Of all the odd concepts to come out of the green movement, carbon offsets have to be near the top of the list. The idea is that you pay someone else to plant a tree, invest in a solar-energy start-up, or otherwise help reduce the amount of carbon you emit into the atmosphere. Then, you can just go about your business more or less as usual, confident that you have offset whatever carbon you may be emitting.

Carbon offsetting has won the backing of sustainability champions. "Offsetting is a common solution," says Brian Mullis, president of Sustainable Travel International, a Hood River, Oregon, nonprofit education organization. "Investing in renewable energy is a good thing, as is preserving ancient forests and investing in replanting areas where deforesting has occurred."

The average citizen of a developed country produces anywhere from six to 23 tons of carbon per year. The cost of buying credits to mitigate those emissions varies widely, starting as low as $1 per ton and going as high as $30 per ton. At San Francisco- based TerraPass, a leading carbon-offsets retailer, credits cost about $10 per ton. So canceling out all the carbon your car emits during a year costs between $30 and $80, depending on the vehicle, according to Tom Arnold, TerraPass's chief environmental offi cer. Offsetting your entire house would run anywhere from $30 to $400, depending on size, location, and other factors.

One of the biggest draws to carbon offsetting is the convenience of it. You can fi nd TerraPasses and other carbon-offset options on the shelves at many retailers as well as through the Internet. In fact, most of the 67,000 TerraPasses that have been sold so far have been purchased online, Arnold says. "Business is very good," he adds. "It's been roughly doubling every year."

TerraPass takes the revenues from its sales and invests in wind-energy projects, projects that upgrade municipal landfi lls in order to reduce the landfills' emissions, and biomass projects that aim to create energy from cow manure. Those investments add up to a lot of carbon offsets. Last year, Arnold says, the carbon-credit industry offset about 30 million tons of carbon emissions.

If you want to play the carbon offset credit game, start by checking out an online carbon calculator. The one on the TerraPass website ( calculates the amount of carbon that needs to be offset for a car, home, dorm room, and even a wedding. Enter a few tidbits of information such as your home's zip code and recent monthly energy bills and you'll be told approximately how much carbon you're emitting and what it will cost to counteract it.

The trading of carbon-offset credits has survived controversy, some of which has stemmed from questions about whether certain credit sellers are offering offsets of little or no value. Today, companies like TerraPass engage third-party auditors to verify the quality of the offsets. In the future, says Arnold, big changes will come in the way consumers buy carbon offsets. "Just think about how easy it is to buy travel insurance," he says. "You can buy from the airline, at the airport - wherever you want. We have to make carbon credits mainstream - a quick, easy, check-the-box thing." And, it's already happening. General Electric recently announced a new credit card that lets cardholders earn not shopping or travel rewards - or even cash back, for that matter - but carbon-offset credits.