An addendum of sorts to Katherine's post below on surprising energy-efficiency conclusions when it comes to "growing local": it could be that walking generates more carbon than driving-at least if beef (or milk) is what's for dinner. How so?
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. Provided, of course, they remembered to switch off the TV rather than leaving it on standby.
The sums were done by Chris Goodall, campaigning author of How to Live a Low-Carbon Life, based on the greenhouse gases created by intensive beef production. "Driving a typical UK car for 3 miles [4.8km] adds about 0.9 kg [2lb] of CO2 to the atmosphere," he said, a calculation based on the Government's official fuel emission figures. "If you walked instead, it would use about 180 calories. You'd need about 100g of beef to replace those calories, resulting in 3.6kg of emissions, or four times as much as driving.
What if, instead of beef, the walker drank a glass of milk? The average person would need to drink 420ml – three quarters of a pint – to recover the calories used in the walk. Modern dairy farming emits the equivalent of 1.2kg of CO2 to produce the milk, still more pollution than the car journey.
Chris Goodall's personal site with more low-carbon life info.