One of the (many) problems with the original Kyoto climate treaty is that it didn't address deforestation—a glaring omission, seeing as how one-fifth of greenhouse-gas emissions come from cutting down trees, especially rainforests. Back in 1997, a big question was whether we had enough satellites to actually monitor deforestation. Now that that's resolved, the big debate is how to convince developing countries not to hack down their forests. Should the world community set up some sort of fund to help pay for conservation programs, or should forests be folded in a bigger carbon market. Nature has a good rundown of this new debate:

An international fund such as that backed by Brazil might be useful to help pay for infrastructure issues as nations develop the expertise to track and police deforestation, sceptics argue, but the resources necessary to address such a problem can be raised only if avoiding deforestation becomes a private economic enterprise.

"In global markets, forests are worth more dead than alive, and this is what we need to turn around," says Andrew Mitchell, director of the Global Canopy Programme in Oxford, UK. "Philanthropy and governments won't do it. You have to look to markets to overturn what is in fact a market failure."

Moreover, having developing nations sign up to cap-and-trade commitments in the forestry sector will build momentum and increase pressure on countries such as China as well, says Stuart Eizenstat, a partner with Covington and Burling who served as chief negotiator for the US delegation to Kyoto. "This could open up a way of breaking this impasse between developed and developing countries."

Last year, The Wall Street Journal had a good piece on a proposal for wealthy countries to pay Indonesia for "avoided deforestation." The details can get thorny, but the market strategy seems like a much more effective solution. Realistically, any approach that centered on some sort of "international fund" to pay for conservation programs would end up being badly underfunded and likely ineffective. On the other hand, one problem with a market approach is figuring out how much to pay people not to cut down their trees—measuring the amount of carbon a given forest absorbs can get complicated.

There's also the issue of mixed signals: The biofuels craze in the developed world is driving Indonesia to hack down its rainforests in order to make way for new palm-oil plantations—making the climate problem worse, not better. (A similar situation's unfolding in Brazil due to a rising demand for beef.) So it's a fairly tricky issue, but it'll be extremely difficult to curb global warming without figuring out some way to address deforestation.

P.S. See also this recent Time dispatch about Canopy Capital, a British firm that's offering to buy up Guyana's rainforests: "For now, Canopy will pay simply to protect Iwokrama's ecosystem services, but in the future it's wagering that the world will get desperate enough to limit climate change—and deforestation—that it will pay Canopy for its stake." 

--Bradford Plumer