NUSA DUA, Indonesia - The fight against global warming has given a boost to a long-stymied environmental cause: saving the rain forests.

Under a scenario that has gained widespread support, developing countries would be paid billions of dollars a year to not raze their trees.

The money would come from rich industrial nations, paying to offset their greenhouse gas emissions from burning fossil fuels.

"Every single effort that has been attempted to conserve tropical forests for the last 35 years has failed," said Don Melnick, a Columbia University biologist who is advising a coalition of rain forest countries. "We're looking at perhaps one of the last great opportunities."

The idea has the backing of environmentalists and some of the world's most voracious destroyers of forest, including Brazil and Indonesia. Getting the idea launched would be a major achievement of negotiations at the U.N. Climate Change Conference here on the Indonesian island of Bali to create a successor to the Kyoto Protocol, which expires in 2012.

But it has faced concerns that it could evolve into a multibillion-dollar crusade that will do little to save forests or reduce carbon emissions.

The destruction of tropical forests accounts for 20 percent of yearly greenhouse gas emissions.

Vast amounts of carbon stored in wood, leaves and soil are released into the atmosphere when forests are burned to clear land for cattle grazing or farming. The carbon also flows into the atmosphere as trees, stumps, branches and other scraps are left to rot.

Logging and burning, however, have been tough to restrict, because they have been so lucrative and necessary for some countries.

"You need to provide alternatives to communities," said Thelma Krug, a lead climate-change negotiator from Brazil.

Brazil wants wealthy countries to create a fund to help poor countries absorb the economic blow. But the proposal has received little support from the industrialized world.

A more popular scenario involves a trading market for greenhouse gas emissions. Industrialized European countries are required under the Kyoto Protocol to reach certain emissions targets. They can accomplish that by emitting less or by buying somebody else's reductions to claim as their own.

By preserving their forests, nations with a history of deforestation could sell the amount of carbon they kept out of the atmosphere.

One report has estimated that sales of these carbon credits could amount to $15 billion a year.

The rewards could be huge for Indonesia and Brazil, which account for about half of yearly deforestation worldwide. When emissions from deforestation are included in the national rankings of the world's biggest greenhouse gas emitters, Indonesia and Brazil rocket to the top of list - just behind the United States and China.

Dozens of smaller countries could cash in as well.

"It can present quite an interesting amount of money," said Ludovic Mpili, an environmental adviser to the president of the Republic of Congo.

Industrial nations like the idea because, compared with the hard and expensive work of actually reducing emissions, buying reduction on the market is relatively easy and inexpensive.

"It's one of the cheapest games in town," Melnick said.

The problem is that the market strategy - involving vast, difficult-to-monitor expanses of jungle and billions of dollars in credits - seems to beg for abuse.

Who could know whether a forest would have been left standing even without the money? Or would protecting one piece of forest simply drive deforestation somewhere else?

An Indonesian environmental group has been handing out fliers here suggesting that the system simply would subsidize companies that control huge swaths of land while restricting peasants from the forests.

Ken Caldeira, a Stanford University climate scientist, said that including forestry in a vast global trading scheme would open myriad possibilities for gaming the system.

"What might be created is an unworkable system, where real emissions get traded off semi-fictional offsets," he said.

The same objections killed attempts to include forest preservation in the drafting of the Kyoto Protocol a decade ago.

Some countries have scowled at the idea because it rewards the most environmentally abusive countries, which have cut down their rain forests with abandon, while shortchanging countries whose forests are intact.

Not every country with a rain forest has a deforestation problem. In the Democratic Republic of Congo, for example, a lack of roads has made logging difficult. Other nations, such as Costa Rica, have managed to protect their forests through conservation.

These countries argue that they shouldn't miss out on a potential financial bonanza simply because they didn't mow down their forests.

"We want recognition for our efforts over the past two decades," said Jorge Mario Rodriguez, a negotiator from Costa Rica.

Doug Boucher, an ecologist at the Union of Concerned Scientists who has followed discussions on the issue, said he believed the plan could be managed to provide both money for developing nations and real emissions reductions for the planet.

The plan would be to establish a baseline for each country, based on historical deforestation rates.

Progress would be measured by the national total, limiting the possibility of shifting deforestation patterns without reducing it overall.

"If you make a commitment to save forests and you are going to get paid for it, presumably you're going to preserve it for a long time," Boucher said.