Delegates from 174 countries and 230 nongovernment interest groups meet from Monday through June 12 in Bonn, Germany, in the second of five negotiating conferences to culminate with a final agreement being adopted in December in Copenhagen, Denmark.

For the first time negotiators will feel the full weight of the United States, which sat on the sidelines during most of the Bush administration's eight years in government. President Barack Obama's delegates, though low keyed at their debut appearance in March, have won an enthusiastic response to their promise to "make up for lost time."

The documents drafted as working texts by two U.N. committee chairmen present a series of options on core issues: by how much must industrial countries cut emissions of greenhouse gases blamed for global warming; how can they help developing nations adapt to climate change; how can tropical countries be compensated for ending deforestation; and how billions of dollars should be raised and spent annually.

"It is not a neat, streamlined text," said Michael Zammit Cutajar, who authored one document. "It is quite messy, in fact."

Problematic words and phrases are set off in brackets that need further discussion. Even the words "shall/should" are repeatedly bracketed, indicating uncertainty about the level of obligation certain actions would require.

"This meeting will not be an easy meeting," said the chief US delegate Jonathan Pershing. "People will begin to express the hard differences and try to figure out how to find common ground," he said in a conference call with reporters Friday.

UN scientists warn that unrestrained emissions of greenhouse gases will lead to changes in temperatures and rainfall, putting millions of people at risk of more severe droughts and storms, rising sea levels that could flood coastlines and sink entire islands, cause the extinction of plant and animal species and increase health hazards for man.

To avoid the worst of those effects, the scientists of the Intergovernmental Panel on Climate Change said in a seminal 2007 report that emissions, mainly from fossil fuels from heavy industry and vehicles, should peak within the next decade and then quickly decline to limit the warming of the Earth by an average 2 degrees Celsius (3 degrees Fahrenheit).

Negotiations have been hamstrung by a persisting argument between the developing countries, which put the onus on the rich world that caused the problem, and the industrial countries that demand every nation must do its share.

The Copenhagen agreement will succeed the 1997 Kyoto Protocol, which expires in four years. The Kyoto accord required 37 industrial countries to cut their emissions by a total 5 percent from 1990 levels, but required nothing from any other nations _ a fact cited by President George W. Bush when he rejected the treaty.

Now, the obligations taken on by fast-expanding economies like China, India and Brazil is a critical point, said Todd Stern, Obama's special climate change envoy. "We need to have commitments to take major steps than can be quantified," Stern said. Those countries cannot remain "on the voluntary side of the equation forever."

Developing nations are pledging to slow the growth of their emissions, as long as they get the funds and technology from advanced countries to keep their economies growing sustainably. They also demand that together the rich countries reduce their emissions within a 25 to 40 percent range from 1990 levels by 2020.

Stern acknowledged the U.S. cannot meet those targets, arguing the 1990 base line is irrelevant anyway. What counts is what each country is mandated to do over the next 10 to 40 years, he said.

"I don't think you are going to see a 25 to 40 percent aggregate number (from) 1990, although it's possible when you add everything up you won't be very far away from it. But I don't think you're going to get all the way up there."

With so many holes in the agreement remaining to be closed, doubts have been raised that the deal can be done by December in Denmark.

"There's going to have to be a lot of work after Copenhagen, no matter what," said Alden Meyer, of the Washington-based Union of Concerned Scientists. "But the question is whether we can lay out the aggressive, ambitious and equitable elements that we need to get a political agreement, and then fill in the details afterward."