Past Few Decades Warmest on Record, Study Confirms

By Juliet Eilperin
Washington Post Staff Writer
Friday, June 23, 2006; 12:00 AM

An independent scientific panel largely ratified the findings of a controversial climate study yesterday, saying the past few decades amount to the hottest period in the last 400 years.

But the National Academy of Sciences report on the "hockey stick graph" -- a much-discussed chart showing a sudden rise in temperatures in the Northern Hemisphere since the Industrial Revolution began -- voiced less confidence about the graph's conclusion that the climate is hotter now than it has been in 1,000 years. As a result, the academy report is not likely to resolve the fierce debate over the extent to which human-generated greenhouse gas emissions are responsible for warming the earth.

The new report provides ammunition to those who say the evidence is overwhelming that industrial activity is transforming the planet by spewing carbon dioxide into the atmosphere, as well as to those who see it as confirmation that significant uncertainty still exists in climate change science.

The report concludes "it can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries." The academy is chartered to advise the government on scientific issues.

NASA scientists have concluded from direct temperature measurements that 2005 was the hottest year on record, with 1998 a close second. Because direct temperature readings only date back to 1860, however, Penn State University climatologist Michael Mann and two colleagues used "proxy" data from ice cores, coral reefs and lake sediments to estimate temperatures back 1,000 years in creating the "hockey stick" -- which gained its name from its distinctive shape.

Panel member Kurt M. Cuffey, a geography professor at the University of California at Berkeley, said at a news briefing that the report "essentially validated" the conclusions Mann reported in 1998 and 1999 using temperature records. The panel also estimated there is a roughly 67 percent chance that Mann is right in saying the past 25 years were the warmest in a 1,000 years. But it noted it is difficult to draw conclusions on temperatures before 1600 because, Cuffey said, "you start relying more and more on data from fewer geographic locations."

House Science Committee Chairman Sherwood L. Boehlert (R-N.Y.) sought the study last year after Energy and Committee Chairman Joe Barton (R-Tex.), a global-warming skeptic, subpoenaed Mann's computer programs, funding sources and other documents.

Boehlert said in a statement yesterday that the academy "shows the value of Congress handling scientific disputes by asking scientists to give us guidance. The report clearly lays out a scientific consensus position on the historic temperature record."

But Sen. James M. Inhofe (R-Okla.), another skeptic, said the academy's report highlights the flaws in Mann's conclusions: "Today's NAS report reaffirms what I have been saying all along, that Mann's 'hockey stick' is broken."

Two non-climate academics in Canada -- mathematician and industry consultant Steve McIntyre and University of Guelph economist Ross McKitrick -- have questioned Mann's methodology. They published a paper in the journal Geophysical Research Letters two years ago suggesting that Mann's approach underplayed the importance of natural variability and that temperatures in the 15th century rivaled current levels.

Mann said in an interview that some news reports and advocates had taken his work "out of context" by minimizing the uncertainty he and his co-authors had acknowledged about their findings.

Researcher Rena Kirsch contributed to this report.

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