Two years ago, it seemed like a bad idea: Let rich nations earn credit for trimming greenhouse-gas emissions under the Kyoto Protocol by helping countries in Latin America, Asia, and Africa save their vanishing tropical forests.
People thought it would be an easy out that would be difficult to monitor and present too many potential loopholes to be effective.
But now, delegates to global climate talks here have dusted off the concept. Deforestation and other land-use changes account for up to 25 percent of the CO2 that human activities release into the atmosphere each year.
By conserving the forests, analysts say, countries can maintain a natural storehouse for a significant amount of carbon dioxide from the atmosphere.
Avoided deforestation, as it's known, "is a very salient question" for future emission-reduction regimes, says Nigel Purvis, a former US negotiator to the protocol who is now with the Nature Conservancy.
The Kyoto Protocol's first commitment period ends in 2012. Industrial countries are bracing for new emissions targets far tougher than the ones they agreed to reach between 2008 and 2012 - an average of some 5 percent below 1990 levels.
Countries in the European Union, for example, are exploring the possibility of cuts in CO2 emissions from 15 to 30 percent below 1990 levels for a post-2012 commitment period. Adding avoided-deforestation projects to the tool kit - allowing industrial countries to receive credits against their emissions targets - could help the heavier polluters.
Some variations on the idea would allow developing countries as well to earn carbon credits for each ton of emissions avoided through such projects. These countries could sell their credits on the growing international carbon market. In principle, the money would flow back to local residents who otherwise would have cut trees for a living or cleared forests to grow crops.
Jeorg Siefert-Grazin, with the Fundación Amigos de la Naturaleza (Friends of Nature Foundation) in Bolivia, points to the Noel Kempff Climate Action Project in his country as an example of what can be done.
Since 1997, the project has grown to embrace slightly more than 2,300 square miles of tropical forest as a national park. The partners have devised ways to compensate residents for getting out of the timber business. The project's partners include the Nature Conservancy, the Bolivian government, and three energy companies.
The project falls outside anything currently acceptable under Kyoto's "clean-development mechanism" for earning carbon credits. But it has potential, supporters say.
Last month, a Dutch firm that certifies CDM projects finished its review of the Noel Kempff project and gave it a thumbs up. They calculate that the effort to date has ensured that nearly 1 million tons of carbon that would have been released to the atmosphere has remained locked up in the forest's trees and undergrowth.
Using avoided deforestation as an approach to meeting emissions targets still arches eyebrows. Bill Hare, climate policy director for Greenpeace International, notes that several peer-reviewed studies have cast doubt on some of the project's claims for success.
In addition, he points to the patchwork nature of the project - protecting some of Bolivia's forests while leaving other sections unprotected. Such projects provide no net benefit to the atmosphere if loggers "leak" from protected to unprotected forests.
To resolve this, delegates from Papua New Guinea and Costa Rica have proposed that if a developing country wants to attract avoided-deforestation projects, it must include all its forests in a national program.
The idea, widely seen as one example of developing countries' willingness to do more to reduce emissions and help shape the future of the protocol and the 1992 framework convention, has met with generally favorable responses.
This approach "is a very constructive proposal," Mr. Hare says. "It would not solve everything, but it would solve a lot of problems" that many people currently see with avoided deforestation.