But “a strong commitment” must be made to more research, improvement of computer models, and development of a global climate observation system in order to more accurately assess the phenomenon.

“Global warming could well have serious adverse societal and ecological impacts by the end of this century,” the report says, including more frequent and heavier rainfall/precipitation, particularly over the higher latitudes. Among U.S. areas that could be affected the most are the major farming areas of the Great Plains.

Eleven of the nation’s top climate scientists, including a Nobel prize winner, produced the report for the Bush Administration, in time for review before his meeting with European Union leaders in Sweden June 14-15.

The president, who has been under fire from environmentalists worldwide and by leaders of many developed nations, for his March announcement that the U.S. would withdraw from the Kyoto protocol on global warming, was expected to offer an alternative plan at the meeting. The Kyoto treaty, signed by then Vice President Al Gore, would impose strict limits on greenhouse gas emissions by developed nations.

“Greenhouse gases are accumulating in Earth’s atmosphere, causing surface temperatures to rise,” said NAS Committee Chairman Ralph Cicerone, chancellor at the University of California, Irvine. “We don’t know precisely how much of this rise to date is from human activities, but based on physical principles and highly sophisticated computer models, we expect the warming to continue because of these greenhouse gas emissions” - chiefly carbon dioxide, generated by burning of fossil fuels, but also methane from livestock and other agricultural sources, ozone, and chlorofluorocarbons (CFCs), such as Freon used for decades in air conditioning systems.

The report notes that Earth’s surface temperatures rose about 1 degree Fahrenheit in the 20th century, but computer models suggest they could go up from 2.5 degrees to 10.4 degrees by the end of the current century if greenhouse gas emissions aren’t curbed.

The warming process “has intensified” in the past 20 years, accompanied by retreating glaciers, thinning arctic ice, rising sea levels, longer growing seasons in many areas, and changing migratory bird patterns, the report says.

While the scientists say the theory that the greenhouse gases are causing the climate change reflects “the current thinking of the scientific community,” they nonetheless caution that “uncertainties about this conclusion remain” because of the wide variability in climate when viewed on the scale of decades and centuries “and the degree of confidence that can be placed on estimates of temperatures going back thousands of years, based on evidence from tree rings or ice cones.”

The best information about past climate variability is obtained from ice cores drilled miles deep in Antarctica and Greenland, which show that temperatures “have changed substantially” over the past 400,000 years. While most occurred over thousands of years, some rapid warming also took place over a period of decades.

“National policy decisions made now, and in the longer term future, will influence the extent of any damage suffered by vulnerable human populations and ecosystems later in this century,” the report said.

Philip Clapp of the National Environmental Trust said the nation’s “highest scientific body has reiterated that the situation is serious and getting worse, and now the president has no excuse not to act,” and Sen. John Kerry, D-Mass., said it “underscoresÖthe vacuum in the administration’s leadership on this issue.”

The Greenpeace organization, which has vigorously criticized the president for pulling out of the Kyoto Treaty, was expected to mount its usual very visible public protests at the meeting in Sweden. “Bush and his corporate allies want to abandon Kyoto - a crime against the climate and the global environment,” said Gerd Leipold, the group’s executive director. In the weeks leading up to a United Nations meeting on the Kyoto Treaty in Bonn, Germany in July, he said, “We will let Bush know what we think of him - knowing that we have the support of millions around the world.”<