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?
Did the Medieval Warm Period exist? If so, what was its spatial and temporal extent?
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 described[3] 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[3]
"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. [4]. In 2004 Mann, Bradley, and Hughes published a corrigendum to their 1998 article, correcting a number of mistakes in the online supplementary information that accompanied their article but leaving the actual results unchanged.
Hans von Storch and colleagues claimed that the method used by Mann et al. probably underestimates the temperature fluctuations in the past by a factor of two or more;[5] however, this conclusion rests at least in part on the reasonableness of the global climate model (GCM) simulation used, which has been questioned;[6][7] Wahl et al. assert errors in the reconstruction technique that von Storch used.[8]. The IPCC AR4 reports that the extent of any such biases in specific reconstructions... is uncertain ... It is very unlikely, however, that any bias would be as large as the factor of two suggested.
Anders Moberg and his Swedish and Russian collaborators have also generated reconstructions with significantly more variability than the reconstructions of Mann et al.[9][10]
On February 12, 2005, Stephen McIntyre and Ross McKitrick published a paper in Geophysical Research Letters that claimed various errors in the methodology of Mann et al. (1998). The paper claimed that the "Hockey Stick" shape was the result of an invalid principal component method.[11] They claimed that using the same steps as Mann et al., they were able to obtain a hockey stick shape as the first PC in 99 percent of cases even if trendless red noise was used as input.[12] This paper was nominated as a journal highlight by the American Geophysical Union,[13] which publishes GRL, and attracted international attention for its claims to expose flaws in the reconstructions of past climate.[14]. 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.)"[15]
In turn, Mann (supported by Tim Osborn, Keith Briffa and Phil Jones of the Climatic Research Unit) has disputed the claims made by McIntyre and McKitrick,[16][17] saying the
"...so-called 'correction' was nothing more than a botched application of the MBH98 procedure, where the authors (MM) removed 80% of the proxy data actually used by MBH98 during the 15th century period... Indeed, the bizarre resulting claim by MM of anomalous 15th century warmth (which falls within the heart of the "Little Ice Age") is at odds with not only the MBH98 reconstruction, but, in fact the roughly dozen other estimates now published that agree with MBH98 within estimated uncertainties...".[18]
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. ,[27] 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." [28]
One point of contention relates to McIntyre's requests for Mann to provide him with the data, methods and source code McIntyre needed to "audit" MBH98.[19] Mann provided some data and then stopped. After a long process - in which the National Science Foundation supported Mann - the code was made publicly available [20]. It happened because Congress investigated after an article in the Wall Street Journal [21] detailed criticisms raised by McIntyre.[22] Congress was especially concerned about Mann’s reported refusal to provide data. In June 2005, Congress asked Mann to testify before a special subcommittee. The chairman of the committee (Joe Barton, a prominent global warming skeptic) wrote a letter to Mann requesting he provide his data, including his source code, archives of all data for all of Mann's scientific publications, identities of his present and past scientific collaborators, and details of all funding for any of Mann's ongoing or prior research, including all of the supporting forms and agreements. [23] The American Association for the Advancement of Science viewed this as "a search for some basis on which to discredit these particular scientists and findings, rather than a search for understanding."[24] 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 [25] and the National Academy of Sciences [26] both published reports. McIntyre and McKitrick claim their findings have been largely confirmed by these reviews. [27] Nature reported it as "Academy affirms hockey-stick graph." [28] and Mann said that Wegman "uncritically parrots claims by [M&M]." [29]
http://en.wikipedia.org/wiki/Hockey_Stick_graph
