Our goal sounded simple: Determine the "greenest" way to travel. So we went to Web sites that help consumers calculate carbon output. Problem was, the sites' calculations for emissions, or "footprints," for the same trip varied from a ballerina's to Bigfoot's.

Take air travel. Although many experts agree that planes have an especially strong impact on global warming due to the nature of emissions at high altitudes, they argue over quantities, which vary with aircraft model (usually, newer planes burn less fuel), route, congestion, and load. Depending on the calculator (we used 11), a flight from New York to Los Angeles was said to emit from 1,924 pounds of CO2 to 6,732 pounds (see chart).

So it makes sense to know how each calculator does the math. Generally, the more explanation it gives and the more info you're asked to input, the better. To learn more, go to www.GreenerChoices.org/calculators.cfm, a Web site from Consumers Union, the nonprofit publisher of Consumer Reports.