WASHINGTON - As 12,000 people gathered in Bali to begin framing a global response to Earth's warming climate, efforts to close a deal that would slow destruction of tropical forests appear to be the best prospect for a concrete achievement from the historic assemblage.

But deforestation is also Exhibit A for the disputes that have made climate negotiations divisive despite widening agreement that global warming is real and largely man-made. While scientific dispute over what causes global warming has ended, the debate over how to address it has just begun.

Deforestation is one of the biggest drivers of the buildup of greenhouse gases in the atmosphere. Each year, tropical forests covering an area at least the size of New York state are destroyed. The carbon dioxide that those trees would have absorbed amounts to 20 percent of the world's greenhouse-gas emissions, about the same as total U.S. emissions.

The bargain is being championed by a dozen of the world's developing countries at the conference, whose ultimate goal is to map out a two-year path aimed at forging a global system for imposing and enforcing reductions in greenhouse-gas emissions.

But the hoped-for compromise - which would give financial rewards to poor nations that slow or halt the destruction of their forests - could still founder amid divisions over who bears how much responsibility for slowing climate change and who should pay for it.

Developing countries that profit from logging or expanded farming and construction are seeking incentives and assistance for preserving their forests or slowing the rate of destruction. But many developed countries do not want to pay other nations for actions that are not taken, and they worry that it would be hard to measure the amount of avoided deforestation.

"The problems tend to start when you get down to the small print," said Yvo de Boer, executive secretary of the U.N. Framework Convention on Climate Change, the treaty organization that oversees international climate negotiations.

Deforestation aside, much of the focus on the Indonesian island will be on the large print. "If things go wrong in Bali, I think we are in deep trouble," de Boer said.

Developed countries may be asked to cut greenhouse-gas emissions 25 to 40 percent during the negotiations among 200 nations, de Boer said.

The goal is climate accords that would take effect after the 2012 expiration of the Kyoto Protocol, which was negotiated a decade ago. Under that treaty, a cap-and-trade system for limiting and creating a market for emissions is in effect in Europe and has become a multibillion-dollar-a-year business.

The United States is to come up with its own plan to cut global-warming gases by mid-2008 and will not commit itself to mandatory caps at the U.N. climate conference, the chief U.S. negotiator said yesterday.

"We're not ready to do that here," said Harlan Watson, the State Department's senior climate negotiator and special representative. "We're working on that, what our domestic contribution would be, and again we expect that sometime before the end of the major-economies process."

Environmentalists accuse the Bush administration of using last September's talks in Washington to subvert the U.N. negotiations and the spirit of the Kyoto Protocol, which requires 36 industrial nations to make relatively modest cuts in "greenhouse" gases. The United States is the only major industrial country to have rejected the Kyoto Protocol.