The North Report was a 2006 report evaluating reconstructions of the temperature record of the past two millennia , providing an overview of the state of the science and the implications for understanding of global warming . It was produced by a National Research Council committee, chaired by Gerald North , at the request of Representative Sherwood Boehlert as chairman of the U.S. House of Representatives Committee on Science .
133-406: These reconstructions had been dubbed " hockey stick graphs " after the 1999 reconstruction by Mann , Bradley and Hughes (MBH99), which used the methodology of their 1998 reconstruction covering 600 years (MBH98). A graph based on MBH99 was featured prominently in the 2001 IPCC Third Assessment Report (TAR), and became a focus of the global warming controversy over the 1997 Kyoto Protocol . It
266-549: A Medieval Warm Period from around 900 to 1300, followed by Little Ice Age . This was the basis of a "schematic diagram" featured in the IPCC First Assessment Report of 1990 beside cautions that the medieval warming might not have been global. The use of indicators to get quantitative estimates of the temperature record of past centuries was developed, and by the late 1990s a number of competing teams of climatologists found indications that recent warming
399-598: A principal component analysis step to summarise these proxy networks, but from 2001 Mann stopped using this method and introduced a multivariate Climate Field Reconstruction (CFR) technique based on the RegEM method which did not require this PCA step. In May 2002 Mann and Scott Rutherford published a paper on testing methods of climate reconstruction which discussed this technique. By adding artificial noise to actual temperature records or to model simulations they produced synthetic datasets which they called "pseudoproxies". When
532-501: A 1991 study and a 1991 book showing methodology and examples of how to produces maps showing climate developments in North America over time. These methods had been used for regional reconstructions of temperatures, and other aspects such as rainfall. As part of his PhD research, Michael E. Mann worked with seismologist Jeffrey Park on developing statistical techniques to find long term oscillations of natural variability in
665-840: A co-author on the paper, had confirmed agreement with these points, and a later paper by Cook, Esper and D'Arrigo reconsidered the earlier paper's conclusions along these lines. 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. In March 2001 Tapio Schneider published his regularized expectation–maximization (RegEM) technique for analysis of incomplete climate data. The original MBH98 and MBH99 papers avoided undue representation of large numbers of tree ring proxies by using
798-455: A decadal basis rather than showing individual years, and produced a single time series so did not show a spatial pattern of relative temperatures for different regions. The IPCC Second Assessment Report (SAR) of 1996 featured Figure 3.20 showing the Bradley & Jones 1993 decadal summer temperature reconstruction for the northern hemisphere, overlaid with a 50-year smoothed curve and with
931-469: A focus of dispute for those opposed to the strengthening scientific consensus that late 20th century warmth was exceptional. In 2003, as lobbying over the 1997 Kyoto Protocol intensified, a paper claiming greater medieval warmth was quickly dismissed by scientists in the Soon and Baliunas controversy . Later in 2003, Stephen McIntyre and Ross McKitrick published McIntyre & McKitrick 2003b disputing
1064-553: 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". The first quantitative reconstruction of Northern Hemisphere (NH) annual mean temperatures was published in 1979 by Brian Groveman and Helmut Landsberg. They used "a short-cut method" based on their earlier paper which showed that 9 instrumental stations could adequately represent an extensive gridded instrumental series, and reconstructed temperatures from 1579 to 1880 on
1197-544: A generally consistent picture of temperature trends during the preceding millennium", including the Medieval Warm Period and the Little Ice Age , "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 concluded "with a high level of confidence that global mean surface temperature was higher during
1330-420: A high-resolution global reconstruction. To relate this data to measured temperatures, they used principal component analysis (PCA) to find the leading patterns, or principal components, of instrumental temperature records during the calibration period from 1902 to 1980. Their method was based on separate multiple regressions between each proxy record (or summary) and all of the leading principal components of
1463-493: A rigorous review process involving 15 independent experts. The report provided a summary and an overview, followed by 11 technical chapters covering the instrumental and proxy records, statistical procedures, paleoclimate models, and the synthesis of large scale temperature reconstructions with an assessment of the "strengths, limitations, and prospects for improvement" in techniques used. The NRC committee's report (the North report)
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#17327874728191596-404: A separate curve plotting instrumental thermometer data from the 1850s onwards. It stated that in this record, warming since the late 19th century was unprecedented. The section proposed that "The data from the last 1000 years are the most useful for determining the scales of natural climate variability". Recent studies including the 1994 reconstruction by Hughes and Diaz questioned how widespread
1729-431: 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: in 2010 the lack of a clarity about this change of data
1862-482: A statistician considered all the choices of data processing and methods to have been "quite reasonable" in a "first of its kind study". He said "I would not have been embarrassed by that work at the time if I'd been involved in it". In response to a question from Edward Wegman on the MBH use of principal components analysis, Bloomfield said this had been reviewed by the committee along with other statistical issues, and "while
1995-548: A study of 1,000 years of tree ring data from Tasmania which, like similar studies, did not allow for possible overestimate of warming due to increased CO 2 levels having a fertilisation effect on tree growth. It noted the suggestion of Bradley et al. 1991 that instrumental records in specific areas could be combined with paleoclimate data for increased detail back to the 18th century. Archives of climate proxies were developed: in 1993 Raymond S. Bradley and Phil Jones composited historical records, tree-rings and ice cores for
2128-418: A temperature drop of almost 0.5 °C during the Little Ice Age , and increased solar output might explain the rise in early 20th century temperatures. A reconstruction of Arctic temperatures over four centuries by Overpeck et al. 1997 reached similar conclusions, but both these studies came up against the limitations of the climate reconstructions at that time which only resolved temperature fluctuations on
2261-648: Is Distinguished Professor and Holder of the Harold J. Haynes Endowed Chair in Geosciences at Texas A&M University , and previous Head of the Department of Atmospheric Sciences. His interests include climate change using simplified climate models. North was born in Sweetwater, Tennessee to Sanford and Marjorie Hill North. He grew up in nearby Knoxville and received his Bachelor's Degree in physics from
2394-406: Is a certain amount of skill. We can actually say something, although there are large uncertainties." In considering the 1998 Jones et al. reconstruction which went back a thousand years, Mann, Bradley and Hughes reviewed their own research and reexamined 24 proxy records which extended back before 1400. Mann carried out a series of statistical sensitivity tests , removing each proxy in turn to see
2527-503: Is broken. Today's report refutes Mann's prior assertions that there was no Medieval Warm Period or Little Ice Age." Al Gore commented that Inhofe and other deniers "will seize on anything to say up is down and black is white." 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." Similarly, Roger A. Pielke Jr. said that
2660-542: Is quite a bit of work to be done in reducing these uncertainties." Climatologist Tom Wigley welcomed the progress made in the study, 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 told the New York Times he was doubtful about adding the 150-year thermometer record to extend
2793-535: Is unprecedented for at least the last 400 years and potentially the last several millennia." In an article titled "Science Panel Backs Study on Warming Climate", Andrew Revkin of the New York Times wrote that "A controversial paper asserting that recent warming in the Northern Hemisphere was probably unrivaled for 1,000 years was endorsed today, with a few reservations, by a panel convened by
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#17327874728192926-537: The IPCC Third Assessment Report (TAR) included a subsection on multi-proxy synthesis of recent temperature change. This noted five earlier large-scale palaeoclimate reconstructions, then discussed the Mann, Bradley & Hughes 1998 reconstruction going back to 1400 AD and its extension back to 1000 AD in Mann, Bradley & Hughes 1999 (MBH99), while emphasising the substantial uncertainties in
3059-508: The National Oceanic and Atmospheric Administration 's Geophysical Fluid Dynamics Laboratory , Jerry Mahlman nicknamed the graph the "hockey stick", with the slow cooling trend the "stick", and the anomalous 20th century warming the "blade". 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
3192-536: The Subcommittee on Oversight and Investigations , wrote joint letters referring to issues raised by the Wall Street Journal article, and demanding that Mann, Bradley and Hughes provide full records on their data and methods, finances and careers, information about grants provided to the institutions they had worked for, and the exact computer codes used to generate their results. Boehlert said this
3325-696: The Synthesis Report - Questions . The Working Group 1 scientific basis report was agreed unanimously by all member government representatives in January 2001 at a meeting held in Shanghai , China. A large poster of the IPCC illustration based on the MBH99 graph formed the backdrop when Sir John T. Houghton , as co-chair of the working group, presented the report in an announcement shown on television, leading to wide publicity. The Huang, Pollack & Shen 2000 borehole temperature reconstruction covering
3458-623: The University of Tennessee . In 1966, he earned a Ph.D. in Physics from the University of Wisconsin–Madison . After a two year post-doc at the University of Pennsylvania , he obtained a tenure track position at the University of Missouri-St. Louis leading to full professor. He spent 1974–75 as a Senior Visiting Scientist at the National Center for Atmospheric Research . He moved to NASA Goddard Space Flight Center in 1978 where he
3591-535: The instrumental temperature record of global surface temperatures over the last 140 years; Mann & Park 1993 showed patterns relating to the El Niño–Southern Oscillation , and Mann & Park 1994 found what was later termed the Atlantic multidecadal oscillation . They then teamed up with Raymond S. Bradley to use these techniques on the dataset from his Bradley & Jones 1993 study with
3724-535: The polar regions and the tropics , they used principal component analysis (PCA) to produce PC summaries representing these large datasets, and then treated each summary as a proxy record in their CFR analysis. Networks represented in this way included the North American tree ring network (NOAMER) and Eurasia . The primary aim of CFR methods was to provide the spatially resolved reconstructions essential for coherent geophysical understanding of how parts of
3857-403: The "Composite Plus Scaling" (CPS) method which was subsequently used by most large-scale climate reconstructions of hemispheric or global average temperatures. In this method, also known as "Composite Plus Scale", selected climate proxy records were standardized before being averaged (composited), and then centred and scaled to provide a quantitative estimate of the target temperature series for
3990-401: The "Medieval Climatic Optimum" from the "late tenth to early thirteenth centuries (about AD 950-1250)", followed by a cooler period of the Little Ice Age which ended only in the middle to late nineteenth century. The report discussed the difficulties with proxy data, "mainly pollen remains, lake varves and ocean sediments, insect and animal remains, glacier termini" but considered tree ring data
4123-417: The "first systematic, statistically based synthesis of multiple climate proxies", and noted that the MBH reconstructions "were the first to include explicit statistical error bars". The report's overview section said that, despite the wide error bars, the hockey stick graph "was misinterpreted by some as indicating the existence of one 'definitive' reconstruction with small century-to-century variability prior to
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4256-411: 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. When Mann gave a talk about the study to
4389-454: The 1960s that accurate use of tree rings as climate proxies for reconstructions was pioneered by Harold C. Fritts . 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. In 1972 he became
4522-524: The 1988 formation of the Intergovernmental Panel on Climate Change to produce reports subject to detailed approval by government delegates. The IPCC First Assessment Report in 1990 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)",
4655-421: The 1990s was the warmest decade and 1998 the warmest year" in the past 1,000 years. 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
4788-461: 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
4921-463: The 19th century physicists John Tyndall and Svante Arrhenius who found the greenhouse gas effect of carbon dioxide (CO 2 ) in the atmosphere to explain how past ice ages had ended. From 1919 to 1923, Alfred Wegener did pioneering work on reconstructing the climate of past eras in collaboration with Milutin Milanković , publishing Die Klimate der geologischen Vorzeit ("The Climates of
5054-447: The 20th century. After this around a decade elapsed before Gordon Jacoby and Rosanne D'Arrigo produced the next quantitative NH reconstruction, published in 1989. This was the first based entirely on non-instrumental records, and used tree rings. They reconstructed northern hemisphere annual temperatures since 1671 on the basis of boreal North American tree ring data from 11 distinct regions. From this, they concluded that recent warming
5187-513: The Asian monsoon region, the El Niño–Southern Oscillation region and the Atlantic region. Areas where more data was needed were to be identified, and there was a need for improved data exchange with computer-based archiving and translation to give researchers access to worldwide paleoclimate information. The IPCC supplementary report, 1992 , reviewed progress on various proxies. These included
5320-466: The Geological Past") together with Wladimir Köppen, in 1924. In the 1930s Guy Stewart Callendar compiled temperature records to look for changes. Wilmot H. Bradley showed that annual varves in lake beds showed climate cycles, and A. E. Douglass found that tree rings could track past climatic changes but these were thought to only show random variations in the local region. It was only in
5453-472: The MBH98 methodology to extend their study back to 1000. A version of the MBH99 graph was featured prominently in the 2001 IPCC Third Assessment Report (TAR), which also drew on Jones et al. 1998 and three other reconstructions to support the conclusion that, in the Northern Hemisphere, the 1990s was likely to have been the warmest decade and 1998 the warmest year during the past 1,000 years. The graph became
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5586-409: The MBH99 study had attracted considerable attention because of its conclusion that recent decades were the warmest in the northern hemisphere in 1000 years, and "Controversy arose because many people interpreted this result as definitive evidence of anthropogenic causes of recent climate change, while others criticized the methodologies and data that were used." Chapter 11 of the report described MBH98 as
5719-619: The McIntyre and McKitrick methodology. Political disputes led to the formation of a panel of scientists convened by the United States National Research Council , their North Report in 2006 supported Mann's findings with some qualifications, including agreeing that there were some statistical failings but these had little effect on the result. More than two dozen reconstructions , using various statistical methods and combinations of proxy records, support
5852-471: The Medieval Warm Period had been at any one time, thus it was not possible "to conclude that global temperatures in the Medieval Warm Period were comparable to the warm decades of the late 20th century." The SAR 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
5985-579: The National Academies NRC report had discredited Mann's theory and shown the "hockey stick" graph to be incorrect. This was disputed by John Holdren , the president's science adviser, who said that although the panel had found minor issues with Mann's methods, they had confirmed his results. When the question was subsequent put to the NRC panel chairman Gerald North , he agreed with Holdren and said "The conclusions that we came to were essentially
6118-567: The National Research Council publication constituted a "near-complete vindication for the work of Mann et al. "; McIntyre's blog Climate Audit published a review of the report by Hans von Storch , Eduardo Zorita and Jesus Rouco , who said that it supported their view that the MBH methodology was questionable. On the day of publication, the Associated Press highlighted the panel's finding that "recent warmth
6251-430: The Northern Hemisphere from 1400 up to the 1970s to produce a decadal reconstruction. Like later reconstructions including the MBH "hockey stick" studies, the Bradley & Jones 1993 reconstruction indicated a slow cooling trend followed by an exceptional temperature rise in the 20th century. Their study also used the modern instrumental temperature record to evaluate how well the regions covered by proxies represented
6384-529: The aim of finding long term oscillations of natural variability in global climate. The resulting reconstruction went back to 1400, and was published in November as Mann, Park & Bradley 1995 . They were able to detect that the multiple proxies were varying in a coherent oscillatory way, indicating both the multidecadal pattern in the North Atlantic and a longer term oscillation of roughly 250 years in
6517-468: The aspect that got the most attention. Their original draft ended in 1980 as most reconstructions only went that far, but an anonymous peer reviewer of the paper suggested that the curve of instrumental temperature records should be shown up to the present to include the considerable warming that had taken place between 1980 and 1998. The Mann, Bradley & Hughes 1998 (MBH98) multiproxy study on "Global-scale temperature patterns and climate forcing over
6650-407: The available long term proxy climate series, "and if the new multivariate method of relating these series to the instrumental data is as good as the paper claims, it should be statistically reliable." He discussed some of the difficulties, and emphasised that "Each paleoclimatic discipline has to come to terms with its own limitations and must unreservedly admit to problems, warts and all." The study
6783-479: The back of an envelope", a "rather dodgy bit of hand-waving". In Bradley 1991 , a working group of climatologists including Raymond S. Bradley , Malcolm K. Hughes , Jean Jouzel , Wibjörn Karlén , Jonathan Overpeck and Tom Wigley proposed a project to improve understanding of natural climatic variations over the last two thousand years so that their effect could be allowed for when evaluating human contributions to climate change. Climate proxy temperature data
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#17327874728196916-463: The basis of their compilation of 20 time-series. These records were largely instrumental but also included some proxy records including two tree-ring series. Their method used nested multiple regression to allow for records covering different periods, and produced measures of uncertainty. The reconstruction showed a cool period extending beyond the Maunder Minimum , and warmer temperatures in
7049-469: The borehole reconstruction published by Pollack, Huang and Shen gave independent support to the conclusion that 20th century warmth was exceptional for the past 500 years. Jones, Keith Briffa , Tim P. Barnett and Simon Tett had independently produced a "Composite Plus Scale" (CPS) reconstruction extending back for a thousand years, comparing tree ring, coral layer, and glacial proxy records, but not specifically estimating uncertainties. Jones et al. 1998
7182-576: The broad consensus shown in the original 1998 hockey-stick graph, with variations in how flat the pre-20th century "shaft" appears. The 2007 IPCC Fourth Assessment Report cited 14 reconstructions, 10 of which covered 1,000 years or longer, to support its strengthened conclusion that it was likely that Northern Hemisphere temperatures during the 20th century were the highest in at least the past 1,300 years. Further reconstructions, including Mann et al. 2008 and PAGES 2k Consortium 2013 , have supported these general conclusions. Paleoclimatology influenced
7315-718: The case for a thousand or more years". Tim Barnett of the Scripps Institution of Oceanography was working towards the next IPCC assessment with Phil 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". A divergence problem affecting some tree ring proxies after 1960 had been identified in Alaska by Taubes 1995 and Jacoby & d'Arrigo 1995 . Tree ring specialist Keith Briffa 's February 1998 study showed that this problem
7448-432: 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. A University of Massachusetts Amherst news release dated 3 March 1999 announced publication in the 15 March issue of Geophysical Research Letters , "strongly suggesting that
7581-466: The climate of the region or hemisphere over time. This method was implemented in various ways, including different selection processes for the proxy records, and averaging could be unweighted, or could be weighted in relation to an assessment of reliability or of area represented. There were also different ways of finding the scaling coefficient used to scale the proxy records to the instrumental temperature record. John A. Eddy had earlier tried to relate
7714-430: The climate system varied and responded to radiative forcing , so hemispheric averages were a secondary product. The CFR method could also be used to reconstruct Northern Hemisphere mean temperatures, and the results closely resembled the earlier CPS reconstructions including Bradley & Jones 1993 . Mann describes this as the least scientifically interesting thing they could do with the rich spatial patterns, but also
7847-405: 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", though there were substantial uncertainties before about 1600. It added that "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
7980-402: The committee's view of increased uncertainties. They called for further research into methods and a search for more proxies for earlier periods. At the press conference the three NRC panellists said they found no evidence supporting the allegations of inappropriate behaviour such as data manipulation, or "anything other than an honest attempt to construct a data analysis procedure". Bloomfield as
8113-435: The conclusion that climatic warming is occurring in response to human activities, and they are not the primary evidence." At the press conference, North said of the MBH papers that "we do roughly agree with the substance of their findings. There is a small disagreement over exactly how sure we are." All three from the NRC committee panel said it was probable, though not certain, that current warming exceeded any previous peak in
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#17327874728198246-641: The conclusion 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. An analysis by Crowley published in July 2000 compared simulations from an energy balance climate model with reconstructed mean annual temperatures from MBH99 and Crowley & Lowery (2000). While earlier reconstructed temperature variations were consistent with volcanic and solar irradiation changes plus residual variability, very large 20th-century warming closely agreed with
8379-480: The controversy, the publisher of Climate Research upgraded Hans von Storch from editor to editor in chief, but 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 though the publisher of Climate Research agreed that the paper should not have been published uncorrected, he rejected von Storch's proposals to improve
8512-525: The data used in MBH98 paper. In 2004 Hans von Storch published criticism of the statistical techniques as tending to underplay variations in earlier parts of the graph, though this was disputed and he later accepted that the effect was very small. In 2005 McIntyre and McKitrick published criticisms of the principal components analysis methodology as used in MBH98 and MBH99. Their analysis was subsequently disputed by published papers including Huybers 2005 and Wahl & Ammann 2007 which pointed to errors in
8645-484: The debate, and Broecker's criticism that MBH99 did not show a clear MWP. They concluded that the MWP was likely to have been widespread in the extratropical northern hemisphere, and seemed to have approached late 20th century temperatures at times. In an interview, Mann said the study did not contradict MBH as it dealt only with extratropical land areas, and stopped before the late 20th century. He reported that Edward R. Cook ,
8778-563: The dominant climate forcing during the 20th century. In a review in the same issue, Gabriele C. Hegerl described their method as "quite original and promising", which could help to verify model estimates of natural climate fluctuations and was "an important step towards reconstructing space–time records of historical temperature patterns". Release of the paper on 22 April 1998 was given exceptional media coverage, including questioning as to whether it proved that human influences were responsible for climate change . Mann would only agree that it
8911-406: The earlier period. The MBH99 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, with "likely" defined as "66-90% chance", was supported by reconstructions by Crowley & Lowery 2000 and by Jones et al. 1998 using different data and methods. The Pollack, Huang & Shen 1998 reconstruction covering
9044-577: The editorial process, and von Storch with three other board members resigned. Senator James M. Inhofe stated his belief that "manmade global warming is the greatest hoax ever perpetrated on the American people", and a hearing of the United States Senate Committee on Environment and Public Works which he convened on 29 July 2003 heard the news of the resignations. Gerald North Gerald R. North (June 28, 1938 – )
9177-701: The effect its removal had on the result. He found that certain proxies were critical to the reliability of the reconstruction, particularly one tree ring dataset collected by Gordon Jacoby and Rosanne D'Arrigo in a part of North America Bradley's earlier research had identified as a key region. This dataset only extended back to 1400, and though another proxy dataset from the same region (in the International Tree-Ring Data Bank ) went further back and should have given reliable proxies for earlier periods, validation tests only supported their reconstruction after 1400. To find out why, Mann compared
9310-659: The final reconstructions, and other methods produced similar results. At the request of the U.S. Congress, initiated by Representative Sherwood Boehlert as chairman of the U.S. House of Representatives Committee on Science , a special "Committee on Surface Temperature Reconstructions for the Past 2,000 Years" was assembled by the National Research Council to quickly prepare a concise report. The NRC Committee, chaired by Gerald North , consisted of 12 scientists and statisticians from different disciplines. Its task
9443-464: The first eigenvector -based climate field reconstruction (CFR). This showed global patterns of annual surface temperature, and included a graph of average hemispheric temperatures back to 1400 with shading emphasising that uncertainties (to two standard error limits) were much greater in earlier centuries. Jones et al. 1998 independently produced a CPS reconstruction extending back for a thousand years, and Mann, Bradley & Hughes 1999 (MBH99) used
9576-567: 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. During the cold years of the 1960s, Lamb had anticipated that natural cycles were likely to lead over thousands of years to
9709-419: The global or hemispherical mean temperature record of the past 500 to 2000 years as shown by quantitative climate reconstructions based on climate proxy records. These reconstructions have consistently shown a slow long term cooling trend changing into relatively rapid warming in the 20th century, with the instrumental temperature record by 2000 exceeding earlier temperatures. The term hockey stick graph
9842-423: The globe, including a rich resource of tree ring networks for some areas and sparser proxies such as lake sediments, ice cores and corals, as well as some historical records. Their global reconstruction was a major breakthrough in evaluation of past climate dynamics, and the first eigenvector -based climate field reconstruction (CFR) incorporating multiple climate proxy data sets of different types and lengths into
9975-476: The graphs. In his view, "None of the statistical criticisms that have been raised by various authors unduly influence the shape of the final reconstruction. This is attested to by the fact that reconstructions performed without using principal components yield similar results." In a debate in the United States House of Representatives on 2 December 2009, Representative Jim Sensenbrenner said that
10108-587: The historic temperature record." He noted that the report raised no doubts about global climate change or the legitimacy of any paper, and concluded that "The report does show, unsurprisingly, that scientists need to continue to work to develop a more precise sense of what global temperatures were between the beginning of the last millennium and about 1600. Congress ought to let them go about that work without political interference." In an opposing statement Senator Jim Inhofe said that "Today's NAS report reaffirms what I have been saying all along, that Mann's ‘hockey stick'
10241-458: The important step of validation calculations , which showed that the reconstructions were statistically meaningful, or skillful . A balance was required over the whole globe, but most of the proxy data came from tree rings in the Northern mid latitudes , largely in dense proxy networks. Since using all of the large numbers of tree ring records in would have overwhelmed the sparse proxies from
10374-437: 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. The reconstruction found significant variability around a long-term cooling trend of –0.02 °C per century, as expected from orbital forcing , interrupted in the 20th century by rapid warming which stood out from
10507-485: The instrumental record. The least squares simultaneous solution of these multiple regressions used covariance between the proxy records. The results were then used to reconstruct large-scale patterns over time in the spatial field of interest (defined as the empirical orthogonal functions , or EOFs) using both local relationships of the proxies to climate and distant climate teleconnections . Temperature records for almost 50 years prior to 1902 were analysed using PCA for
10640-399: The issues are real, they had a very minimal effect, not a material effect on the final reconstruction." The U.S. House Science Committee Chairman Sherwood Boehlert issued a statement on the day of publication: "I think this report shows the value of Congress handling scientific disputes by asking scientists to give us guidance. The report clearly lays out a scientific consensus position on
10773-533: The last 1,000 years. In the Soon and Baliunas controversy , two scientists cited in the papers said that their work was misrepresented, and the Climate Research paper was criticised by many other scientists, including several of the journal's editors. 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
10906-525: The last few decades of the 20th century than during any comparable period during the preceding four centuries", justified by consistent evidence from a wide variety of geographically diverse proxies, but "Less confidence can be placed in large-scale surface temperature reconstructions for the period from 900 to 1600", and very little confidence could be assigned to hemispheric or global mean surface temperature estimates before about 900. The NRC committee stated that "The basic conclusion of Mann et al. (1998, 1999)
11039-426: The last thousand years. When asked if they could quantify "less confidence" and "plausible", Bloomfield explained that their wording reflected the panel's scientific judgements rather than well defined statistical procedures, and "When we speak of 'less confidence' we're more into a level of sort of 2 to 1 odds, which IPCC, they interpreted 'likely' as that level, roughly 2 to 1 odds or better." The NRC report said that
11172-524: The mid-19th century". When questioned about who had said MBH99 was definitive when the paper itself emphasised the uncertainties, North said his opinion was that, "The community probably took the results to be more definitive than Mann and colleagues originally intended." Kurt Cuffey said the context was that the MBH work "...was really the first of its kind," and had generated debate in the normal process of science where ideas are put forward then challenged over time. The IPCC 2001 report had been careful to give
11305-459: The nation's pre-eminent scientific body." Next day, the BBC News coverage on 23 June 2006 headed "Backing for 'hockey stick' graph" said the report "largely vindicates the researchers' work, first published in 1998." The Boston Globe reported that the panel had concluded that the evidence presented in the hockey stick graph was "probably true". The Wall Street Journal story, written by
11438-500: The northern hemisphere average, and compared the instrumental record with the proxy reconstruction over the same period. It concluded that the "Little Ice Age" period was complex, with evidence suggesting the influence of volcanic eruptions. It showed that temperatures since the 1920s were higher than earlier in the 500-year period, an indication of other factors which could most probably be attributed to human caused changes increasing levels of greenhouse gases . This paper introduced
11571-429: The past 500 years gave independent support for this conclusion, which was compared against the independent (extra-tropical, warm-season) tree-ring density NH temperature reconstruction of Briffa 2000 . 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
11704-430: The past five centuries supported the conclusion that 20th century warming was exceptional. In a perspective commenting on MBH99, Wallace Smith Broecker argued that the Medieval Warm Period (MWP) was global. He attributed recent warming to a roughly 1500-year cycle which he suggested related to episodic changes in the Atlantic's conveyor circulation . A March 2002 tree ring reconstruction by Jan Esper et al. noted
11837-532: The past six centuries" was submitted to the journal Nature on 9 May 1997, accepted on 27 February 1998 and published on 23 April 1998. The paper announced a new statistical approach to find patterns of climate change in both time and global distribution, building on previous multiproxy reconstructions. The authors 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
11970-410: The period from 900 to 1600". It broadly agreed with the basic findings of the original MBH studies, which subsequently been supported by other reconstructions and proxy records, while emphasising uncertainties over earlier periods. The principal component analysis methodology that McIntyre and McKitrick had contested had a small tendency to bias results so was not recommended—but it had little influence on
12103-477: The point that bristlecone pines from the Western U.S. could have been affected by pollution such as rising CO 2 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
12236-416: 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." The New York Times report had a colored version of the graph, distinguishing
12369-424: The predicted effects of greenhouse gas emissions. Reviewing twenty years of progress in palaeoclimatology, Jones noted the reconstructions by Jones et al. (1998), MBH99, Briffa (2000) and Crowley & Lowery (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. The Working Group 1 (WG1) part of
12502-523: 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. A commentary on MBH98 by Jones was published in Science on 24 April 1998. He noted that it used almost all
12635-426: The rarity of sunspots during the Maunder Minimum to Lamb's estimates of past climate, but had insufficient information to produce a quantitative assessment. The problem was reexamined by Bradley in collaboration with solar physicists Judith Lean and Juerg Beer , using the findings of Bradley & Jones 1993 . The Lean, Beer & Bradley 1995 paper confirmed that the drop in solar output appeared to have caused
12768-458: The reconstruction procedure was used with these pseudoproxies, the result was then compared with the original record or simulation to see how closely it had been reconstructed. The paper discussed the issue that regression methods of reconstruction tended to underestimate the amplitude of variation. While the IPCC Third Assessment Report (TAR) drew on five reconstructions to support its conclusion that recent Northern Hemisphere temperatures were
12901-504: The reconstruction useful for investigating natural variability and long-term oscillations as well as for comparisons with patterns produced by climate models. The CFR method made more use of climate information embedded in remote proxies, but was more dependent than CPS on assumptions that relationships between proxy indicators and large-scale climate patterns remained stable over time. Related rigorous statistical methods had been developed for tree ring data, with Harold C. Fritts publishing
13034-433: The reconstructions have been taken up by fossil fuel industry funded lobbying groups attempting to cast doubt on climate science. Paleoclimatology dates back to the 19th century, and the concept of examining varves in lake beds and tree rings to track local climatic changes was suggested in the 1930s. In the 1960s, Hubert Lamb generalised from historical documents and temperature records of central England to propose
13167-506: The region to produce a corrected version of this dataset. Their reconstruction using this corrected dataset passed the validation tests for the extended period, but they were cautious about the increased uncertainties. The Mann, Bradley and Hughes reconstruction covering 1,000 years (MBH99) was submitted in October 1998 to Geophysical Research Letters which published it in March 1999 with
13300-448: The report. 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
13433-460: The same as the hockey stick," and the scientific conclusion that the world was warming was independent of Mann's research. 1965 1978 1979 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Hockey stick graph (global temperature) Hockey stick graphs present
13566-418: The same reporter whose announcement of the McIntyre and McKitrick 2005 paper had featured on its front page, had the muted headline "Panel Study Fails To Settle Debate On Past Climates" for a piece on an inside page, which said the panel found that the key conclusion of MBH99 "is 'plausible' but not proved." On 28 June 2006 Nature reported the outcome as "Academy affirms hockey-stick graph. But it criticizes
13699-411: The shape of the reconstructions, it found that "In practice, this method, though not recommended, does not appear to unduly influence reconstructions of hemispheric mean temperature", and reconstructions using other methods were qualitatively similar. Some of the criticisms of validation techniques were more valid than others, these issues and the effect on robustness of the choice of proxies contributed to
13832-633: The statistician Peter Bloomfield and ice sheet/borehole specialist Kurt M. Cuffey . In its summary, the NRC committee noted the development of large-scale surface temperature reconstructions, especially MBH98 and MBH99, and highlighted six recent reconstructions: Huang, Pollack & Shen 2000 , Mann & Jones 2003 , Hegerl et al. 2006 , Oerlemans 2005 , Moberg et al. 2005 and Esper, Cook & Schweingruber 2002 . Its main findings were; 20th century instrumentally measured warming showed in observational evidence, and can be simulated with climate models, large-scale surface temperature reconstructions "yield
13965-546: The surrounding region. Their study did not calibrate these proxy patterns against a quantitative temperature scale, and a new statistical approach was needed to find how they related to surface temperatures in order to reconstruct past temperature patterns. For his postdoctoral research Mann joined Bradley and tree ring specialist Malcolm K. Hughes to develop a new statistical approach to reconstruct underlying spatial patterns of temperature variation combining diverse datasets of proxy information covering different periods across
14098-499: 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." 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
14231-526: The two datasets and found that they tracked each other closely from 1400 to 1800, then diverged until around 1900 when they again tracked each other. He found a likely reason in the CO 2 " fertilisation effect " affecting tree rings as identified by Graybill and Idso, with the effect ending once CO 2 levels had increased to the point where warmth again became the key factor controlling tree growth at high altitude. Mann used comparisons with other tree ring data from
14364-740: The two-year-old paper's conclusion fairly low confidence as "likely" at 2 to 1 odds, but use of the graph as a visual had created a misleading impression that this research was more resolved. The graph had been used as a visual in several places, including the IPCC summary for policymakers. Various criticisms of the MBH statistical methods were discussed in Chapter 11, in the context of more recent research that explored ways to address these problems, and showed greater amplitude of temperature variations over 1000 to 2000 years. Recent papers cited included Wahl & Ammann 2007 . On McIntyre and McKitrick's criticism of principal component analysis as tending to bias
14497-483: The uncertainties surrounding their work. They suggested that "poor communication by others" had led to the "subsequent confusion". At the Wegman Report hearings in July 2006, Gerald North testified that, "Dr. Wegman's criticisms of the statistical methodology in the papers by Mann et al. were consistent with our findings," referring to the NRC report that found that the methodology did not have an undue effect on
14630-408: The warmest in the past 1,000 years, it gave particular prominence to an IPCC illustration based on the MBH99 paper. The hockey stick graph was subsequently 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. From an expert viewpoint the graph was, like all newly published science, preliminary and uncertain, but it
14763-436: 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." It noted that "Surface temperature reconstructions for periods prior to the industrial era are only one of multiple lines of evidence supporting
14896-400: The way the controversial climate result was used." The article said that the NRC 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 this had been a comment made by North at the press conference, and was not stated in
15029-433: The whole period, with the 1990s "the warmest decade, and 1998 the warmest year, at moderately high levels of confidence." This was illustrated by the time series line graph Figure 2(a) which 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
15162-423: Was criticised as misleading . 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. In February 2000 Thomas J. Crowley and Thomas S. Lowery 's reconstruction incorporated data not used previously. It reached
15295-424: Was "highly suggestive" of that inference. He said 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
15428-411: Was "not yet sufficiently easy to assess nor sufficiently integrated with indications from other data to be used in this report." A "schematic diagram" of global temperature variations over the last thousand years has been traced to a graph based loosely on Lamb's 1965 paper, nominally representing central England, modified by Lamb in 1982. Mike Hulme describes this schematic diagram as "Lamb's sketch on
15561-424: Was "to summarize current scientific information on the temperature record for the past two millennia, describe the main areas of uncertainty and how significant they are, describe the principal methodologies used and any problems with these approaches, and explain how central is the debate over the paleoclimate temperature record to the state of scientific knowledge on global climate change." The NRC report went through
15694-530: Was a "misguided and illegitimate investigation" into something that should properly be under the jurisdiction of the Science Committee, and in November 2005 after Barton dismissed the offer of an independent investigation organised by the U.S. National Academy of Sciences , Boehlert requested the review, which became the North Report. The North Report went through a rigorous review process, and
15827-548: Was adapted from MBH99. 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. This iconic graph adapted from MBH99 was featured prominently in the WG1 Summary for Policymakers under a graph of the instrumental temperature record for the past 140 years. The text stated that it was "likely that, in the Northern Hemisphere,
15960-466: Was anomalous over the 300-year period, and went as far as speculating that these results supported the hypothesis that recent warming had human causes. Publicity over the concerns of scientists about the implications of global warming led to increasing public and political interest, and the Reagan administration , concerned in part about the political impact of scientific findings, successfully lobbied for
16093-449: Was disputed by contrarian Pat Michaels with the claim that all of the warming took place between 1920 and 1935, before increased human greenhouse gas emissions. The George C. Marshall Institute alleged that MBH98 was deceptive in only going back to 1400, and so not covering the Medieval Warm Period which predated industrial greenhouse gas emissions. The same criticisms were made by Willie Soon and Sallie Baliunas . In October 1998
16226-538: Was disputed by various contrarians, and in the politicisation of this "hockey stick controversy" the New York Times of 14 February 2005 hailed a paper by businessman Stephen McIntyre and economist Ross McKitrick (MM05) as undermining the scientific consensus behind the Kyoto agreement. On 23 June 2005, Rep. Joe Barton , chairman of the House Committee on Energy and Commerce , with Ed Whitfield , Chairman of
16359-496: Was exceptional. Bradley & Jones 1993 introduced the "Composite Plus Scaling" (CPS) method which, as of 2009, was still being used by most large-scale reconstructions. Their study was featured in the IPCC Second Assessment Report of 1995. In 1998 Michael E. Mann , Raymond S. Bradley and Malcolm K. Hughes developed new statistical techniques to produce Mann, Bradley & Hughes 1998 (MBH98),
16492-446: Was more widespread at high northern latitudes, and warned that it had to be taken into account to avoid overestimating past temperatures. Variations on the "Composite Plus Scale" (CPS) method continued to be used to produce hemispheric or global mean temperature reconstructions. From 1998 this was complemented by Climate Field Reconstruction (CFR) methods which could show how climate patterns had developed over large spatial areas, making
16625-474: Was needed at seasonal or annual resolution covering a wide geographical area to provide a framework for testing the part climate forcings had played in past variations, look for cycles in climate, and find if debated climatic events such as the Little Ice Age and Medieval Warm Period were global. Reconstructions were to be made of key climate systems, starting with three climatically sensitive regions:
16758-524: Was not unusual. In March they published an extended paper in Energy & Environment , with additional authors. The Bush administration 's Council on Environmental Quality chief of staff Philip Cooney inserted references to the papers in the draft first Environmental Protection Agency Report on the Environment , and removed all references to reconstructions showing world temperatures rising over
16891-485: Was popularized by the climatologist Jerry Mahlman , to describe the pattern shown by the Mann, Bradley & Hughes 1999 (MBH99) reconstruction, envisaging a graph that is relatively flat with a downward trend to 1900 as forming an ice hockey stick 's "shaft" followed by a sharp, steady increase corresponding to the "blade" portion. The reconstructions have featured in Intergovernmental Panel on Climate Change (IPCC) reports as evidence of global warming . Arguments over
17024-426: Was published on 22 June 2006. Committee member John Michael Wallace said that "Our conclusion is that this recent period of warming is likely the warmest in the last millennium", and added that "This doesn't change the scientific landscape in terms of the greenhouse warming debate". At a press conference, held on the same day, clarifications were given by three members of the NRC committee science panel: Gerald North ,
17157-424: Was published on 22 June 2006. It concluded "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", justified by consistent evidence from a wide variety of geographically diverse proxies , but "Less confidence can be placed in large-scale surface temperature reconstructions for
17290-552: Was submitted to The Holocene on 16 October 1997; their revised manuscript was accepted on 3 February and published in May 1998. 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
17423-449: Was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators". It said "Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence,
17556-560: Was the initial proposer and first Study Scientist for the Tropical Rainfall Measuring Mission . He joined Texas A&M University in 1986. In 2005 to 2006 he chaired a United States National Research Council committee investigating surface temperature reconstructions for the last 2,000 years, set up at the request of Representative Sherwood Boehlert , chairman of the U.S. House of Representatives Committee on Science . Their report, published in July 2006,
17689-499: Was widely used to publicise the issue of global warming, and it was targeted by those opposing ratification of the Kyoto Protocol on global warming. A literature review by Willie Soon and Sallie Baliunas , published in the relatively obscure journal Climate Research on 31 January 2003, used data from previous papers to argue that the Medieval Warm Period had been warmer than the 20th century, and that recent warming
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