There are about 40 operations worldwide that offer carbon offsets, but it’s difficult for a company to understand exactly where the money is going, the Seattle Post-Intelligencer reports.

Here’s how they work: Companies and consumers look at how much carbon they put into the environment and pay to subsidize renewable energy projects, or pay for tree planting or other eco-friendly activities to offset the environmental effects. Since 2006, the global carbon trading market has more than doubled to $22 billion.  

Corporations of all types are getting involved for a number of reasons – least of which is the marketing and PR potential – here are just a few recent announcements:

Expedia.com announced that consumers could “fly green” at $6 for a short flight to $30 for international travel through a program with TerraPass.

Last week, Four retailers, BetterWorld.com, Evogear.com, 3r Living, and Alonovo.com, announced they were using a new program from Carbonfree.org and Carnegie Mellon University’s Green Design Institute that offsets shipping-related carbon-dioxide emissions.

The Home Depot announced it would offset all carbon emissions created this year by the company’s Atlanta headquarters and a portion of emissions created by employees commuting to work and traveling on business.

Whole Foods Market signed up with Renewable Choice Energy to sell a Wind Power Card, available at approximately 80 Whole Foods Market locations.

Companies that sell to consumers pass the “donations” to another firm – like Carbonfree.org. This company can be either for- or non-profit and can charge between $4 and $30 to offset 1 metric ton of carbon.

But there’s little official oversight. The Center for Resource Solutions is developing a certification program for carbon reductions similar to its Green-e program that is used to verify renewable energy certificates.

Many carbon-offset groups also buy and retire carbon dioxide emissions reductions (The Seattle Post goes into more detail on this) from the Chicago Climate Exchange. If regulations are put in place, the retired credits will prevent companies from buying allowances. Companies can trade carbon offsets and allowances similar to stocks in the stock market.