"Walking does more than driving to cause global warming, a leading environmentalist has calculated.

Food production is now so energy-intensive that more carbon is emitted providing a person with enough calories to walk to the shops than a car would emit over the same distance. The climate could benefit if people avoided exercise, ate less and became couch potatoes."
http://www.timesonline.co.uk/tol/news/uk/science/article2195538.ece