for National Geographic News
The last two decades of the 20th century were the hottest in 400 years and quite likely the warmest for several millennia, a leading U.S. scientific body concludes in a new report.
The National Academies' National Research Council report also said "human activities are responsible for much of the recent warming."
The U.S. Congress had requested the report after controversy arose last year over surface-temperature reconstructions published in the 1990s by climatologist Michael Mann, now at Pennsylvania State University, University Park, and colleagues.
The reconstructions resulted in the widely cited "hockey stick" graph, which shows Earth's temperature sharply rising in recent decades after a thousand years of stability. The graph looks like a hockey stick lying on its side.
To create the graph, Mann and his colleagues pulled together temperature evidence from specimens such as tree rings, corals, and cores of sediment and ice. They had to rely on this natural evidence because thermometer records go back only about 150 years.
The graph gained prominence when the United Nations published it in a 2001 report that concluded that greenhouse gases from human activities had probably caused most of the warming measured since 1950, according to the New York Times.
Critics of the graph have said that Mann and his colleagues based it on cherry-picked and erroneous data.
Gerald North is a geoscientist at Texas A&M University in College Station. He chaired the National Research Council panel that produced the new report, which was released yesterday.
At a press conference Thursday at the National Academies headquarters in Washington, D.C., North said Mann and his colleagues prepared their data in a professional manner.
"I certainly did not see anything inappropriate. I mean, there might have been things that maybe they could have done differently, better, but I have no cause to think there was anything inappropriate," he said.
Mann, in an interview with National Geographic News, said the new report "nailed the coffin on the skeptics."
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