22 June 2006The New York Times
A controversial paper asserting that recent warming in the Northern Hemisphere was probably unrivaled for 1,000 years was endorsed today, with a few reservations, 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 it particularly challenged the authors' conclusion that the decade of the 1990's was probably the warmest 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 E. Mann, a climatologist now 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 business-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 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 today, 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 A.D.
In the report, the panel stressed that the significant remaining 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 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.
The 1999 paper is part of a growing body of work trying to pull together widely disparate clues of climate conditions before the age of weather instruments.
The paper includes a graph of temperatures in the Northern Hemisphere that gained the nickname "hockey stick" because of its vivid depiction of a long period with little temperature variation for nearly 1,000 years, followed by a sharp upward hook in recent decades.
The hockey stick has become something of an environmentalist icon. It was prominently displayed in a pivotal 2001 United Nations report concluding that greenhouse gases from human activities had probably caused most of the warming measured since 1950. A version of it is in the Al Gore documentary "An Inconvenient Truth."
Senator James M. Inhofe, Republican of Oklahoma, and Representative Joe Barton, Republican of Texas, have repeatedly criticized the Mann study, citing several peer-reviewed papers challenging its methods.
The main critiques were done by Stephen McIntyre, a statistician and part-time consultant in Toronto to minerals industries, and Ross McKitrick, an economist at the University of Guelph in Ontario.
They contended that Dr. Mann and his colleagues selected particular statistical methods and sets of data, like a record of rings in bristlecone pine trees, that were most apt to produce a picture of unusual recent warming. They also complained that Dr. Mann refused to share his data and techniques.
In an interview, Dr. Mann expressed muted satisfaction with the panel's findings. He said it clearly showed that the 1999 analysis has held up over time.
But he complained that the committee seemed to forget about the many caveats that were in the original paper. "Even the title of the paper on which all this has been based is as much about the caveats and uncertainties as it is about the findings," he said.
Raymond S. Bradley, a University of Massachusetts geoscientist and one of Dr. Mann's co-authors, said that the caveats were dropped mainly as the graph was widely reproduced by others. (The other author of the 1999 paper was Malcolm K Hughes of the University of Arizona.)
The report was done at the request of Representative Sherwood Boehlert, the New York Republican who is chairman of the House Science Committee, who called last November for a review of the 1999 study and related research to clear the air.
In a statement, Mr. Boehlert, who is retiring at the end of the year, expressed satisfaction with the results, saying, "There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change — which doesn't rest primarily on these temperature issues, in any event — or any doubts about whether any paper on the temperature records was legitimate scientific work."
