Check out this site:  www.carbonfootprint.com and use the calculator to assess your carbon footprint.  My carbon footprint is currently 1.936 tonnes of CO2.  The US average is 20.4 tonnes!  The only way to maintain a carbon footprint this low is to not own a car.  My commute by bus and rail is essentially carbon-neutral.  By switching my commuting method to the car I used to own (2005 Nissan Xterra), my carbon footprint would more than  triple to a whopping 6.489 tonnes!