If you buy carbon offsets, your money is supposed to go toward solar farms, wind turbines, planting trees and other projects that mitigate carbon dioxide.

But paying for projects you'll probably never visit to counter emissions you can't always see might seem nebulous.

"Usually folks out there will ask, 'How do I know it's real?'" said Russell Simon, a spokesman for Carbon Fund, a nonprofit carbon-offset seller.

The key is to buy offsets from outfits that are certified by a third party, said Simon, a former Journal North reporter now living in Washington, D.C.

Some Carbon Fund-backed projects are certified under the widely used Green-e program, which verifies how much carbon is offset by renewable-energy projects. With forestation projects, the Rain Forest Alliance is brought in to check out how many trees are planted, how much carbon they can absorb, who owns the land and whether the land is protected.

"You don't get the greenhouse-gas reductions in the mail. It happens somewhere other than where you are, but that's the point," Simon said.

Carbon Fund sells offsets at $5.50 per metric ton. It says offsetting 10 tons of carbon — roughly what a sedan spews over 30,000 miles on the road — is about the same as conserving more than 1,100 gallons of gasoline or what 8.3 acres of forest absorbs in a year.

Green Energy New Mexico, a Santa Fe-based nonprofit, also sells offsets certified by Green-e. Its offsets, which it calls green tags, sell for $20 for every megawatt hour produced by renewable energy through the Bonneville Environmental Foundation.

Twenty percent of green-tag revenues in New Mexico goes to an in-state renewable-energy fund. Nothing has been built here with the money, but if all goes as Green Energy director David Griscom envisions, the money could help fund a solar kiosk in the Santa Fe Railyard that could power surrounding buildings while teaching people about renewable energy.

"That's just a start," Griscom said. "I want to get solar on schools and community centers."