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Prius v. Vegan

December 10, 2007

Posted by Dan Lashof in Solving Global Warming

Tags:

CAFE, carbonfootprint, co2, vegan, vegetarian

^{_{No, this is not the title of a WWE title bout (or even a WWF fact sheet). Its a question I was asked by someone wanting to know which would make a bigger difference in lowering his personal carbon footprint. I knew that personal vehicles produce about 20 percent of U.S. greenhouse gas emissions, and that according to a recent Food and Agriculture Organization report raising farm animals is responsible for almost 20% of global emissions, but I didn't know how the numbers would break down on a personal level for the average American. With the help of research assistant Jamie Consuegra, here is what I found out:}}

^{_{A paper published last year by two researchers at the University of Chicago (Gidon Eshel and Pamela Martin, Earth Interactions Vol. 10, Paper No. 9) suggests that adopting a vegan diet would reduce personal greenhouse gas emissions more than driving a Prius. The paper received a lot of media attention at the time and the claim has been circulating on the web ever since. Unfortunately, the paper relies on quite generic calculations and drastically underestimates emissions from vehicles. Correcting for this, it looks to me like switching to a Prius has about 3 times the impact of switching to a vegan diet. The numbers are of the same order of magnitude, however, and there is a lot of uncertainty in the food calculation. The bottom line will depend on individual factors, such as how much red meat you eat to start with, how many miles you drive, and whether the vehicle you are trading in is a Camry or a Hummer. More details below:}}  

^{_{Eshel and Martin consider all of the greenhouse gas emissions released when raising animals (with the exception of emissions from fertilizer used to grow feed.) They go on to rank the emissions associated with five meat and dairy diets. Assuming you get nearly 30% of your calories from animal sources, poultry is the least carbon intensive diet releasing ~0.8 tons of CO2 eq per year per person more than a vegan diet. Dairy and eggs follow poultry at ~1 ton of CO2 eq/yr/person. Fish comes in third at ~1.1 tons CO2 eq/yr/person and is a tricky case because some species (like swordfish and tuna) require enormous amounts energy to catch while you can get a better return with schools of fish that live close to shore. The mean American diet, which includes poultry and red meat, is ranked 4th releasing about 1.5 tons of CO2 eq/yr/person more than a vegan diet. Finally, the red meat diet is the most carbon intensive, contributing over 2 tons of CO2 eq/yr/person extra emissions.}}

^{_{For the purposes of their comparison the authors subtract their estimate of the CO2 emissions of a Prius (1.19 ton CO2 /year) from the emissions of a Camry (2.24 tons CO2 /year) and they conclude that you would only reduce your CO2 emissions by 1.05 tons per year by investing in a hybrid. These estimate are too low by a factor of 3 to 4, however.}}

^{_{According to www.fueleconomy.gov a Prius releases 4 tons of CO2 eq per year and a Camry releases 7.3 tons of CO2 eq per year. Using the average American light duty vehicle for comparison implies that switching to a Prius reduces emissions by 56%, or 5 tons CO2 eq/yr/vehicle.}}

^{_{There are several reasons for this discrepancy. First, Eshel and Martin calculate that one gallon of gasoline releases 16 lbs of CO2, which is way too low. The EPA looks at the full life cycle of the fuel and estimates that one gallon of gasoline releases 25 lbs of CO2 eq. Second, the EPA estimates that the average driver puts 15,000 miles on his car per year while Eshel and Martin use a per capita number of about 8,000 miles per year (there is room for argument about the appropriate assumption here: in practice buying a hybrid dictates the vehicle emissions for your passengers as well as yourself; choosing your diet only influences yourself unless you dictate the menu for your household). Finally, Eshel and Martin overestimate the fuel economy of all vehicle types because they applied test data without adjusting for real world driving conditions.}}

^{_{The bottom line is that an American eating an average diet and driving 15,000 miles per year in a 20 miles-per-gallon-vehicle will reduce his or her carbon footprint about 3 times as much by switching to a Prius compared to adopting a vegan diet. Nonetheless, raising farm animals does have a huge environmental impact, so reducing your meat consumption, particularly red meat, is good for the health of the planet, and its healthier for you too.}}

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