Prestigious scientific panel backs global-warming data

After a comprehensive review of climate-change data, the pre-eminent U.S. scientific body found average temperatures on Earth have risen about 1 degree over the past century, a development that "is unprecedented for the last 400 years and potentially the last several millennia."

The report from the National Academies of Science also concluded that "human activities are responsible for much of the recent warming."

Coupled with a report last month from the Bush administration's Climate Change Science Program that found "clear evidence of human influences on the climate system," the new study signals a growing acceptance in Washington of widely held scientific views on the causes of global warming.

The academies' review focused on the controversial "hockey stick" graph, which shows Earth's temperature rising abruptly to its highest point in 1,000 years after a long period of stability.

The panel dismissed critics' charges that fraud and error were responsible for the graph's sharp upward swing, noting many studies had confirmed its essential conclusions in the eight years since it was first published in the journal Nature.

The finding was a rebuke to skeptics and some conservative politicians, who have repeatedly attacked the hockey stick as the work of overzealous scientists determined to shame the government into imposing environmental rules on big business.

"There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change ... or any doubts about whether any paper on the temperature records was legitimate scientific work," said Rep. Sherwood Boehlert, R-N.Y., chairman of the House Science Committee, who had requested the study.

Geophysicist Michael Mann of Pennsylvania State University, lead author of the study that debuted the hockey-stick graph, said it's time "to put this sometimes silly debate behind us and move forward, to do what we need to do to decrease the remaining uncertainties."

Though scientists have documented global warming in myriad ways — including the melting of polar ice caps — the hockey stick stood out for encapsulating the issue in an instantly recognizable way.

The graph shows a stretch of stable temperature lasting for 900 years that suddenly arcs upward in the last century, resembling a hockey stick laid on its side.

One attack came from Rep. Joe Barton, R-Texas, who last year launched an investigation of Mann and his colleagues. The chairman of the House Energy and Commerce Committee demanded information about their data and funding sources, an effort widely viewed as an attempt to intimidate the scientists.

The crux of the dispute is that humans have had thermometers for only 150 years. To determine temperatures before that time, scientists have to rely on indirect measurements, or proxies, such as tree-ring data, cores from boreholes in ice, glacier movements and lake sediments.

The academies panel affirmed that proxy measurements made over the past 150 years correlate well with actual measurements during that period, giving confidence the proxies provide an accurate picture of earlier times.