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The Warmest in 1000 Years? Revisiting the Hockey Stick

By Roy Spencer - January 27, 2005 12:00 AM

A science article that has been accepted by Geophysical Research Letters casts serious doubt on the oft-cited claim that global temperatures are warmer now than they have been anytime in the last 1,000 years.

Stephen McIntyre and Ross McKitrick examined the methodology that led Mann et al. (1998) to publish in the popular science journal Nature the famous "hockey stick" shaped temperature curve, which was a centerpiece of the Third Assessment Report of the U.N.'s Intergovernmental Panel on Climate Change (IPCC) in 2001. The hockey stick curve showed a gradual cooling since around 1400 A.D. (the hockey stick handle) then a sharp warming since about 1900 (the blade of the stick). This was taken as proof that the major climatic event of the last 1,000 years was the influence of humans in the 20th century.

As you might imagine, it's a little difficult to construct a temperature history for a period of record that, for the most part, had no reliable thermometer measurements. Since good thermometer measurements extend back to only around the mid-1800's, "proxy" measurements, primarily tree ring data, have been used to extend the temperature record back additional centuries.

McIntyre & McKitrick found that the Mann et al. methodology included a data pre-processing step, one which was not reported in the original study, that essentially guaranteed that a hockey stick curve would result from their analysis. They demonstrated this by applying the same methodology to many synthetic temperature records that were constructed with random noise. In almost every case, a hockey stick curve resulted. The claim of unprecedented warmth and the hockey stick shape appear to hinge on the treatment of one species of tree, the bristlecone pine, from North America in the 1400's. Further statistical tests showed that this critical signal in the early 15th century lacked statistical significance. This suggests that the results of Mann et al. were simply a statistical fluke, which greatly exaggerated a characteristic of the bristlecone pines, which may or may not be related to global temperatures.

The new article, like so much published science, simply points out errors in previously published science, which is the way science should work. So why should there be so much fuss this time? Because the original Mann et al. article has had huge repercussions. The hockey stick, along with the "warmest in 1,000 years" argument, has become a central theme of debates over the Kyoto Protocol, a treaty to limit emissions of greenhouse gases, in governments around the world. The question begging to be answered is: Why did the IPCC so quickly and uncritically accept the Mann et al hockey stick analysis when it first appeared? I cannot help but conclude that it's because they wanted to believe it.

The IPCC leadership can always fall back on the claim that they were only using published research, which is true. The criterion for scientific results to be included in governmental reports has usually been publication in the scientific literature, or in some cases the work only needs to be accepted for publication. But it now appears this is not sufficient. Unusual claims in science should be met with unusual skepticism, and this did not happen with the Mann et al. study. An increasing number of researchers have anecdotal evidence that the science tabloids, Nature and Science, select reviewers of some manuscripts based upon whether they want those papers to be accepted or rejected. In other words, it seems like the conclusions of a paper are sometimes more important that the scientific basis for those conclusions. Since those periodicals have profit and popularity motives that normal scientific journals do not, maybe the time has come to downgrade the scientific weight of publications in those journals, at least for some purposes.

It took two non-climate people -- the global warming debate's equivalent of internet bloggers -- to do what should have been done by an independent paleoclimate research group. Both are Canadians: McIntyre is a consultant for mineral exploration, and McKitrick is a professor of economics. They received no outside funding for their work, so they can not be accused of being bought off. It surely won't help the IPCC's reputation that this latest development follows on the heels of the recent resignation of the IPCC's leading hurricane expert over editorial bias in the IPCC leadership.

It will be interesting to see if the IPCC, and its member countries, continue to rally around the hockey stick, or discard it. At the very least, the IPCC process, now starting working toward its next report, will likely be a little more careful about what conclusions it draws from published research. A multi-governmental panel like the IPCC is necessary, I believe, to provide guidance on the state of global warming science. The trouble is that it is run by humans, all of whom have their own biases. I don't have a solution for that problem. After watching the Terminator movies, I don't think I would want to put machines in charge of it, either.

The good news is that the scientific method works. Independent testing of scientific results, especially controversial ones, adds confidence to the science and lends credibility in the minds of the public and policymakers. As it stands now, however, today's article by McIntyre and McKitrick will only further damage the reputation of the IPCC leadership and call into question its objectivity.

(Additional information about the McIntyre and McKitrick work, including referenced publications, can be found at )



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