Scientific American: Going home for Thanksgiving is a time-honored American tradition–and one that exemplifies the environmental burden created by all our hectic traveling. We spewed more than 2 billion metric tons of carbon dioxide from our trains, planes and automobiles in 2007, according to the U.S. Department of Energy. I love my beat up old junker of a car, but at 30 miles-per-gallon, I’m not doing the atmosphere any favors. Turning to ethanol or biodiesel doesn’t help much, given that those fuels …