I've been meaning to figure this out for a while: Just how much CO2 did we add to the atmosphere by driving our old minivan? In the time we owned it, from 2003 to 2009, we drove 76,000 miles. At an average of about 18 mpg, that means we used about 4,220 gallons of gas in the six years we owned our car. (Which is 564 cubic feet, in case you're interested, or a cube of gasoline about 8 feet on a side, or something a little bigger than a minivan, but not much).
On average, a gallon of gas burned in a car generates about 19.56 pounds of CO2. So in our time owning the car, we generated about 82,540 pounds of CO2, or 41 tons, just to get ourselves from place to place (about 7 tons per year). 41 tons! That definitely seems like something worth changing.
And that's only looking at CO2, not the other emissions in terms of chemicals and particulates that come from driving cars.
Now that we've gotten rid of our car, let's just say our environmental impact has declined drastically. We still put a little extra CO2 out from our Zipcar jaunts, but otherwise, our transportation methods are doing very little in terms of dumping CO2 into the air (outside of huffing and puffing while pedaling our bikes up hills).
I'm not saying that everyone in the U.S. can suddenly give up their cars. But if more of those who have the ability to make the choice actually do it, there's an impact to be made.
(P.S. In case you're interested, Slate ran an article a while back about how one gallon of gas produces so much CO2.