If you participate in the carbon offset programs sponsored by the major airlines, you generally do not have a choice of specific projects in which to invest. Each airline does provide a description of the types of projects that the money is used for. The program sponsored by American Airlines uses the carbon offsets for renewable energy, forest sequestration, and energy efficiency projects. The program sponsored by Delta Airlines uses the carbon offsets for planting trees. The program sponsored by Northwest Airlines uses the carbon offsets for land conservation projects. The program sponsored by Continental Airlines uses the carbon offsets for reforestation and renewable energy projects. About.com provides additional information about the carbon offset programs sponsored by the major airlines.

http://airtravel.about.com/od/safetysecurity/a/earthair.htm

If you wish to choose specific programs to spend your carbon offset dollars on, you can do the following: (1) visit the sites of the major firms active in the carbon offset market; (2) calculate how much carbon you need to offset for your flight using their carbon offset calculators; (3) choose a project for your carbon offsets; and (4) purchase your carbon offsets online. The Environmental Defense Fund (EDF) provides recommendations for specific carbon offset projects and links to eight companies that are active in the carbon offset market at its Innovation Exchange program site.

http://innovation.edf.org/page.cfm?tagID=23994