"There is going to be large change," said atmospheric scientist David Battisti of the University of Washington in Seattle. "The risks are very large."

The group met at the American Association for the Advancement of Science to try to drive home the message that climate change is already under way.

"You hope that somehow people will understand that we have got to do something now," Joyce Penner, an atmospheric scientist at the University of Michigan, said in an interview. "Some people get it - some people are driving hybrids. But there is a problem with the American public."

Climate experts around the world agree one first step to battling the buildup of polluting gases that is warming the Earth is an agreement called the Kyoto Protocol, a 1997 pact aimed at reducing greenhouse emissions.

More than 120 nations have ratified the pact or acceded to it. President Bush pulled out in 2001, arguing Kyoto was too expensive and unfairly excluded developing nations. The United States is the world's biggest polluter, producing 36 percent of warming emissions.

Bush's advisers also cite doubts about models that predict the course of global warming.

UNDERESTIMATING THE THREAT

The climate experts agreed there is debate about the models but say if anything, they underestimate the extent of the problem.

"The models ... are good enough to tell us we ought to be starting now to do what we can to reduce emissions," said Michael Oppenheimer, a professor of environmental science at Princeton University.

Oppenheimer said sea levels have risen 4 inches (9 cm) already over the past century and could rise between 4 and 40 inches (9 to 88 cm) more in the next century.

Both the Greenland and the West Antarctic ice sheets are "highly vulnerable" to global warming, Oppenheimer said. If completely melted, the Greenland ice sheet would add 25 feet to overall sea level and the West Antarctic Ice Sheet would raise it by 16 feet.

This would be enough to swamp most of Florida, Bangladesh and Manhattan, he said.

"The sea level rise over the past century appears greater than what the model says it should be," Oppenheimer said. "The ice sheets may be contributing more than the models predict."

The researchers said they hoped to convince the U.S. public to pressure politicians into policy changes.

"In this country it depends a lot on what happens in the next election," said geochemist Daniel Schrag of Harvard University. "I don't think we can expect to change the minds of this administration in the next couple of months."

Schrag said the current concentration of carbon dioxide in the atmosphere is 380 parts per million - higher than it has been for at least the past 430,000 years.

"In the next 100 years, unless immediate action is taken, carbon dioxide levels will rise to between 800 and 1,000 parts per million. The last time carbon dioxide was that high was during the Eocene, 55 to 36 million years ago," Schrag added.

At that time he said "palm trees lived in Wyoming, crocodiles lived in the Arctic, Antarctica was a pine forest and sea level was at least 300 feet higher than today."