Expedia.com has become the first online travel agency to offer travelers the ability to purchase carbon offsets to help cancel out the greenhouse gas emissions that lead to global warming... Airline travel currently accounts for about 13 percent of U.S.- transportation-based emissions of carbon dioxide, the primary greenhouse gas responsible for global warming. To help address this, Expedia is partnering with TerraPass to make it simple for environmentally conscious travelers to be carbon-balanced travelers by purchasing a TerraPass from Expedia as part of their trip.