Ditching your gas guzzler is a great way to reduce your carbon footprint, but if you really want to do something about global warming, get a used car. You'll be putting less carbon dioxide into the atmosphere.
As Matt Power notes in this month's issue of Wired, hybrids get great gas mileage but it takes 113 million BTUs of energy to make a Toyota Prius. Because there are about 113,000 BTUs of energy in a gallon of gasoline, the Prius has consumed the equivalent of 1,000 gallons of gasoline before it reaches the showroom. Think of it as a carbon debt -- one you won't pay off until the Prius has turned over 46,000 miles or so.
There's an easy way to avoid that debt -- buy a used car. The debt has already been paid. But not just any used car will do.