There was an article published that stated that walking to the store causes more environmental damage than driving there. The theory is that by walking you burn more calories that if you drove. If you replace those extra calories burned by eating beef, more CO2 is released into the atmosphere. It is pretty sketchy math, and makes a lot of assumptions. The article here does a pretty good job of ripping it apart.
The real point of the article is that far too much energy is used in the production and transportation of beef and other food. The walking vs. driving was just a nice attention grabber.