Climate change is already occurring and immediate steps are needed to both slow it down and adapt to the changes that will occur anyway, scientists said Tuesday.

There is no question there will be effects from climate change, Chris Field of the Carnegie Institution of Washington (search) said at a briefing at the American Association for the Advancement of Science (search).

"We are already seeing impacts, the question is, at what level will we decide it is a problem," Field said.

William Easterling of Pennsylvania State University said: "The time to act is now." He spoke at a separate briefing held by the Pew Center on Global Climate Change (search).

Climate change, specifically warming, has become a growing concern for many scientists, who worry that industrial exhaust and other gases in the atmosphere are raising temperatures and will damage crops and human health, raise the sea level and cause other problems.

They cite records showing an average worldwide temperature increase of about 0.6 degrees Fahrenheit over the past century.

Some scientists disagree, however, saying the computer models that forecast climate changes are not yet accurate enough to be used as a basis for policy changes.

The climate models aren't good enough to say exactly how global warming is going to come out, but "they are good enough to tell us we should be doing something." Michael Oppenheimer of Princeton University said at the AAAS briefing.

Policy-makers make decisions all the time without having complete proof, argued Thomas Crowley of Duke University.

In December, the Bush administration said it is planning a five-year program of research on climate change.

And in April, White House science adviser John H. Marburger III denied charges that the administration refuses to accept the reality of global warming.

He noted that Bush said in a 2001 Rose Garden speech that the concentration "of greenhouse gases, especially CO2, have increased substantially since the beginning of the Industrial Revolution."

The National Academy of Sciences (search) has indicated that the increase is due in large part to human activity, Marburger said, and "while scientific uncertainties remain, we can now begin to address the factors that contribute to climate change."

Daniel Schrag of Harvard University likened prompt action to insurance, saying there is a risk of catastrophe in the future and steps need to be taken to help avoid it.

David Battisti of the University of Washington, also speaking at the AAAS session, said the amount of carbon dioxide in the air is likely to triple in the next 100 to 150 years.

The result, he said, could melt some polar ice, raising sea levels and severely damaging low-lying areas.

That rise could inundate low-lying parishes in the delta area of Louisiana and as much as one-third of Florida, warned Easterling.

Commented Oppenheimer: If sea level rise occurs gradually, a sea wall could be built to protect places like New York, but not large areas like Florida or Bangladesh.

Eileen Claussen, president of the Pew Center, said its report drew four conclusions:

— Even if measures to reduce global warming are put into place today some increase will still occur and ways will be needed to adapt to it.

— Adapting will be challenging, costly and imperfect.

— Ecosystems around the world are already being affected by global warming.

— Acting in advance of problems is necessary to reduce damage.