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Companies and nonprofits that sell carbon offsets usually provide calculators at their Web sites that allow you to figure out your impact not only from a given flight, but from your daily life, including in your car and home. They will be glad, of course, to sell you an offset to reduce those footprints, too.

But whether an offset buyer is a good citizen or a fool being parted from his money might depend on how well the buyer chooses his offset provider.

Quality Counts

Later this year, the California-based Center for Resource Solutions hopes to have in place a program to certify carbon offsets. If all goes as planned, the nonprofit will award a "Green-E" seal of approval to carbon offsets that meet a certain standard. Green-E certification won't be a guarantee that a particular provider is efficient, but it will certify that a carbon offset being sold actually exists. Additionally, providers who want Green-E certification will be required to disclose specific project information to buyers.

Meanwhile, there's a bit of a Wild West atmosphere in the carbon-offset market.

Different providers have very different ways of calculating emissions, depending on, for example, whether they agree with scientists who calculate that emissions released in the air are more damaging than those released on the ground. They also charge widely different amounts to offset -- anywhere from about $4 to $25 a metric ton. (Carbon offsets are typically sold as metric tons, which are about 10 percent more than a U.S. "short" ton.)

"In the absence of an accepted standard, almost anyone can offer to sell you almost anything and claim that this purchase will make you carbon-neutral," says a report prepared for the nonprofit environmental group Clean Air -- Cool Planet, based in New Hampshire. "It is very difficult for consumers, even environmentally savvy ones, to differentiate between a high-quality and low-quality offering."

Even Environmental Defense, one of the nation's leading environmental groups, found it too time- and resource-intensive to investigate the 30 or so carbon-offset providers operating as of last year, says Tom Murray, an Environmental Defense economist. Instead, the nonprofit organization asked the providers to nominate their two best projects for evaluation.

Environmental Defense studied those projects and endorsed five, which are detailed at http://www.fightglobalwarming.com. One of the projects is offered by Carbonfund.org. The other four are run by for-profit firms: AtmosClear Climate Club, DrivingGreen.com, e-BlueHorizons and Natsource LLC.

Travelers who buy offsets are most likely to do so when booking trips online through Expedia, Orbitz or Travelocity, since doing so requires only the click of a mouse. (See story at right for a list of offset providers linked to those travel sites, none of which makes any money on offset sales.)

Murray is working toward the day when the U.S. government will step in, as most governments in the industrialized world have done, to set standards and track compliance with them. Meanwhile, consumers on their own can improve their odds of choosing a good offset provider by asking a few questions:

· Does the provider offer detailed information about the projects your dollars fund?

· Does the provider break down how the money is spent, perhaps even provide an audit?

· Does the provider have a credible third party evaluate the projects to ensure "additionality," meaning the projects wouldn't have been done without offset funding, and that the same offset isn't being sold multiple times?

· Does the price seem reasonable?

"In a voluntary market, you can't assume a higher price equals a higher-quality offset," Murray says. Then again, cheap is no guarantee of quality, either.

The projects chosen by Environmental Defense all compensated for a metric ton of carbon for $4 to $8. That's not an official rule of thumb, but for now anyway, it's the closest thing to a standard that we've got.