After a comprehensive review of climate change data, the nation's preeminent scientific body found that average temperatures on Earth had risen by about 1 degree over the last century, a development that "is unprecedented for the last 400 years and potentially the last several millennia."
The report from the National Research Council 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 from the council, part of the National Academy of Sciences, signals a growing acceptance in Washington of widely held scientific views on the causes of global warming.
The council's review focused on the controversial "hockey stick" graph, which shows Earth's temperature remaining stable for 900 years then suddenly arching upward in the last century. The curve resembles a hockey stick laid on its side.
The panel dismissed critics' charges that fraud and statistical error were responsible for the graph's sharp upward swing, noting that many studies had confirmed its essential conclusions in the eight years since it was first published in the journal Nature.
"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 House Science Committee Chairman Sherwood Boehlert (R-N.Y.), who requested the study in November.
The finding was a rebuke to global warming skeptics and some conservative politicians who have attacked the hockey stick as the work of overzealous scientists determined to shame the government into imposing environmental regulations on big business.
Geophysicist Michael E. Mann of Pennsylvania State University, lead author of the study that debuted the graph, said it was 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 cited various factors as evidence of global warming -- including the melting of polar ice caps and measurements of atmospheric carbon dioxide -- the hockey stick encapsulated the issue in an instantly recognizable way.
"It's a pretty profound, easy-to-understand graph," said Roger A. Pielke Jr., director of the University of Colorado's Center for Science and Technology Policy Research. "Visually, it's very compelling."
The chart drew little attention until it was highlighted in a 2001 report by the United Nations Intergovernmental Panel on Climate Change.
After that, "the hockey stick was everywhere," Pielke said.
It also became an easy target.
"If you are someone who's interested in critiquing climate science," he said, "the hockey stick would be a lightning rod."
One prominent attack came from the House Energy and Commerce Committee chairman, Rep. Joe L. Barton (R-Texas), who last year launched an investigation of Mann and his colleagues. Barton demanded information about their data and funding sources -- an effort widely viewed as an attempt to intimidate the scientists.
Barton's committee has launched an inquiry into the statistical validity of the hockey stick. Larry Neal, the committee's deputy staff director, criticized the National Research Council panel Thursday for having only one statistician among its 12 members.
The crux of the dispute is that thermometers have been used for only 150 years. To determine temperatures before that, scientists rely on indirect measurements, or proxies, such as tree ring data, cores from boreholes in ice, glacier movements, cave deposits, lake sediments, diaries and paintings.
Mann and his collaborators tried to integrate data from many such sources to produce climate records for the last 1,000 years. Their report was filled with caveats and warnings about the uncertainties of their conclusions -- caveats that were overlooked as the research achieved more celebrity.
The panel affirmed that proxy measurements made over the last 150 years correlated well with actual measurements during that period, lending credence to the proxy data for earlier times.
It concluded that, "with a high level of confidence," global temperatures during the last century were higher than at any time since 1600.
Although the report did not place numerical values on that confidence level, committee member and statistician Peter Bloomfield of North Carolina State University said the panel was about 95% sure of the conclusion.
The committee supported Mann's other conclusions, but said they were not as definitive. For example, the report said the panel was "less confident" that the 20th century was the warmest century since 1000, largely because of the scarcity of data from before 1600.
Bloomfield said the committee was about 67% confident of the validity of that finding -- the same degree of confidence Mann and his colleagues had placed in their initial report.
Panel members said Mann's conclusion that the 1990s were the warmest decade since 1000 and that 1998 was the warmest year had the least data to support it.
The use of proxies, they said, does not readily allow conclusions based on such narrow time intervals.
The report said that establishing average temperatures before 1000 was difficult because of the lack of data, but said the trend appeared to indicate that stable temperatures could extend back several thousand years.