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Academy affirms hockey-stick graph

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But it criticizes the way the controversial climate result was used.

Washington DC

It's probably the most politicized graph in science — an icon of the case for climate change to some, and of flawed science in the service of that case to others — and it has coloured the climate-change debate for nearly a decade. Now the US National Academy of Sciences (NAS) has weighed in with a report on the ‘hockey-stick’ plot, which it hopes will finally lay the controversy to rest.

The graph purports to chart global temperatures over the past millennium; a sharp rise at the current end is the ‘blade’ that makes the otherwise flattish line look like a hockey stick. Climate groups have claimed it as evidence of dangerous global warming; sceptics, especially in the United States and Canada, have questioned the study's merit and statistical methodology.

Cause for controversy: Michael Mann used proxies for climate change, such as tree rings, to produce a picture of Earth's changing climate over the past millennium. Note: We recommend downloading a pdf to view this graphic. Credit: IPCC

In its report, released on 22 June, the NAS committee more-or-less endorses the work behind the graph. But it criticizes the way that the plot was used to publicize climate-change concerns. And it leaves open big questions about whether researchers should be obliged to make their data available (see Plotting a course).

“We roughly agree with the substance of their findings,” says Gerald North, the committee's chair and a climate scientist at Texas A&M University in College Station. In particular, he says, the committee has a “high level of confidence” that the second half of the twentieth century was warmer than any other period in the past four centuries. But, he adds, claims for the earlier period covered by the study, from AD 900 to 1600, are less certain. This earlier period is particularly important because global-warming sceptics claim that the current warming trend is a rebound from a ‘little ice age’ around 1600. Overall, the committee thought the temperature reconstructions from that era had only a two-to-one chance of being right.

The graph arose from the work of Michael Mann, a climatologist now at Pennsylvania State University in University Park, and two colleagues. In two papers published in 1998 and 1999, Mann's team examined tree rings, ice cores and other ‘proxies’ of past climate, and used them to reconstruct the Northern Hemisphere's temperature over the past millennium (M. E. Mann et al. Nature 392, 779–787; 1998 and M. E. Mann et al. Geophys. Res. Lett. 26, 759–762; 1999).

The analysis was complex because the proxies were geographically dispersed and contained uncertainties that are often difficult to gauge. For example, the growth of bristle-cone pine trees, which played an important role in the Mann study, depends on temperature, but also rainfall. The researchers concluded in their 1999 paper that “the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium”, and included a graph showing a sharp upturn in temperature from about 1900 onwards. The plot soon became known as the hockey stick, and was featured prominently in the executive summary for policy-makers in the 2001 report on global warming from the Intergovernmental Panel on Climate Change (IPCC).

Shortly after it appeared in the report, two Canadians, economist Ross McKitrick and mineral-exploration consultant Stephen McIntyre, attacked the methodology behind the graph, claiming that it was based on insufficient data and flawed statistical analysis. US politicians amplified their complaints, most prominently Representative Joe Barton (Republican, Texas), who in 2005 wrote to Mann demanding he share his data with critics and congressional overseers. In an effort to quell the controversy, the chairman of the House Committee on Science, Representative Sherwood Boehlert (Republican, New York), commissioned the academy to examine the earlier work.

The academy essentially upholds Mann's findings, although the panel concluded that systematic uncertainties in climate records from before 1600 were not communicated as clearly as they could have been. The NAS also confirmed some problems with the statistics. But the mistakes had a relatively minor impact on the overall finding, says Peter Bloomfield, a statistician at North Carolina State University in Raleigh, who was involved in the latest report. “This study was the first of its kind, and they had to make choices at various stages about how the data were processed,” he says, adding that he “would not be embarrassed” to have been involved in the work.

Panel members were less sanguine, however, about whether the original work should have loomed so large in the executive summary of the IPCC's 2001 report. “The IPCC used it as a visual prominently in the report,” says Kurt Cuffey, a panel member and geographer at the University of California, Berkeley. “I think that sent a very misleading message about how resolved this part of the scientific research was.”

“No individual paper tells the whole story,” agrees North. “It's very dangerous to pull one fresh paper out from the literature.”

Mann says that he is “very happy” with the committee's findings, and agrees with the core assertion that more must be done to reduce uncertainties in earlier periods. “We have very little long-term information on the Southern Hemisphere and large parts of the ocean,” he says. As for the report's effect on the policy debate, Mann says: “Hopefully this is the beginning of us, as a community, putting that silliness behind us.”

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