Science academy supports global warming findings

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

But the National Academy of Sciences report on the so-called “hockey stick graph” – a much-discussed chart showing a sudden rise 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 141-page report, written by a dozen prominent scientists, 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 by Congress to provide the government with advice on scientific issues.

NASA scientists have concluded from direct temperature measurements that 2005 was the hottest year on record, with 1998 as a close second. Because direct temperature readings only date back to 1860, however, Pennsylvania 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 Cuffey, a geography professor at the University of California at Berkeley, said at a press briefing that the report “essentially validated” the conclusions Mann reported in 1998 and 1999 papers using temperature records. The NAS panel also estimated there is a roughly 67 percent chance that Mann was right in saying the past 25 years were the warmest in 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 Boehlert, R-N.Y., sought the NAS study last year after Energy and Committee Chairman Joe Barton, R-Texas, a global warming skeptic, subpoenaed Mann’s computer programs, funding sources and other documents.

Boehlert said in a statement Thursday that the NAS “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 Senate Environment and Public Works Committee Chairman James Inhofe, R-Okla., another skeptic, said the academy’s report highlighted 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 nonclimate academics in Canada – mathematician and industry consultant Steven 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. Other scientists found errors in their analysis.