Transportation is responsible for approximately a third of the average American's global warming impact. It's also one of the factors that we have the most control over — where little changes can make a big difference.

America is car country...
Burning fuel produces carbon dioxide, one of the most important greenhouse gasses responsible for the global warming we're now experiencing. Most transportation — by car, bus, or airplane — requires burning fuel. In fact, Americans use more than 380 million gallons of gasoline per day! For each gallon of gasoline burned, about 20 pounds of carbon dioxide is released into the atmosphere. For the average car that drives 12,500 miles per year, that's almost a pound of CO2 per mile — or 11,450 pounds per year — about enough to fill an Olympic swimming pool!

...but we're jet-setters too
Americans also like to fly a lot. We take more than 500 million airplane rides per year. It's harder to calculate the CO2 emissions of an airplane flight since it depends on the length of the flight, (airplanes burn more fuel on takeoff than when cruising at 30,000 feet) the type of the airplane, how many people are aboard and other factors. A round-trip flight from New York to Chicago results in about 650 pounds of CO2 emissions; a cross-country trip produces about 2,000 pounds.

Cutting down is easier than you might think
Reducing your family's transportation CO2 emissions is easier than you might think. The best way to cut down is to just skip the car when possible — walking or biking is better for you and the planet. In addition, try to combine your errands into a single trip to save time, gas and CO2 emissions. If you're lucky to live in a place with good public transportation, use it, and enjoy the time you'll have to read a book or have a heart-to-heart with your kids.

But even when you have to drive, there are things you can do to get better gas mileage — and reduce your contribution to global warming. Scheduled maintenance keeps your car running at its best. Tire pressure is particularly important — keeping them at the recommended pressure (see the sticker in the driver's door frame or on your gas cap lid) can save up to 4 percent on your gasoline bill. Less gas burned equals less CO2 produced.

When it comes time to trade up to your next set of wheels, consider your needs carefully. Are you currently driving a bigger vehicle than you need? If you're in the market for a new car, be sure to check out the new car-based "crossover" SUVs that get better mileage than their truck-based competitors. Hybrids and the new batch of clean diesels coming soon are also a good bet. But do the math first; the price premium you pay for a hybrid or diesel won't necessarily translate into lower costs over the long run. You might be better off with a more conventional, less expensive car that gets good, if not great, mileage.

Flying presents a tough challenge, because it can be hard to skip these important trips. Taking the train (if it's an option) is much better for the environment and can be a lot of fun, too — it's certainly less stressful than airport security. If you're serious about reducing your carbon footprint and have to fly, consider adding a carbon offset to your ticket through online travel sites like Expedia.com.