This week, I took public transportation several times when I would normally drive and was feeling very virtuous. But yesterday I blew all that green goodness by stepping onto one of the biggest carbon contributors out there–an airplane. For a cross-country flight. And I’m not alone–there were 809MM passengers on airplanes in 2008.

Why does flying have such a big environmental impact? Planes emit carbon dioxide & nitrous oxide, very potent greenhouse gasses. According to Greenskies.org, a single flight across the Atlantic would produce the same amount of carbon per person as their home does in a year. In addition, the CO2 and water vapors emitted at high altitude also has a bigger environmental impact–experts suggest anywhere from 2-2.7X the impact at ground level.

Yes, reducing your flights is the biggest way to reduce that impact, but that is often not practical. Are there other ways to green your flying? Pattie Prairie, the CEO of Brighter Planet, had some good tips in a post this week that included packing lightly, fly non-stop, picking the greenest airline you can (JetBlue is the best of the major carriers), and flying during the day. I hit two for four on my trip–going non-stop with a small carry on. But my airline was mediocre and the flight was at night. Now what?

Brighter Planet also sells carbon offsets and their mantra is to conserve what you can and offset the rest. My flight offset is $28 and that money goes to projects like the Greensburg Windfarm. Carbon offsets have gotten some mixed reviews recently, but that’s not entirely surprising given how new the market is. If you want to make sure you are purchasing a high quality offset, here is a ranking of providers.

I have to be honest and say I’m not 100% there yet on offsets. I like that the calculations give me a number that I should “contribute” to offset something ungreen and make it easy. But I wish I had a better sense for where the money is going and more transparency about “middle man” costs. So I’ve purchased them here and there, but am by no means a regular.

What about you? Do you purchase offsets? Who do you buy from? What do you think?