Do carbon offsets live up to their promise?

"You put a bunch of climate wonks in a room, it's the one [topic] they're going to talk about most," says Mr. Bayon. "And it's the one that has bedeviled every single climate discussion I've ever seen."

But while experts disagree on the effectiveness of the carbon market at averting global warming, nearly everyone agrees on two points. First, the fact that people are beginning to factor in the cost of their carbon footprint when doing business is good. "You're starting to put a price on the emissions of carbon," says Bayon. "That cost begins to filter into your operations. And you start saying to yourself, 'Should I throw that 10 or 20 bucks out of the window?' "

Second, the more money invested in renewable energy, the better. "That has an important effect in the aggregate," says Bogdan Vasi, assistant professor at Columbia's School of International and Public Affairs in New York City. "As more and more people make these choices, they are creating a market, and slowly it's shifting the proportion of renewable energy to fossil-fuel energy."

But ultimately the carbon-offset market is more a phase than a destination, says Jonathan Isham, professor of international environmental economics at Middlebury College in Vermont. "We really want a world where, in a generation, we don't need offsets anymore," he says. "Once we get the legislation we need, prices will reflect the social costs of carbon."

Everybody loves trees. They're beautiful, big, and green. Unfortunately, planting them may not be the best approach to reduce global warming, say scientists. While a tree does suck up carbon, its net cooling effect depends on latitude, according to a collaborative study from Lawrence Livermore National Laboratory in Livermore, Calif. Only trees planted at tropical latitudes have a net cooling effect. Those at temperate latitudes actually warm the planet.

And unless a forest is permanent (and who can guarantee that?), trees only temporarily sequester atmospheric carbon. When they burn or decompose, the carbon they contain is released back into the atmosphere. In tropical countries, where trees are most effective as a cooling agent, they're often up against poverty and political instability. "Does some guy wake up and say, 'Now I'm the dictator of the country. I want a golf course?' " says Michael Dorsey, a professor of environmental studies at Dartmouth College in Hanover, N.H. "There's the big issue."

Still, a tree's value shouldn't be discounted. While not the ideal carbon solution, they do increase biodiversity and decrease soil erosion. Most important, their natural appeal makes them ready-made symbols. "We do support tree-planting projects to get our employees engaged," says Erin Meezan, director of environmental affairs at Interface Inc., a textile company with an environmental bent. "It's one of the easiest things for people to understand. If you start getting into anaerobic digesters and underground injection, we lose them."

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