Here's how it works. An individual buys offsets from a company that in turn invests in projects such as solar and wind power, reforestation or buying offsets from the Chicago Climate Exchange, where companies buy and sell rights to produce carbon emissions. Two popular offset sites are carbonfund.org and NativeEnergy (nativeenergy.com). Former vice president Al Gore, star of the Oscar-winning documentary An Inconvenient Truth, is known to buy offsets in an effort to be "carbon neutral."

Environmental author Bill McKibben says he "double- and triple-offsets" his travel. Lily Fessenden, director at the Audubon Expedition Institute in Cambridge, Mass., also buys offsets. But McKibben and Fessenden say offsets must be accompanied by action: reducing waste first. Offsets "are a great way to spread awareness," says Chip Giller, founder of online environmental magazine Grist (grist.org).

At best, they can be a "nice gesture," adds Amy Kaleita, assistant professor of agriculture and biosystems engineering at Iowa State University. But, "if I had the money, I would put it in the bank to save up for my high-efficiency washer and dryer rather than buy a carbon offset."

Others liken offsets to the old Catholic practice of buying indulgences for the forgiveness of sins. But offsets can lead a person to believe it's OK to pollute — as long as they pay for it later.

"There is no way of negating anything you do," says Danny Seo, author of the Simply Green series. "You can't counterbalance something bad you've done by doing something good."

Environmental humorist Lou Bendrick dislikes offsets for the same reason. "You get to sin, but you get to buy your way out of it. It's sort of silly. It's like buying tofu offsets to be able to eat a hamburger."