Academy Affirms Hockey-Stick Graph

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Vol 441|29 June 2006 NEWS Academy affirms hockey-stick graph

WASHINGTON DC IPCC 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 tempera- tures 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. In its report, released on 22 June, the NAS committee more-or-less endorses the work behind the graph. But it criticizes the way Cause for controversy: Michael Mann used proxies for climate change, such as tree rings, to produce a that the plot was used to publicize climate- picture of Earth’s changing climate over the past millennium.

change concerns. And it leaves open big ques- COGHILL T. tions about whether researchers should be warming sceptics claim that the current warm- and M. E. Mann et al. Geophys. Res. Lett. 26, obliged to make their data available (see ing trend is a rebound from a ‘little ice age’ 759–762; 1999). ‘Plotting a course’). around 1600. Overall, the committee thought The analysis was complex because the prox- “We roughly agree with the substance of their the temperature reconstructions from that era ies were geographically dispersed and con- findings,” says Gerald North, the committee’s had only a two-to-one chance of being right. tained uncertainties that are often difficult to chair and a climate scientist at Texas A&M Uni- The graph arose from the work of Michael gauge. For example, the growth of bristle-cone versity in College Station. In particular, he says, Mann, a climatologist now at Pennsylvania pine trees, which played an important role in the committee has a “high level of confidence” State University in University Park, and two the Mann study, depends on temperature, but that the second half of the twentieth century colleagues. In two papers published in 1998 also rainfall. The researchers concluded in was warmer than any other period in the past and 1999, Mann’s team examined tree rings, ice their 1999 paper that “the 1990s are likely the four centuries. But, he adds, claims for the cores and other ‘proxies’ of past climate, and warmest decade, and 1998 the warmest year, in earlier period covered by the study, from used them to reconstruct the Northern Hemi- at least a millennium”, and included a graph AD 900 to 1600, are less certain. This earlier sphere’s temperature over the past millennium showing a sharp upturn in temperature from period is particularly important because global- (M. E. Mann et al. Nature 392, 779–787; 1998 about 1900 onwards. The plot soon became PLOTTING A COURSE The US National Academy of demanding that, among other Barton and his committee have that the NAS is putting together a Sciences (NAS) report may help things, they list their funding commissioned their own statistical panel to examine issues of data put the ‘hockey-stick’ debate to agencies and provide access to analysis, which is ongoing. sharing in all fields. The panel, rest, but it leaves open a larger their raw data and algorithms. The NAS panel largely demurred which will convene later this question about who should have Letters were also sent to the on questions of data sharing, saying summer, “will be a broad look at access to researchers’ data. National Science Foundation and that although openness is generally access across the board”, says Mann’s critics complained the Intergovernmental Panel on good, such issues are “discipline Kearney. that he would not share the Climate Change demanding dependent”. “We thought that For now, politicians will probably methodological details or raw data information on the work (see question was a little big for what intervene as they see fit, says David that would enable them to assess Nature 436, 7; 2005). Many in the we were trying to do here,” says Goldston, chief of staff for the his work. In June 2005, scientific community were shocked committee chair Gerald North. House Committee on Science: “As congressman Joe Barton by the letters’ aggressive tone, but That doesn’t mean the matter is the scientific community continues (Republican, Texas) wrote to the Mann did supply information about closed. Bill Kearney, the academy’s to wrestle with this, it will continue hockey-stick study’s authors how to access the data he used. director of media relations, says to be an issue.” G.B.

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NATURE|Vol 441|29 June 2006 NEWS

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 KAEHLER/CORBIS W. 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 com- plaints, 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 chair- man of the House Committee on Science, Representative Sherwood Boehlert (Repub- lican, New York), commissioned the academy to examine the earlier work. The academy essentially upholds Mann’s findings, although the panel concluded that Experts comb tropics for clues to vCJD systematic uncertainties in climate records from before 1600 were not communicated Some people in Papua New transmitted from cows infected infected. The longest as clearly as they could have been. The NAS Guinea who once feasted on with bovine spongiform incubation time was calculated also confirmed some problems with the their own relatives did not encephalopathy, might cause a to be at least 56 years, and statistics. But the mistakes had a relatively succumb to the prion disease similar epidemic among those perhaps seven years longer — minor impact on the overall finding, says kuru until 50 years later, say who ate infected meat in the although the average Peter Bloomfield, a statistician at North researchers who have 1980s and 1990s. So far, only incubation time seems to be 12 Carolina State University in Raleigh, who laboriously tracked down the 156 deaths from vCJD have years (J. Collinge et al. Lancet was involved in the latest report. “This study last sufferers in remote villages. been reported in Britain, the 367, 2068–2074; 2006). “For was the first of its kind, and they had to The discovery renews concern worst-affected country, and the the first time we can see the make choices at various stages about how that another human prion number of new cases peaked in extraordinary incubation the data were processed,” he says, adding disease, variant Creutzfeldt– 2000, suggesting that vCJD period in human prion disease,” that he “would not be embarrassed” to have Jakob disease (vCJD), might be takes about ten years to says Collinge. “It’s sobering been involved in the work. incubating silently in some incubate. But debate continues that, half a century on, this Panel members were less sanguine, how- populations and could rear its about whether another wave of disease has not disappeared.” ever, about whether the original work head decades from now. cases could yet appear. Collinge says that vCJD should have loomed so large in the executive Neurologists have long been John Collinge at University could have a much longer summary of the IPCC’s 2001 report. “The fascinated by kuru, which College London and his average incubation time, of IPCC used it as a visual prominently in the caused an unprecedented colleagues went to Papua New 30 years or more, because the report,” says Kurt Cuffey, a panel member epidemic of neurodegenerative Guinea to find out. Most people prions are passing from cows and geographer at the University of disease in the Fore people of with kuru have already died, but to humans rather than between California, Berkeley. “I think that sent a very Papua New Guinea that peaked the team ramped up existing humans. A species barrier misleading message about how resolved this in the 1950s and early 1960s. In disease monitoring to find the extends incubation times in part of the scientific research was.” death rituals, families steamed last of the epidemic. Working animal tests. People who have “No individual paper tells the whole and ate the bodies of their with local communities, they already succumbed to vCJD story,” agrees North. “It’s very dangerous to relatives — along with, it was scoured isolated villages that might have been particularly pull one fresh paper out from the literature.” later discovered, infectious are typically more than 2,000 genetically susceptible, as Mann says that he is “very happy” with prion proteins that caused the metres above sea level, lost in other evidence has suggested. the committee’s findings, and agrees with debilitating and fatal disease. dense rainforest and connected Mathematical models used the core assertion that more must be done to The ritual was prohibited in the only by tracks. “It’s arduous to predict the size of a vCJD reduce uncertainties in earlier periods. “We mid-1950s by the Australian trekking,” says Collinge. epidemic could now include have very little long-term information on authorities who governed that Between 1996 and 2004, the these findings. “Most people the Southern Hemisphere and large parts of part of Papua New Guinea, and researchers found what they seem to think we’re over the the ocean,” he says. As for the report’s effect the disease eventually became believe are the last 11 cases of worst,” Collinge says. “We have on the policy debate, Mann says: “Hopefully less frequent. kuru. Patients’ histories were to be cautious about assuming this is the beginning of us, as a community, Interest in kuru reawakened collected, to piece together this disease is going away.” ■ putting that silliness behind us.” ■ with the realization that vCJD, when they were probably Helen Pearson Geoff Brumfiel