11/28/2011 09:47 am ET | Updated Jan 28, 2012

To Fly or Drive?

It's the holidays, which means it's that time of year when we all pack on a few extra pounds.

Of CO2, that is.

Many young Americans live far from our families. It's the downside of the young American dream of "Go west, young woman!" -- sung more appropriately by the Dixie Chicks in Wide Open Spaces (I couldn't listen to this song without crying for several years after college.) Take my family, for example: parents in Massachusetts, in-laws in Tennessee, brother in Utah, I'm in California. So we fly to see each other every Christmas.

Truth is, it's better to use just about any other form of transportation than flying. is one of several websites that lets you calculate if it's more cost-effective to fly or drive for your trip and throws in your pounds of CO2 emitted (in tiny letters at the bottom) to boot. Generally, it's both cheaper and greener to drive, especially if you've got company in the car for the trip.

In the situation of driving by yourself in a car that gets poor gas mileage (about less than 16 mpg), flying produces less CO2 than driving. Add one person to the car, though, and you cut your carbon footprint in half, so driving wins again.

But these calculations are only considering the actual CO2 emissions from the airplane and not another big piece of warming caused by air travel -- contrails. Yep, contrails contribute to climate change.

The contrails come from water vapor and other particles that come out of the plane's engine into the cold, high air. There they soon freeze, forming a contrail. You've probably noticed before how at first a narrow contrail can gradually broaden into a long thin cloud. These are cirrus clouds, high clouds that don't reflect sunlight like most clouds, but instead absorb heat from the Earth and trap it, keeping us warmer. The 2007 IPCC report estimated that contrails add 25-60 percent to the warming from the plane's CO2 emissions and a recent paper went even further, estimating that the warming from the contrail is actually greater than from the CO2 emissions.

That's the simplified story, but there's a bunch of complicating factors to add to the mix: Sometimes the bigger cirrus clouds do form from the contrail, sometimes they don't. Sometimes the contrail-cirrus cloud actually prevents other cirrus clouds from forming, so that means less warming. At first, the contrail actually causes cooling, when it's still made of water droplets that reflect incoming sunlight. And contrails only last for a period of hours, whereas the CO2 will contribute to climate change for around a century.

It also depends on when you fly. If you take a red-eye, there's no incoming sunlight to be reflected, so all the contrail can do is the warming part.

So, if you're traveling over the holidays and you can't pack the whole fam into the car (or better yet, bus or, my personal favorite, train), here are a few things you can do to lighten your carbon load:

- Take a nonstop flight if possible, since lots of fuel gets burned in take-offs and landings.
- Fly during the day rather than at night.
- Buy carbon offsets for your trip. One such site is TerraPass.
- Take public transportation to and from the airport. It's way cheaper than paying for parking, too.

We all should be lucky enough to spend the holidays with our friends and family. Here's to making your holiday travels as green and safe as possible.