Sunday, June 10, 2007; P07

All of the most prominent online travel agencies -- Expedia, Orbitz and Travelocity -- offer a convenient way to voluntarily pay to offset CO2emissions caused by a trip. Just click to have the price of the offset added to your bill. All three companies say they pass 100 percent of the offset payment to the partners providing the offset service, but the partners have different priorities, procedures and prices. Here's a primer on the offset program connected with each travel service.

CONTRACTS WITH: TerraPass ( http://www.terrapass.com/), a private company based in San Francisco.

YOUR COST: A purchase of $5.99 is said to offset 1,000 pounds of CO2-- which the company estimates is the "carbon footprint" that one passenger is responsible for on a 2,200-mile round-trip flight. For $16.99 you are said to offset 6,500 miles; for $29.99, 13,000 miles.

PROJECTS: Terrapass funds methane-capture plants on several dairy farms and domestic clean energy projects such as wind farms. It also buys, then resells to consumers, carbon offsets from the Chicago Climate Exchange. The CCX is a financial institution whose members, generally businesses that emit greenhouse gases, voluntarily agree to reduce their emissions by a certain amount. Those reductions are then considered to be like a commodity that can be traded and sold. Currently, wholesale prices for carbon offsets on the CCX are $3.65 a metric ton, which is 2,200 pounds.

TRANSPARENCY: As a private company, Terrapass is not required to open its books. It will not reveal how much of a cut it takes for profit, how much goes to other middlemen and how much directly to carbon offsetting projects. The offsets the company buys on the climate exchange, says its chief environmental officer, Tom Arnold, are verified as legitimate by the nonprofit Center for Resource Solutions, based in San Francisco.

CONTRACTS WITH: Carbonfund.org ( http://www.carbonfund.org/), a Silver Spring nonprofit focused solely on fighting climate change. To buy an offset with your ticket at present, you must go to http://www.eco.orbitz.com/, but company officials say that in coming days customers will have the option of donating to an offset program at the main site, http://www.orbitz.com/.

YOUR COST: For a regional flight, $3.25 offsets 1,300 pounds of CO2. Cross-country: $6.25. International: $12.50. Customers can get info about their flight's mileage at the site.

PROJECTS: You may choose from among three types of projects: Renewable energy, including wind farms and solar energy; reforestation, meaning tree planting; and energy efficiency projects. Carbonfund.org provides detailed information about the renewable energy and reforestation projects at its Web site. If you choose an energy efficiency project, Carbonfund.org will buy a carbon offset on your behalf from the Chicago Climate Exchange.

TRANSPARENCY: An audit of Carbonfund.org projects conducted by the nonprofit Environmental Resources Trust is available at Carbonfund's Web site. The nonprofit's basic financial reporting to the IRS is available online at http://www.guidestar.org/.

CONTRACTS WITH: The Conservation Fund ( http://www.conservationfund.org/gozero), a nonprofit land conservation group headquartered in Arlington.

YOUR COST: Travelocity doesn't precisely calculate the amount of carbon released from a given flight but charges $10 to offset an average trip for one, said to cover the flight, one night's hotel and a rental car. If you wish to be more precise, you can go to the Conservation Fund's calculator and type in specifics. If, for example, you say you will be flying 2,200 miles, the calculator suggests contributing $6.92 to offset your flight.

PROJECTS: Trees are planted to offset carbon emissions. Currently money goes to a project in the Bogue Chitto National Wildlife Refuge in Louisiana, where Hurricane Katrina killed about 70 percent of the trees. Jeffrey Glueck, a Travelocity spokesman, says the $10 funneled to the Conservation Fund through Travelocity pays for the planting of two trees, which over their lifetime will remove two tons of carbon from the atmosphere -- assuming, of course, that they thrive.

TRANSPARENCY: The Conservation Fund says it keeps 4 percent of every dollar funneled to it via Travelocity for overhead and fundraising and sends the remainder to the U.S. Fish and Wildlife Service to spend for trees at Bogue Chitto. It doesn't provide an audit to the public but says a third party -- Environmental Resources Trust -- verifies its projects, checking, for example, to make sure the number of trees paid for are actually planted. Basic information about the nonprofit is reported to the IRS; a report is available at http://www.guidestar.org/.

-- C.L.