A car that travels 15,000 miles a year and averages 25 miles per gallon exhales about 5.5 metric tons of CO₂. Anyone feeling a twinge of guilt over that might be interested in buying carbon dioxide credits, a novel way to offset your CO₂ trail.

Carbon credits give the buyer the right to release atmospheric CO₂, a prime contributor to global climate change. They’re basically a tax on CO₂ that’s used to help develop cleaner technologies. The Kyoto Protocol, an international treaty designed to limit global warming, spawned a worldwide market in carbon credits. Now you can get in on it.

How it works. In the U.S., a trading system for big players is operated by the Chicago Climate Exchange. If member companies can prove they’ve cut emissions below a cap set by the exchange, they can trade or sell the difference as “credits.” Polluters that can’t meet caps buy credits to balance their carbon budget. Other parties can buy credits, too--to remove them from the market and, in effect, reduce pollution.

Now individuals can reduce their carbon footprint. Organizations such as nonprofits Carbonfund.org and Drive­Neutral, and TerraPass, a for-profit group, let consumers reduce CO₂ emissions by buying and retiring CO₂ credits. They help calculate your carbon dioxide emissions, suggest how many credits will offset them, spell out what their credits mean, and identify the entity that verifies reductions. Prices vary. Carbon­fund.org says that a tax-deductible donation of $99 will eliminate 23 metric tons of CO₂. Terra­Pass charges $50 to offset 5.4 metric tons.

CR’s take. Buying carbon credits is one way to cut CO₂ emissions. More direct ways include buying a high-mpg vehicle and choosing energy-efficient appliances.