According to Friends of the Earth, air travel is the world’s fastest growing source of the greenhouse gases which cause global warming. In fact, studies suggest commercial aircraft generate almost as much carbon dioxide annually as the entire continent of Africa.

For the concerned traveler, purchasing carbon offsets is one relatively simple and inexpensive way to counterbalance emissions from airline travel (and from everyday activities such as driving, home heating and more). Carbon offsets have been available to travelers and non-travelers for years as a standalone purchase. However, in August, Travelocity and Expedia announced the ability to purchase offsets through their online travel services.

Carbon offsets are a reduction in carbon dioxide emissions from a third party, aimed at canceling out the emissions arising from a particular activity. The methods of carbon offsetting can vary widely, from planting trees and restoring wetlands to investing in alternative energy sources. Carbon neutral companies offer calculators that allow an individual to estimate emissions from a particular activity and in turn purchase offsets of the appropriate size to cancel that output.

Through a TerraPass-Expedia partnership, travelers can choose offsets for airline travel and/or packages. Prices begin at $5.99 to offset roughly a 2,200 mile round trip flight and range up to $29.99 for a 13,000 mile flight. TerraPass is audited by the Center for Resource Solutions, an independent non-profit organization that conducts green power industry monitoring and certification. According to Expedia, all proceeds go directly to climate change reduction efforts.

Through Travelocity’s Go Zero program, customers can purchase offsets when reserving travel packages. Pricing for offsets ranges from $10 to $40. According to Travelocity, all proceeds go directly to The Conservation Fund, a non-profit organization that works to regenerate forests and wetlands. Travelocity is also purchasing offsets for its own employee travel program and is planting its first grove of trees in the Bogue Chitto National Wildlife Refuge 45 miles north of New Orleans.