When the rock band Coldplay became concerned that the production and distribution of its CD’s were contributing to global warming, it didn’t switch to solar-powered studios. Instead, the group planted 10,000 mango trees in southern India to offset its carbon emissions.

Coldplay’s calculation was part of a growing market in voluntary carbon offsets—when environmentally concerned consumers pay for steps that theoretically should remove an amount of carbon from the atmosphere equal to that which the consumers emit. Airlines, hotels, and online booking sites are making it easier for passengers and guests to participate.

British Airways gives travelers the option to purchase offsets for flights on its home page. Guests at the San Juan Marriott Resort & Stellaris Casino can choose to pay an extra dollar each night. Travelocity users need only click a button: a $10 donation can offset a quick weekend trip, including air travel, a one-night hotel stay, and a rental car.

But as Coldplay found out, carbon trading doesn’t necessarily result in a reduction in greenhouse gases. When journalists visited the band’s trees last year, they found that two out of every five had died without any extra carbon in their trunks.

What’s a concerned traveler to do?Many environmentalists argue that offsets are just a cheap fix that puts off real reform and that the ultimate goal should be to reduce emissions. When possible, stick to public transportation (buses, trains, and ferries are the most eco-friendly, in that order). If you purchase credits, make sure that they come from a reputable vendor, audited by independent third parties, and choose programs that invest in renewable energy and were created for the sole purpose of carbon trading. See the sidebar for a list of top companies.