Another group of scientists has estimated the environmental burden of beef. The researchers suggest that meat from cows contributes 10 kilos of greenhouse gases (expressed as CO2 equivalents) for every 1000 calories of food. Put in a less scientific way, a Big Mac® a day will represent more than a tonne of global warming emissions a year, using up your entire carbon budget by the middle years of the century.

Seven years ago I wrote an article (covered in the New York Times blog here) that suggested that walking to the shops and then eating beef to replace the calories used would generate more greenhouse gases than driving a car to make the purchases. This little piece of ad hoc research was cruelly dismissed as utter nonsense by all right-thinking people. Well, if you believe the figures published today, I’ve finally got my revenge. Beef turns out to be twice as carbon intensive as driving.

The policy implications are, of course, completely non-existent. Most of us don’t exercise enough, and if we do walk to the shops we usually don’t need to replace the calories. In fact, we are probably walking because we want to lose weight. Nevertheless, it still seems interesting to me that a (quite inefficient) fossil fuel engine moving the best part of a tonne of metal is less greenhouse gas intensive than the docile bovines grazing in the field next to my office.

This is the form of the calculation. (A longer and more complicated version can be found in my book How to Live a Low Carbon Life).

Walking

1) Assume that the individual is in calorie balance. That is, she doesn’t want to lose weight and therefore any calories used in exercise will be replaced by new food calories.

2) She walks to the shops. The distance is 1.5 miles and she walks at 3 miles per hour. Therefore the round trip takes an hour. If she’d been sitting watching TV, she’d use about 60 calories an hour (approximately the ‘basal metabolic rate’ for a 60 kg woman).

3) A person of this weight walking at 3 miles per hour uses about 220 calories in an hour’s walk.

4) So the incremental effect of walking to the shops and back is about 160 calories.

5) The global warming footprint of beef is 10 kilos per thousand calories, says the new paper. So 160 calories of beef represents 1.6 kilos of CO2 equivalent emissions.

To walk to the shops 1.5 miles away, come back, and replace the calories lost with beef would add 1.6 kilos to global warming emissions.

Driving

1) A reasonably new mid-sized car generates about 130 grams of emissions per kilometre or roughly 200 grams per mile.

2) To drive to the shops and back is 3 miles. So the CO2 emissions would be about 600 grams.

3) Add a little to reflect the lower engine efficiency of driving a short distance and increase the figure by 33%

To drive to the shops would add about 800 grams to global warming emissions, half the figure from walking and then replacing calories with beef.

Beef is about 5 times as bad as pork or dairy products. So my assertion only works for meat from cows. Nevertheless after all the scorn of seven years ago, I’m really pleased to have some academic justification for my piece of research.

And I should really stop crowing about being a vegetarian - dairy products may be much better than beef but they're actually worse than poultry per calorie.