Authors: Mary-Elena Carr; Kate Brash; Robert Anderson; Madeleine Rubenstein
On September 8, 2010, Deutsche Bank Climate Change Advisors (DBCCA) and the Columbia Climate Center (CCC) published a report responding to the major claims of climate change skeptics. The report, entitled “Climate Change: Addressing the Major Skeptic Arguments,” aims to examine the many claims and counter-claims surrounding the current climate change debate. The report presents responses to the most prominent arguments of climate change skeptics, including claims that Earth is not warming; that any warming is not human-induced; and that warming is not harmful and does not require mitigation.
On September 13, 2010, Professor Ross McKitrick of the University of Guelph issued a response to the DBCCA/CCC publication, entitled Response to Misinformation from Deutsche Bank. In his response, Professor McKitrick offers several critiques of the DBCCA/CCC report. Specifically, Professor McKitrick criticizes two sections within the DBCCA/CCC report: an introductory section which presents a brief history of the so-called “hockey stick controversy”; and a later section which comments on the controversy surrounding the phrase “hide the decline,” from e-mails stolen from the University of East Anglia.
We thank Professor McKitrick for noting a few instances where we mischaracterized conclusions from the work of other groups. However, we disagree strongly with several of his points. In this two-part blog series, we respond to Professor McKitrick’s comments. In addition, the DBCCA/CCC report has been amended with a brief erratum to reflect necessary changes, and can be found on the DBCCA website.
In this first blog post, we address the first of Professor McKitrick’s critiques: our assessment of the “hockey stick controversy”. In Part 2 of the series, we will address Professor McKitrick’s comments concerning the “hide the decline” statement.
Part I: The Hockey Stick Controversy
Below, excerpts from Professor McKitrick’s critiques are indicated in italics.
1. The DB refers to both Mann et al. hockey stick papers (the ones in Nature and Geophysical Research Letters) as well as a 2005 paper by Rutherford et al. But despite supposedly presenting a rebuttal of Steve’s and my work on the hockey stick, the DB paper fails to cite our main publications (our 2005 Geophysical Research Letters and Energy and Environment papers) nor does it provide any summary of what those papers argued.
The claims that we addressed in this report are listed in both a table of the editorial section and in the executive summary- the hockey stick is not mentioned among these. This is because our aim was not to rebut McIntyre and McKitrick’s work (hereafter MM), nor to endorse the Mann, Bradley and Hughes studies (hereafter MBH), nor to summarize the bulk of the research of those who disagree or agree with critiques of the hockey stick. As expressed on page 10 of the DBCCA/CCC report, the introductory section on the hockey stick controversy aimed to provide the reader with a historical overview of some events surrounding an “example controversy.”
2. The DB paper states that Mann et al. published a correction in 2004 after our initial publication, and claims “none of the results or analyses were affected.” This is a misleading claim. First, as was acknowledged in the online supplement to the correction, the principal component analysis method used by Mann et al. was affected by the correction insofar as they used a flawed method without properly disclosing in their original paper what they were doing.
Professor McKitrick is incorrect. The corrigendum (Mann et al. 2004) explicitly states that the only corrections are to the list of proxy records. The Supplementary Information provides a more thorough explanation of the methods but fails to indicate any methodological flaw. There is no discussion of any change in the principal components. We plotted the principal components provided in the Supplementary Information of the 2004 corrigendum and they are identical to those shown in Figure 5 of the 1998 paper.
3. Second, subsequent analyses, including those of the Wegman and National Academy of Science panels both concluded Mann’s flawed methods biased the results.
Firstly, the National Academy of Science (NAS) report (2006) concludes that the methodology has a tendency to bias the results (see section 8 below), while the Wegman report (2006) has no specific finding regarding bias.
Secondly, the overarching conclusion of the NAS report is that while the methodology used by MBH may have been flawed, the fundamental conclusion from the MBH studies (that warming in the late 20th century was “unprecedented during at least the last 1,000 years”) is still valid; indeed, the NAS reports that it is “supported by an array of evidence” (NAS 2006, pg. 3). This is the fundamental conclusion which is of the most interest to that section of our report.
4. The DB paper then reports a political timeline without dates or details as follows:
“Until this point the controversy followed the standard pattern of scientific discourse: discovery, publication, attempts at replication, criticism, adjustment and re-publications. The debate entered the political arena when McIntyre and McKitrick met with Senator James Inhofe (R-OK), an outspoken denier of anthropogenic climate change; shortly afterward, Congressman Joseph Barton (R-TX) write to Michael Mann, demanding that he share all his data, methods and associated information with critics and congressional staff (Eilperin 2005).”
We [McIntyre and McKitrick] made a presentation on our work in Washington in November 2003, during which time we briefly met Senator Inhofe, among others. Barton’s letter to Mann and his coauthors was not issued until in July 2005, and it was in response to Mann’s statement in a Wall Street Journal article of 14 February 2005 affirming his refusal to share his computer code with us. Contrary to the DB paper, apart from our brief meeting in fall 2003, McIntyre and I did not meet with Senator Inhofe during the debate over our work and, in particular, during the lead-up to the issuance of Barton’s letter.
Consistent with our goal of providing a brief example of a climate change controversy, we did not provide specific dates but instead a sequence of events. It is quite clear that the Wall Street Journal article by Regalado in February 2005 played a key role in the subsequent letter to Mann from Congressman Barton, a fact which we did not not include.
We do not wish to challenge the dates of meetings we were not privy to; however, Professor McKitrick and Mr. McIntyre have indicated a willingness to participate in the policy-science debate. They presented their work on May 11, 2005 at the George Marshall Institute, an organization which aims to provide technical and scientific information to policy makers. The timing of the 2005 presentation (i.e. following the February 2005 Wall Street Journal article but prior to Congressman Barton’s June letter) is indicative of an engagement in the policy-science dialogue.
5. The DB paper then claims that the 2006 report of the National Academy of Sciences “rejected the claims of McIntyre and McKitrick and endorsed, with a few reservations, Mann et al.’s work.” This is a misrepresentation. It is quite notable that no citations to the NAS report are given to support this statement. The NAS report endorsed every technical criticism we made.
We mischaracterized the results of the NAS report regarding MM’s findings. The sentence that Professor McKitrick cites on page 11 should be re-worded as indicated in the erratum:
“National Academy of Sciences (2006) acknowledged the merit of the critique of McIntyre and McKitrick, who identified methodological errors in the original work of Mann et al., but concluded that the primary findings of Mann et al. were not significantly impacted by these technical errors.”
This updated sentence accurately reflects the full conclusions of the NAS report: that while the MBH study had methodological errors, the fundamental conclusion of their study was still valid. The basis of this re-wording is found in the extracted quotes below from NAS (2006) (emphasis added).
“McIntyre and McKitrick (2003, 2005a,b) question the choice and application of statistical methods, notably principal component analysis; the metric used in the validation step of the reconstruction exercise; and the selection of proxies, especially the bristlecone pine data used in some of the original temperature reconstruction studies. These and other criticisms, explored briefly in the remainder of this chapter, raised concerns that led to new research and ongoing efforts to improve how surface temperature reconstructions are performed.” (pp 112-113)
“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 both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on ice caps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2,000 years. Not all individual proxy records indicate that the recent warmth is unprecedented, although a larger fraction of geographically diverse sites experienced exceptional warmth during the late 20th century than during any other extended period from A.D. 900 onward.” (pp 3-4)
6. Professor McKitrick points out several methodological flaws in the MBH study, including the quantification of uncertainty, different measures of skill and the use of specific datasets. For example, he states that “the hockey stick method, and the test statistics used to validate it, systematically underestimated the uncertainties in the data.”
As stated above, it was not our intention to fully endorse to MBH study, or to evaluate Professor McKitrick’s critique of it. Therefore, we don’t dispute any of these points, or his interpretation of the NAS finding on them. However, a critical outcome of the NAS (2006) report, and of the many studies that jointly make up the modern landscape of climate science, is that northern hemisphere temperatures have been higher in the late 20th century than in the previous thousand years. In the context of the public debate around climate change, this is the conclusion with the most salience, not whether MBH’s methodologies were without flaw.
7. We argued that the flawed principal component methodology used in Mann et al.’s work biased their results. The NAS panel concluded (p. 106): “As part of their statistical methods, Mann et al. used a type of principal component analysis that tends to bias the shape of the reconstructions. A description of this effect is given in Chapter 9.”
Professor McKitrick has truncated this quote and omitted the paragraph’s significant conclusion: that while the methodology introduces a tendency to bias, that the bias does not invalidate the conclusion of the MBH studies. The NAS report goes on to conclude (emphasis added):
“As part of their statistical methods, Mann et al. used a type of principal component analysis that tends to bias the shape of the reconstructions. A description of this effect is given in Chapter 9. In practice, this method, though not recommended, does not appear to unduly influence reconstructions of hemispheric mean temperature; […]” (p 113)
8. The DB paper summarizes the Wegman panel findings with regards to the hockey stick as follows:
“They also concluded that the methodological errors in the original Mann et al. papers had no impact on the scientific conclusion.” The DB Report went on to say “While the uncertainty associated with assessments of past climate might have been understated and there were minor methodological errors in the Mann et al. studies, both NAS (2006) and Wegman et al. (2006) confirmed the soundness of the research and concluded the primary conclusions were unaffected by any methodological problems.”
In addition to misrepresenting the NAS findings, this is a wholly false misrepresentation of the findings of the Wegman report.
As discussed above, we were not assessing whether the MM criticisms have merit or whether the MBH study was without flaw. Instead, we wished to point out that the fundamental conclusion of the MBH publications did not change as a result of MM’s criticisms. Because Wegman et al only address one of the conclusions of MBH, we have amended our statements on page 12:
“While the uncertainty associated with assessments of past climate might have been understated and there were minor methodological errors in the Mann et al. studies, both NAS (2006) and Wegman et al. (2006) confirmed the soundness of the research and concluded the primary conclusions were unaffected by any methodological problems.”
“While the uncertainty associated with assessments of past climate were found to be understated and there were methodological errors in the Mann et al. studies, NAS (2006) concluded that the primary conclusion of unprecedented warming in the late 20th century has been borne out by subsequent studies.”
The statement that Professor McKitrick refers to, also on page 12, has been corrected to say:
“They concluded, as NAS (2006) had already done, that the data employed by MBH was insufficient to assert that 1998 was the warmest year, and the 1990’s the warmest decade, in the past thousand years.”
First and foremost, we wish to re-emphasize the intent and main conclusion of the DBCCA/CCC report. The report aimed to review and respond to the main arguments of those skeptical of climate change in the context of an increasingly polarized debate over climate science and policy. The primary conclusion of the report was that none of the many skeptical claims currently circulated in the debate undermines the conclusion that human-induced climate change is a serious threat to our environment and to humanity. While we thank Professor McKitrick for his comments and have amended the report where necessary, this overall conclusion remains unchanged.
As noted above, the DBCCA/CCC report did not aim to re-examine the specifics of the hockey stick controversy. Since, however, this particular controversy remains a source of concern, we will state again that the principal finding of MBH has been supported by a variety of subsequent studies. The combined force of those studies tells us that temperatures in the late 20th Century have been warmer than at any period of equivalent duration in the past millennium. The methodological errors identified by McIntyre and McKitrick, while valid critiques, do not alter this principal finding.
Ultimately, the specificity of this particular debate over the MBH study has only served to distract public attention from the larger picture. Earth’s climate varies naturally over all time scales, and there have been much warmer periods in Earth history than at present. This does not disprove anthropogenic climate change— it is possible both that Earth used to be warmer than today, and that we are currently experiencing human-induced climate change, since these two principles are not mutually exclusive. The warming observed today is consistent with basic physics of greenhouse gases and the planet’s heat budget. It cannot be reproduced in models that do not include anthropogenic emissions of greenhouse gases—in other words, there is no known natural cause for the warming recently observed. Especially in the context of a public debate, where misinformation has been exaggerated and salient information is hard to extract, it is important not to lose sight of this fundamental observation.
National Academy of Sciences. 2006. Surface Temperature Reconstructions for the Last 2000 Years. Washington, DC. National Academy of the Sciences. 145pp. http://www.nap.edu/catalog/11676.html
Wegman, E., D. Scott, and Y. Said. 2006. Ad hoc committee report on the ‘hockey stick’ global climate reconstruction. A report to Chairman Barton, House Committee on Energy and Commerce and to Chairman Whitfield, House Subcommittee on Oversight and Investigations. 92 pp. http://republicans.energycommerce. house.gov/108/home/07142006_Wegman_Report.pdf