An influential and controversial paper asserting that recent warming in the Northern Hemisphere was probably unrivaled for 1,000 years was endorsed on Thursday by a panel convened by the nation's pre-eminent scientific body.
The panel said that a statistical method used in the 1999 study was not the best and that some uncertainties in the work "have been underestimated," and particularly challenged the authors' conclusion that the 1990s were the warmest decade in a millennium.
But in a 155-page report, the 12-member panel convened by the National Academies said "an array of evidence" supported the main thrust of the paper. Disputes over details, it said, reflected the normal intellectual clash that takes place as science tests new approaches to old questions.
The study, led by Michael Mann, a climatologist at Pennsylvania State University, was the first to estimate widespread climate trends by stitching together a grab bag of evidence, including variations in ancient tree rings and temperatures measured in deep holes in the earth.
It has been repeatedly attacked by Republican lawmakers and some industry-financed groups as built on cherry-picked data meant to create an alarming view of recent warming and play down past natural warm periods.
At a news conference at the headquarters of the National Academies, several members of the panel reviewing the study said they saw no sign that its authors had intentionally chosen data sets or methods to get a desired result.
"I saw nothing that spoke to me of any manipulation," said one member, Peter Bloomfield, a statistics professor at North Carolina State University.
He added that his impression was that the study was "an honest attempt to construct a data analysis procedure."
More broadly, the panel examined other recent research comparing the pronounced warming trend over the last several decades with temperature shifts over the last 2,000 years. It expressed high confidence that warming over the last 25 years exceeded any peaks since 1600. And in a news conference here on Thursday, three panelists said the current warming was probably, but not certainly, beyond any peaks since the year 900.
The experts said there was no reliable way to make estimates for surface-temperature trends in the first millennium.
In the report, the panel emphasized that the signficant uncertainties about climate patterns over the last 2,000 years did not weaken the scientific case that the current warming trend was caused mainly by people, through the buildup of heat-trapping greenhouse gases in the Earth's atmosphere.
"Surface temperature reconstructions for periods prior to the industrial era are only one of multiple lines of evidence supporting the conclusion that climatic warming is occurring in response to human activities, and they are not the primary evidence," the report said.
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