But the dump began its methane-burning efforts long before TerraPass came calling, and only because there were local concerns that methane might be contaminating the groundwater system.

As the magazine wrote, the main effect of the offsets in this instance was to provide the dump's owners, "a $13-billion US company based in Houston, with some extra revenue."

Stavins says there are some simple economic concerns with the offset market that relate to the buyers and sellers being motivated for reasons other than environmental good.

"The seller, typically in a developing country, has an incentive to fabricate, to say, 'If you didn't pay me I was going to cut down the forest,' or, 'Gee, unless you paid me I wasn't going to put in this energy-efficient equipment, even though it's readily available. I was going to seek out a really inefficient piece of equipment.' "

And if the seller of offset credits has an incentive to be misleading, Stavins notes, "the firm that's buying them also has an incentive to look the other way, and worst of all is the intermediary. Whether profit making or a non-profit organization, they have an incentive just to see the exchange take place."

Stavins suggests that unless the buyer of offset credits -- or the intermediary -- imposes strict standards on the seller, there's a good chance the seller will take the offset money and continue to do what they were doing anyway.

"I don't fault them for immorality or anything, they are doing what they are doing and they realize they can also get extra money," he says. "'Hey, I'm putting in this plant, and I can also apply for this subsidy.' No one ever wants to call these things subsidies, though that's essentially what they are.

It's worth noting that most critics of offset programs do not dispute the need to reduce greenhouse-gas emissions -- Stavins, for example, advocates government policy initiatives far stricter than those embraced by the United States and Canada so far.

Gregg Easterbrook, a fellow at the Brookings Institute, a Washington think-tank, recently noted the U.S. House of Representatives is touting its purchase of 30,000 tonnes of offsets to help the U.S. Congress become carbon neutral.