, Roger A. Pielke, Jr. said that the National Research Council publication constituted a "near-complete vindication for the work of Mann et al.";[113] Nature reported it as "Academy affirms hockey-stick graph. But it criticizes the way the controversial climate result was used."[114]
North was reported as saying at the press conference announcing the report that the panel had "concluded that systematic uncertainties in climate records from before 1600 were not communicated as clearly as they could have been". In a letter to Nature on August 10, 2006, Bradley, Hughes and Mann pointed out that the original title of their 1999 paper (MBH99) was "Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations", and it had concluded that "more widespread high-resolution data are needed before more confident conclusions can be reached". They said that "the uncertainties were the point of the article", and that it was "hard to imagine how much more explicit" they could have been about the uncertainties surrounding their work. They suggested that "poor communication by others" had led to the "subsequent confusion".[33][115]
According to Hans von Storch, Eduardo Zorita[116] and Jesus Rouco,[117] reviewing the NAS report on McIntyre's blog Climate Audit, "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."[118] It was noted by their critics, however, that no such statement, explicit or implicit, is present on the two pages cited;[119] 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."[120] 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.[121][122]
Richard Muller said in his December 2006 book Physics for future presidents that "the strongest statement that could be made was that the present years were the warmest in the last 400 years, not 1,000 as Mann had said" and that "In the end, there was nothing new left in Mann's papers that the National Academy supported, other than the idea that using principal component analysis was, in principle, a good one."[123]
At the American Statistical Association 2006 Joint Statistical Meetings, John Michael Wallace reported that the NRC had found the Mann et al. claim that the last two decades were the warmest of the last 1000 years "entirely plausible", supported by a wide range of evidence. They had reported this cautiously, as "plausible" meaning 2:1 odds in favor.[124]
In an editorial dated 14 July 2006, the Wall Street Journal announced that a report commissioned by the House Committee on Energy and Commerce was due to be released that day. It gave a preview of the conclusions of the report, which had been prepared by three statisticians: Edward Wegman, David W. Scott and Yasmin H. Said.[88] The committee chairman U.S. Rep. Joe Barton issued a press release giving a summary of the report's findings, with quotations from the report. The report primarily focused on the statistical analysis used in the MBH paper, but also considered the personal and professional relationships between Mann et al. and other members of the paleoclimate community.[94][96] It became known as the "Wegman Report", and its findings were discussed at hearings of the Energy Subcommittee on Oversight and Investigations under its chairman U.S. Rep. Ed Whitfield.[95]
Wegman's institution, George Mason University, confirmed in October 2010 that they were investigating misconduct charges, following a March 2010 formal complaint by Raymond S. Bradley alleging plagiarism and fabrications in the Wegman Report. A 250-page study by computer scientist John Mashey, posted on the "Deep Climate" website, claims that 35 of the 91 pages in the Wegman Report were plagiarized, and "often injected with errors, bias and changes of meaning." Wegman responded that he was "very well aware of the report", but at the university's request would not comment further until all issues were settled.[137][138] Reviews by outside experts contacted by USA Today found the plagiarism obvious and inappropriate, with social network analysis partly copied from Wikipedia. Wegman said there was "speculation and conspiracy theory" in John Mashey's analysis, and said that "[t]hese attacks are unprecedented in my 42 years as an academic and scholar." He stated that the Wegman Report never "intended to take intellectual credit for any aspect of paleoclimate reconstruction science or for any original research aspect of social network analysis."[138] The investigation was still at the preliminary "inquiry" stage rather than being a full investigation, according to a 26 May 2011 clarification from a George Mason University spokesman.[139]
Wegman and Said authored a paper as an extension of the part of the Wegman Report which used social network analysis to suggest that there had been inappropriate close collaboration between some climate scientists. This paper, which omitted the names of the scientists, was published in the journal Computational Statistics and Data Analysis in 2008. After computer scientist Ted Kirkpatrick of the Simon Fraser University read the "Deep Climate" website allegations of plagiarism, he made a formal complaint to the journal.[129] On 16 March 2011, Wegman sent an email to the journal saying that a student "had basically copied and pasted" work by other authors into the Wegman Report, and this text had been used in the journal paper without acknowledgement. He said that "We would never knowingly publish plagiarized material". In May 2011 the journal's editor, Stanley Azen of the University of Southern California, announced that the journal was retracting the paper, because it used portions of other authors' writings without sufficient attribution.[140] John Dahlberg of the United States Office of Research Integrity indicated that plagiarism could result in sanctions. A George Mason University spokesman declined to comment and said it was a "personnel matter".[129]
The manuscript of the paper had been submitted on 8 July 2007 and accepted for publication on 14 July 2007. The student issued a statement that she had been "Dr. Wegman's graduate student when I provided him with the overview of social network analysis, at his request. My draft overview was later incorporated by Dr. Wegman and his coauthors into the 2006 report. I was not an author of the report." She had met with a GMU misconduct committee, and said that "My academic integrity is not being questioned."[130]
A Nature editorial commented on the implication that the plagiarised material in the retracted paper was likely to also be present in the earlier "infamous" Wegman Report, including allegations against Mann and his co-authors which had frequently been cited by climate-change sceptics. The George Mason University's policies indicated that its initial inquiry should have been completed within 12 weeks of the original complaint, and although 14 months had passed without this being resolved, there were loopholes for extensions. It said that the university should "take the initiative to move investigations along as speedily as possible while allowing time for due process. Once an investigation is complete, the institution should be as transparent as it can about what happened", especially where public funds were involved.[141]
Paleoclimate findings by the IPCC before and after the Hockey Stick Controversy:
Before: 2001 (page 2)[142]
After: SPM statement from 2007 (page 10)[143]
In May 2007, Hans von Storch reviewed the changes in thought caused by the hockey stick controversy writing:
McIntyre was critical of this Nature blog entry because von Storch did not acknowledge the role of McIntyre and McKitrick:
However von Storch replied[146] that:
In a paper published by PNAS on 9 September 2008, Mann and colleagues produced updated reconstructions of Earth surface temperature for the past two millennia.[147] This reconstruction used a more diverse dataset that was significantly larger than the original tree-ring study, at more than 1,200 proxy records. They used two complementary methods, both of which showed a similar "hockey stick" graph with recent increases in northern hemisphere surface temperature are anomalous relative to at least the past 1300 years. Mann said, "Ten years ago, the availability of data became quite sparse by the time you got back to 1,000 AD, and what we had then was weighted towards tree-ring data; but now you can go back 1,300 years without using tree-ring data at all and still get a verifiable conclusion."[148] In a PNAS response, McIntyre and McKitrick said that they perceived a number of problems, including that Mann et al used some data with the axes upside down.[149] Mann et al. replied that McIntyre and McKitrick "raise no valid issues regarding our paper" and the "claim that 'upside down' data were used is bizarre", as the methods "are insensitive to the sign of predictors." They also said that excluding the contentious datasets has little effect on the result.[150]
A study of the changing climate of the Arctic over the last 2,000 years, by an international consortium led by Darrell Kaufman of Northern Arizona University, was published on 4 September 2009. They examined sediment core records from 14 Arctic lakes, supported by tree ring and ice core records. Their findings showed a long term cooling trend consistent with cycles in the Earth's orbit which would be expected to continue for a further 4,000 years but had been reversed in the 20th century by a sudden rise attributed to greenhouse gas emissions. The decline had continued through the Medieval period and the Little Ice Age. The most recent decade, 1999–2008, was the warmest of the period, and four of the five warmest decades occurred between 1950 and 2000. Scientific American described the graph as largely replicating "the so-called 'hockey stick,' a previous reconstruction".[151]
Further support for the "hockey stick" graph came from a new method of analysis developed by Martin Tingley and Peter Huybers of Harvard University, which produced the same basic shape, albeit with more variability in the past, and found the 1990s to have been the warmest decade in the 600 year period the study covered.[152]