In addition to carbonfund.org, certified carbon credits are marketed by nonprofits including the Nature Conservancy and Bonneville Environmental Foundation , along with climate mitigation services firms TerraPass and 3Degreesinc.com .

You can also purchase credits directly from airlines like JetBlue and United, and online travel companies OrbitzandExpedia, when booking tickets online.

Prices per credit can vary, depending on how the carbon emission is measured.

Some calculators, for example, include all the pollutants generated per flight, while others factor in departure dates (it’s more fuel efficient to fly in July than in January), airline carrier and seat class (economy, business, first class.)

For its part, Terrapass in San Francisco aggregates direct donations from its online consumers and corporate partners, like Expedia, and uses those dollars to help emitters, including landfills and dairy farms, reduce their carbon emissions.

“We establish long-term contracts with them, and help them get the money they need to install capital equipment and implement process changes that reduce their emissions,” says Terrapass Chief Executive Erin Craig. She says her group helps complete the modifications as well.

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