WASHINGTON -- Starting Monday, negotiators in Copenhagen will try to nail down all of the main elements of a treaty to curb global warming, but a final agreement won't be possible until the United States figures out what it is willing to do to reduce emissions of heat-trapping pollution.

President Barack Obama plans to visit the talks on their final day to promise that the United States will cut its share of emissions and to press for a strong agreement. The world, however, will be watching to see whether he also signals a willingness to pressure Congress to enact the law that's needed to make that happen.

Despite charges by some critics that data on global temperatures have been altered, the evidence of climate change is conclusive and worsening.

In the past few years, Arctic summer sea ice shrank so much that even glaciologists were startled. For the first time, scientists reported that Greenland and Antarctica are shrinking as ice breaks off into the oceans faster. At the same time, the long-term trend of warming continues, according to temperature data kept by U.S. government agencies.

``It's extremely hard to get a full-blown treaty without the United States,'' said Dan Esty, a professor of environmental law and policy at Yale University. ``We would have had, if the United States had come ready to negotiate on the question of targets and timetables, a chance to bring China and India into the fold.''

China and India have agreed to improve energy efficiency, meaning the growth of their emissions will be reduced. However, they're resisting any deadline for when their emissions should peak and decline.

``I think it's going to take presidential leadership to get the U.S. in a position to really move forward on climate change,'' Esty said.

The Copenhagen meeting, scheduled to last two weeks, was designed to reach agreement on:

• How much industrialized countries will reduce heat-trapping pollution.

• How much major developing countries can limit the growth of emissions.

• How much developed countries will help developing ones adapt to unavoidable climate changes and shift to cleaner energy, in the next three years and over the decades ahead.

The U.S. offer, made Nov. 25, is about a 17 percent reduction from 2005 levels by 2020. Using the 1990 baseline, the international benchmark, that's only a 4 percent reduction. An assessment by climate scientists in 2007 said industrial countries need to cut emissions by 25 percent to 40 percent between the baseline year of 1990 and 2020.

Yvo de Boer, the U.N. official overseeing the negotiations, said the industrialized world's pledges taken together were ``not yet where science says we need to be to avoid the worst impacts of climate change.''

Jonathan Lash, the president of the World Resources Institute, an environmental policy research organization, said it was understandable that Obama didn't want to overstep what Congress had indicated it might do.

``I think the most important thing is to have an unequivocal signal that we're on a path to de-carbonize the U.S. economy,'' Lash said. ``All of the industry leaders I work with in USCAP'' -- U.S. Climate Action Program, an environment-business group -- ``are looking for that certainty. I think that once that certainty is there, the change in the U.S. economy will go very quickly.''

Lash said that Obama's plan to attend the Copenhagen talks was ``a very strong indication of his commitment to action,'' but that he hoped the president would use the international stage to give Congress a deadline to pass a climate and energy law.

Competing for the attentions of Congress and Obama is revamping healthcare in America.

The Energy Action Coalition, a group of 50 youth organizations, started a campaign last month that it calls ``It's Game Time, Obama.''

The organization worked to turn out the youth vote for Obama and appreciates his work on climate change so far, said Jessy Tolkan, the group's executive director.

Nonetheless, the coalition plans to keep up pressure on the president and members of Congress for a strong climate bill, she said, because ``there's no chance to get that legislation if he's not out there selling it to the American people.''