You've probably heard of carbon offsetting—the idea being that you can purchase an "offset" to compensate for the eco-impact of your auto or air travel. A number of companies now offer the service, usually providing mileage calculators on their Web sites to help you gauge your emissions.

But while carbon offsetting is a positive first step in reducing greenhouse gas emissions, Dorfman stresses that it's not a carte blanche for jet-setting around the planet guilt-free.

"Offsets are not a 'get out of jail free' card," he says. "They don't eliminate the carbon emissions we put into the atmosphere by traveling, which is why it's always helpful to seek clean, fuel-efficient travel options."