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I heard recently that a beef eater actually emits more CO2 by biking a mile than driving. Skeptical, I ran a back-of-the-envelope calculation using data provided by the first reasonable-looking results I found after a quick google search of all the necessary assumption. Starting assumptions included:
It takes 43 calories to bike one mile. Beef provides 210 calories per 3 oz serving. Beef production emits 36.4 kg CO2 per kg of beef (Sources linked from here). Driving emits 8.81 * 10 ^-3 met tons per gallon (EPA). City driving returns 20 miles per gallon.
After crunching the numbers, a biker who gets all of her calories from beef would emit .63 kg of CO2 on that mile long trip whereas a driver would let .44 kg of CO2 into the atmosphere.
I think the major lesson here is that eating less beef will be very good for the environment. David Tilman, in a recent talk here at Rutgers, showed figures that suggested that chicken and seafood release far less CO2 than beef, and that plant-based sources of calories use far less than any meat. Let's assume that chicken uses half the CO2 and plant-derived calories use one-tenth. In that scenario, far less CO2 will be given off than if the person drove or used beef to fuel her bike journey.
Finally, city mileage can vary by quite a bit. Large suvs could easily get 10 mpg, thus doubling one's carbon footprint, whereas a Prius or Civic hybrid could halve the carbon cost of the trip. Choose your vehicle purchases accordingly!
