Academy Affirms 'Hockey-Stick' Graph

US National Academy of Science affirms hockey-stick graph but it criticizes the way the controversial climate result was used.

"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.

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.