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

The hockey stick controversy is a dispute over the reconstructed estimates of Northern Hemisphere Mean Temperature Changes Over The Past Millennium ,1
  Title Global-scale temperature patterns and climate forcing over the past six centuries
  First1 Michael E last1=Mann
  First2 Raymond S last2=Bradley
  First3 Malcolm K last3=Hughes
  Author1-link Michael Mann (scientist)
  Author2-link Raymond Bradley
  Journal Nature
  Volume 392 pages=779-787 year=1998
  Url http://wwwcaenvirothoncom/Resources/Mann,%20et%20al%20Global%20scale%20temp%20patternspdf
  Format PDF




NATURE OF THE DISPUTE

The political significance of the scientific controversy over the graph centers on its use as part of the evidence for Anthropogenic Global Warming . The MBH98 reconstruction was prominently featured in the 2001 United Nations Intergovernmental Panel On Climate Change (IPCC) Third Assessment Report (TAR) and as a result has been widely published in the media.

This dispute centered on technical aspects of the methodology and data sets used in creating the MBH98 reconstruction. The issue was originally raised by former mining executive Stephen McIntyre and economist Ross McKitrick . Their criticisms were that Mann ''et al.'''s reconstructed millennial temperature graph (the hockey stick) was an artifact of flawed calculations and serious data defects; in turn, MBH replied that these criticisms were spurious.

The dispute eventually led to an investigation at the behest of U.S. Congress by a panel of scientists convened by the National Research Council (NRC) of the United States National Academy Of Sciences to consider reconstructions of the last 2000 years in general; in addition, an investigation was performed at the behest of Congressman Joe Barton by a panel of three statisticians, chaired by Edward Wegman specifically addressing the MBH work. Both the NRC and Wegman teams issued reports in 2006.

The second graph on the right shows the data from MBH98 and from several other climate reconstructions, subsequent to the 1998 reconstruction. Two of the other temperature reconstructions included on the graph are by Mann and co-authors.

There is an ongoing debate about the details of the temperature record and the means of its reconstruction. The debate centers around several discussion points:

  • How well can past temperatures be reconstructed from the data we have?

  • Was the late 20th century the warmest period during the last 1,000 years?

  • Was the Medieval Warm Period observed in the North Atlantic region part of a broader global or hemispheric warming?

  • Are bristlecone tree rings valid temperature proxies?



DISCUSSION OF THE MBH RECONSTRUCTION


The hockey stick controversy has to a large extent been focussed on Mann and on the MBH98 reconstruction on which he was the lead author. ''Scientific American'' magazine described him as the "Man behind the Hockey Stick," referring to this reconstruction of temperatures. The BBC described2
the "hockey stick" as a term coined for the chart of temperature variation over the last 1,000 years. The chart is relatively flat from the period A.D. 1000 to 1900, indicating that temperatures were relatively stable for this period of time. The flat part forms the stick's "shaft." After 1900, however, temperatures appear to shoot up, forming the hockey stick's "blade." The combination of the two in the chart suggests a recent sharp rise in temperature caused by human activities. The BBC further stated
:"The high-profile publication of the data led to the "hockey stick" being used as a key piece of supporting evidence in the Third Assessment Report by the United Nations' Intergovernmental Panel On Climate Change (IPCC) in 2001."

In 2003, Stephen McIntyre and Ross McKitrick published "Corrections to the Mann ''et al'' (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series" '' Energy And Environment '' 14(6) 751-772, raising concerns about their ability to reproduce the results of MBH. The IPCC AR4 reports that "Wahl and Ammann (2007) showed that this was a consequence of differences in the way McIntyre and McKitrick (2003) had implemented the method of Mann et al. (1998) and that the original reconstruction could be closely duplicated using the original proxy data." . In 2004 Mann, Bradley, and Hughes published a Corrigendum to their 1998 article, correcting a number of mistakes in the online supplementary information that accompanied their article but leaving the actual results unchanged.

  First1 Hans last1=von Storch
  First2 Eduardo last2=Zorita
  First3 Julie M last3=Jones
  First4 Yegor last4=Dimitriev
  First5 Fidel last5=González-Rouco
  First6 Simon F B last6=Tett
  Author1-link Hans von Storch
  Title Reconstructing Past Climate from Noisy Data
  Journal Science
  Volume 306 issue=5696 pages=679-682 year=2004
  Doi 101126/science1096109
  Url http://w3ggkssde/staff/storch/pdf/vonStorch2004sciencepdf


  Title Testing the Fidelity of Methods Used in Proxy-Based Reconstructions of Past Climate
  First1 Michael E last1=Mann
  First2 Scott last2=Rutherford
  First3 Eugene last3=Wahl
  First4 Caspar last4=Amman
  Author1-link Michael Mann (scientist)
  Journal Journal Of Climate
  Volume 18 issue=20 pages=4097-4105 year=2005
  Url http://wwwmeteopsuedu/~mann/shared/articles/MRWA-JClimate05pdf


  First1 Eugene R last1=Wahl
  First2 David M last2=Ritson
  First3 Caspar M last3=Ammann
  Title Comment on "Reconstructing Past Climate from Noisy Data"
  Journal Science year=2006
  Volume 312 issue=5773 pages=529
  Doi 101126/science1120866
  Url http://w3ggkssde/staff/storch/pdf/wahl_060428pdf



  First1 Anders last1=Moberg
  First2 Dmitry M last2=Sonechkin
  First3 Nina M last3=Datsenko
  First3 Wibjörn last3=Karlén
  Title Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data
  Journal Nature year=2005
  Volume 433 pages=612-617
  Doi 101038/nature03265
  Url http://wwwnaturecom/nature/journal/v433/n7026/pdf/nature03265pdf


  Title Hockey sticks, principal components, and spurious significance
  First1 Stephen last1=McIntyre
  First2 Ross last2=McKitrick
  Author1-link Stephen McIntyre
  Author2-link Ross McKitrick
  Journal Geophysical Research Letters
  Volume 32 year=2005
  Url http://wwwclimate2003com/pdfs/2004GL012750pdf
  Doi 101029/2004GL021750
  Format PDF


This paper was nominated as a journal highlight by the American Geophysical Union ,http://www.agu.org/sci_soc/prrl/jh050309.html which publishes GRL, and attracted international attention for its claims to expose flaws in the reconstructions of past climate. Wall Street Journal blog . The IPCC AR4 says this paper ''may have some theoretical foundation, but Wahl and Amman (2006) also show that the impact on the amplitude of the final reconstruction is very small''.

After testing the work of Mann ''et al.'' (1998), McKitrick commented
:"The Mann multiproxy data, when correctly handled, shows the 20th century climate to be unexceptional compared to earlier centuries. This result is fully in line with the borehole evidence. (As an aside, it also turns out to be in line with other studies that are sometimes trotted out in support of the hockey stick, but which, on close inspection, actually imply a MWP as well.)"

  Title Note on paper by McIntyre and McKitrick in Energy and Environment
  First1 Michael E last1=Mann
  First2 Raymond S last2=Bradley
  First3 Malcolm K last3=Hughes
  Author1-link Michael Mann (scientist)
  Author2-link Raymond Bradley
  Year 2003
  Url http://holocenemeteopsuedu/Mann/EandEPaperProblempdf


:"...so-called 'correction' was nothing more than a botched application of the MBH98 procedure, where the authors (MM) removed 80% of the proxy data actually used by MBH98 during the 15th century period... Indeed, the bizarre resulting claim by MM of anomalous 15th century warmth (which falls within the heart of the "Little Ice Age") is at odds with not only the MBH98 reconstruction, but, in fact the roughly dozen other estimates now published that agree with MBH98 within estimated uncertainties...". Michael Mann and Gavin Schmidt on peer reviewing

Mann has been personally involved in the debate over climate change. In testimony before the U.S. Senate in 2003, he stated:
:"It is the consensus of the climate research community that the anomalous warmth of the late 20th century cannot be explained by natural factors, but instead indicates significant anthropogenic, that is human influences... More than a dozen independent research groups have now reconstructed the average temperature of the northern hemisphere in past centuries... The proxy reconstructions, taking into account these uncertainties, indicate that the warming of the northern hemisphere during the late 20th century... is unprecedented over at least the past millennium and it now appears based on peer-reviewed research, probably the past two millennia."

More recently, the National Academy of Sciences considered the matter. On June 22, 2006, the Academy released a pre-publication version of its report Report-Surface Temperature Reconstructions for the Last 2,000 Years, {Link without Title} supporting Mann's more general assertion regarding the last decades of the Twentieth Century, but showing less confidence in his assertions regarding individual decades or years, due to the greater uncertainty at that level of precision.

:"The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes ...
    Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. The substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that "the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium" because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales." {Link without Title}

One point of contention relates to McIntyre's requests for Mann to provide him with the viewed this as "a search for some basis on which to discredit these particular scientists and findings, rather than a search for understanding."http://www.aaas.org/news/releases/2005/0714letter.pdf When Mann complied, all of the data was available for McIntyre. Congress also requested that third party science panels review the criticisms of McIntyre and McKitrick. The Wegman Panel "The Wegman Report" and the National Academy of Sciences "Surface Temperature Reconstructions for the last 2,000 years" by National Academy of Science [http://books.nap.edu/catalog.php?record_id=11676#toc both published reports. McIntyre and McKitrick claim their findings have been largely confirmed by these reviews. "A Scorecard on MM03" by McIntyre and McKitrick [http://www.climate2003.com/blog/050202.scorecard.htm] Nature reported it as "Academy affirms hockey-stick graph." [http://www.nature.com/nature/journal/v441/n7097/full/4411032a.html Academy affirms hockey-stick graph.]


NATIONAL RESEARCH COUNCIL REPORT

At the request of the U.S. Congress, a special "Committee on Surface Temperature Reconstructions for the Past 2,000 Years" was assembled by the National Research Council's Board on Atmospheric Sciences and Climate. The Committee consisted of 12 scientists from different disciplines and was tasked with explaining the current scientific information on the temperature record for the past two millennia, and identifying the main areas of uncertainty, the principal methodologies used, any problems with these approaches, and how central the debate is to the state of scientific knowledge on global climate change.

The panel published its report in 2006.Committee on Surface Temperature Reconstructions for the Past 2,000 Years. ''Surface Temperature Reconstructions for the Past 2,000 Years'' . The National Academies Press, Washington, D.C. 2006. The report agreed that there were statistical shortcomings in the MBH analysis, but concluded that they were small in effect. The report summarizes its main findings as follows:http://fermat.nap.edu/books/0309102251/html/2.html


In response, a group-authored post on :10.1038/4411032a

According to , "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."Hans von Storch, Eduardo Zorita, Fidel González-Rouco: Press release and comment on the NAS report "Surface Temperature Reconstructions for the last 200 Years" ; 22 June 2006 It was noted by their critics, however, that no such statement, explicit or implicit, is present on the two pages cited 91 and 111 ; 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." page 95

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. Huybers (2005) Wahl and Ammann (2006)


COMMITTEE ON ENERGY AND COMMERCE REPORT (WEGMAN REPORT)

A team of statisticians led by Edward Wegman , chair of the National Academy of Sciences’ (NAS) Committee on Applied and Theoretical Statistics, was assembled at the request of U.S. Rep. Joe Barton , an outspoken global warming skeptic. Blog by Eric Berger The report primarily focused on the statistical analysis used in the MBH paper, and also considered the personal and professional relationships between Mann ''et al'' and other members of the paleoclimate community. Findings presented in this report (commonly known as the "Wegman Report" full report fact-sheet ) include the following:


The Wegman report has itself been criticized on several grounds:



UPDATES


In a letter to '' Nature '' on August 10 , 2006 , Bradley, Hughes and Mann pointed at the original title of their 1998 article: "''Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations''"Geophys. Res. Lett. 26, 759–762; 1999 and pointed out "more widespread high-resolution data are needed before more confident conclusions can be reached'' and that the uncertainties were ''the point of the article." Mann and his colleagues said that it was "hard to imagine how much more explicit" they could have been about the uncertainties surrounding their work and blaming "poor communication by others" for the "subsequent confusion." He has further suggested that the criticisms directed at his statistical methodology are purely political and add nothing new to the scientific debate.http://news.mongabay.com/2006/0716-climate.html

Paleoclimate findings by the IPCC before and after the Hockey Stick Controversy:

Before: 2001 (page 2)http://www.ipcc.ch/pub/spm22-01.pdf

:" proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year."

After:
Current SPM statement from 2007 (page 10) IPCC Summary for Policymakers

:"“Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years. Some recent studies indicate greater variability in Northern Hemisphere temperatures than suggested in the TAR, particularly finding that cooler periods existed in the 12 to 14th, 17th, and 19th centuries. Warmer periods prior to the 20th century are within the uncertainty range given in the TAR.”

In May 2007, Hans Von Storch reviewed the changes in thought caused by the hockey stick controversy writing:
In October 2004 we were lucky to publish in Science our critique of the ‘hockey-stick’ reconstruction of the temperature of the last 1000 years. Now, two and half years later, it may be worth reviewing what has happened since then.


At the EGU General Assembly a few weeks ago there were no less than three papers from groups in Copenhagen and Bern assessing critically the merits of methods used to reconstruct historical climate variable from proxies; Bürger’s papers in 2005; Moberg’s paper in Nature in 2005; various papers on borehole temperature; The National Academy of Science Report from 2006 – al of which have helped to clarify that the hockey-stick methodologies lead indeed to questionable historical reconstructions. The 4th Assessment Report of the IPCC now presents a whole range of historical reconstructions instead of favoring prematurely just one hypothesis as reliable.


McIntyre was critical of this ''Nature'' blog entry because von Storch did not acknowledge the role of McIntyre and McKitrick;http://www.climateaudit.org/?p=1518 however von Storch replied Hans von Storch that:

This was on purpose, as we do not think that McIntyre has substantially contributed in the published peer-reviewed literature to the debate about the statistical merits of the MBH and related method. They have published one peer-reviewed article on a statistical aspect, and we have published a response – acknowledging that they would have a valid point in principle, but the critique would not matter in the case of the hockey-stick ... we see in principle two scientific inputs of McIntyre into the general debate – one valid point, which is however probably not relevant in this context, and another which has not been properly documented


As a lot of claims regarding the hockey stick revolve around statistical aspects, "Report on Temperature Trends in the Lower Atmosphere” Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. Thomas R. Karl, Susan J. Hassol, Christopher D. Miller, and William L. Murray, editors, 2006. A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, Washington, DC. and shared his vision of the role of statisticians in the process. The session was summarized by R. Smith in ASA Section on Statistics and the Environment newsletter ASA Section on Statistics and the Environment Newsletter, Spring 2007 .


REFERENCES



EXTERNAL LINKS


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