Many people worry about the climate impacts of their travel, and some even offset it with payments to good-guy companies like Terrapass. But do you actually know whether taking the train is greener than driving? And just how bad is flying, anyway?

According to the National Geographic Green Guide, you roughly double your emissions if you cancel your plane reservations and drive across the country instead. If you take the train, then you’ll cut carbon dioxide (CO2) by half compared to the plane. A key reason is that the train (or the diesel bus) may be a big carbon emitter, but it’s designed to carry a lot of passengers, so the per capita emissions are a lot lower.

Airplanes are about 3 percent of total global climate emissions. A single flight produces three tons of carbon dioxide per passenger, but the amount goes up dramatically if the plane is nearly empty. Further complicating the picture for planes is that they produce vapor trails and emit tropospheric ozone, which have big — but not long-lasting — climate impacts. CO2 from your car’s exhaust, by contrast, will stay in the atmosphere for centuries.

Illustration of different forms of travel

The definitive study on this was produced last year and appeared in Environmental Science and Technology. The big lesson: It pays to carpool.

Driving a car solo has the same basic climate impact as taking an 80-percent-full plane flight a similar distance, the study said. If the plane is full, it beats the car. Add two other people and it’s like you’re traveling on a (half full) bus or train. If your car is a diesel (or a hybrid), the two additional passengers have you looking better than an average train or bus passenger. I’d love to know how a battery electric car compares, but that was beyond the study’s boundaries.

Assuming that your travel is fully booked, the diesel bus comes out on top, followed by the high-speed train, the car with three people in it, then the medium aircraft.

Trains and buses have an average occupancy of only 40 percent, so there’s a lot of room for improvement there. And cars would be much cleaner if they could store the CO2 they generate. A poll shows consumers are willing to pay extra to turn their rides into carbon fighters. And learn more about carbon offsets from this video from the Bonneville Environmental Foundation here:

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