San Francisco's airport has just introduced a new kiosk where you can buy carbon offsets for the flight you are about to take with the swipe of a credit card. Now, no one (including the people quoted here who just bought the carbon offsets) really has any idea what they are buying. And even fewer people know if what they are buying actually works (go here to read GOOD's more full-scale examination of how it is possible that you can pay to remove a set amount of gas from the air).Says NPR: "The kiosks look similar to self check-in machines or small ATMs. Travelers input the number of miles their trip will cover, how long it will take and the number of passengers they plan to buy offsets for.For example, for a 6,000-mile-long, 12 hour flight, the carbon offset cost would be about $34.34 for one traveler. The price varies from a few dollars for a short West Coast flight to as much as $70 for an international trip."Many people, in these post-Inconvenient Truth days, do feel bad about the impact of flying on their carbon footprint, and while it may not necessarily help the environment, buying carbon offsets certainly doesn't hurt it. An extra $30 a flight to plant some trees-that, I can get behind.