The July 1 issue of Nature magazine ran a correction by Michael Mann, Raymond Bradley, and Malcolm Hughes of mistakes in their widely cited 1998 Nature article, which purported to give an accurate reconstruction of global temperatures over the past six centuries. The 1998 article was the initial source for the “hockey stick” graph cited by the United Nations Intergovernmental Panel on Climate Change (IPCC) and others in predicting imminent and significant global climate change.
The hockey stick graph purported to show the global mean temperature was relatively constant through the first nine hundred years of the past millennium and then rose sharply in the twentieth century. That graph accordingly takes the shape of a hockey stick lying on the ground with its blade poking up in the air. It was featured as proof of global warming in the Third Assessment Report of the IPCC, released in 2001.
Since the article’s first appearance, a number of papers have been published challenging either the hockey stick’s reconstruction of past temperatures (as did Esper et al. in the March 2002 edition of Science) or Mann’s overall handling of data (as did Chapman et al. in the April 15, 2004 issue of Geophysical Research Letters).
The brief notice in Nature did not contain any corrections beyond an uninformative list of data errors, but instead referred readers to the Web site http://www.nature.com/nature, where they can find acknowledgment of changes to the study’s methodology (referred to as “an expanded description of the methodological details”).
Errors Undermine Results
This admission of error came as a result of an article by statistics expert Stephen McIntyre and Ross McKitrick, associate professor of economics at the University of Guelph, which exposed serious errors in the data and methodology other than the 1998 article. The editors of Nature agreed and required Mann and his collaborators to acknowledge their mistakes and revise their methodology accordingly.
“Corrigendum: Global-scale temperature patterns and climate forcing over the past six centuries,” the correction published in the July 1 issue of Nature, ends with the statement, “None of these errors affect our previously published results.” McIntyre and McKitrick strongly dispute that claim: “We have done the calculations and can assert categorically that the claim is false. We have made a journal submission to this effect and will explain the matter fully when that paper is published.”
The Mann correction was not published as an Addendum, which, according to Nature‘s published policy, is done when “Authors inadvertently omitted significant information available to them at the time” but which does “not contradict the original publication.” Nature publishes Corrigenda only “if the scientific accuracy or reproducibility of the original paper is compromised.” Nature‘s designation of the correction as a corridengum contradicts the article authors’ claim that the errors in the original paper did not affect the published results.
Theory Losing Support
Mann, an assistant professor of environmental sciences at the University of Virginia, had ferociously defended his hockey stick papers and had launched several personal attacks on McIntyre and McKitrick. The corrigendum listed five references but did not cite the paper by McIntyre and McKitrick (“Corrections to the Mann et al. (1998) Proxy Data Base and Northern Hemisphere Average Temperature Series,” Energy and Environment 14(6)) that first drew attention to the mistakes in the original “hockey stick” article.
In June, Mann had published in the Journal of Geophysical Research a correction to another article by him, which had appeared in a previous issue. The correction came after complaints from other paleoclimatologists that his methodology in the paper did not show as big a warming trend from the end of the Little Ice Age as was necessary to support his conclusions, and in particular the hockey stick graph. The criticisms showed Mann had underestimated how cold the Little Ice Age was.
The full debate over the hockey stick controversy can be followed at Ross McKitrick’s Web site, http://www.uoguelph.ca/~rmckitri/ research/trc.html. Until the issue is resolved more convincingly, skeptics say, the “hockey stick” graph cannot be cited as reliable.
Iain Murray ([email protected]) is a senior fellow of the Competitive Enterprise Institute (http://www.cei.org), where he specializes in the debate over climate change and the use and abuse of science in the political process. Myron Ebell ([email protected]) oversees global warming and international environmental work at CEI and chairs the Cooler Heads Coalition, a subgroup of the National Consumers Coalition (http://www.consumeralert.org/) that focuses on climate-change issues.