654 13th Street, Preservation Park, Oakland, California 94612, U.S.A. 510-251-1600 | fax: 510-251-2203 | [email protected] | www.pacinst.org Driving vs. Walking: Cows, Climate Change, and Choice Michael Cohen and Matthew Heberger Pacific Institute April 2008 Summary Could walking be worse for the planet than driving? This startling idea has recently received coverage in New York Times blogs1 and beyond. In the following, we look at the numbers behind this comparison to determine whether the life-cycle2 greenhouse gas (GHG) emissions of food needed to replace the calories burned in walking a certain distance could exceed the life-cycle emissions of the fuel needed to drive a car the same distance. Our conclusion: it depends on the assumptions.3 Driving turns out to be better only if you compare it to a very greenhouse-gas-intensive food, such as beef. When we consider an average American diet, which is still energy-intensive compared to diets in other countries, walking is better for the planet. While we commend growing efforts to understand the complex implications our purchases, choices, and activities have on the climate, we caution against making hasty behavioral changes based on analyses and comparisons that may be faulty or rely on unrealistic assumptions. Introduction From “paper versus plastic” to “cotton diapers versus disposable diapers,” the environmental impacts of our daily choices have been debated for years. Similarly, the question of whether walking (or bicycling) is better or worse than driving has been discussed for more than two decades.4 The “drive vs. walk” question goes something like this: you’re at home and need to go to a store three-quarters of a mile away (1.5 miles roundtrip).5 Concerned about your potential contribution to climate change and conscious that the production and transportation of the calories you’ll burn in walking or bicycling can be very carbon-intensive, you wonder if driving to the store would release fewer greenhouse gases. In his book, How to Live a Low-Carbon Life,6 Chris Goodall answers the question this way: “It makes more sense to drive than walk, if walking means you need to eat more to replace the energy lost.”7 Goodall—a proponent of reducing GHG emissions—comments that his intent is not to encourage driving, but rather, “to draw attention to the carbon intensity of modern food production, particularly of meat.”8 The media have already repeated Goodall’s claims, with limited or no critical analysis. The Times (U.K.) stated: “Walking does more than driving to cause global warming, a leading environmentalist has calculated.”9 In February 2008, John Tierney posted a New York Times blog essentially repeating Goodall’s driving vs. walking claim, writing: If you walk 1.5 miles, Mr. Goodall calculates, and replace those calories by drinking about a cup of milk, the greenhouse emissions connected with that milk (like methane from the dairy farm and carbon dioxide from the delivery truck) are just about equal to the emissions from a typical car making the same trip.10Pacific Institute 2 Are such claims warranted? Do the assumptions behind Goodall’s findings withstand scrutiny? Let’s look at the numbers. The All-Beef Diet Walking 1.5 miles at a moderately fast 3.0 mph pace requires about 120 calories,11 equivalent to the amount of calories in 67 g (2.4 oz) of top sirloin.12 To estimate the life-cycle GHG emissions of beef, Goodall refers to an article on beef production in Japan.13 Using the estimates from this article, the calories burned by walking the 1.5 miles would be equivalent to 1.9 kg of CO2-equivalent emissions, if replaced by top sirloin.14 Of course, estimating life-cycle GHG emissions for beef cattle production and transportation is much more complex than, say, measuring the caloric value of a given quantity of beef. A variety of factors affect GHG emissions, including the source and means of producing cattle feed (petroleum-based fertilizers, for example, can greatly increase total emissions); the type of feed (methane produced by cow’s digestion contributes to total GHG);15 and the source of electricity used for processing and storing the meat (e.g. burning coal generates more GHG than does hydropower or wind). Given these variable factors, it is not surprising that a British-based estimate of GHG emissions from beef cattle production is only about half that of the Japanese study used by Goodall.16 Using the numbers from the Japanese and British studies, we can estimate that between 1.3 and 2.4 kg of CO2-equivalent emissions are produced to raise, process, store, and transport the top sirloin that powers this 1.5-mile walk. Table 1. Carbon Emissions Estimates, by Calorie Source CO2-equivalent (g) calories low high top sirloin,17 67 g 123 1,300 2,400 ground beef,18 40 g 123 790 1,500 2% milk,19 8.1 oz 123 230 660 nonfat milk,20 11.2 oz 123 330 970 apple,21 lg (237 g) 123 7 170 avg. daily caloric intake CO2-equivalent (g) typical US diet 2,693 5,100 vegan diet 2,693 1,000 Sources: Caloric values of foods22; CO2-e of Beef,23 Milk,24 and Apples25; Daily caloric intake.26 Car GHG Emissions How do the GHG emissions associated with the calories burned by walking compare to those linked to driving? Goodall states that the typical car in the United Kingdom emits 0.29 kg CO2-equivalent/mile.27 Using EPA estimates of GHG emissions/gallon of gasoline,28 this works out to an average fuel economy of 30.3 mpg—50% higher than the average U.S. vehicle.29 Gas mileage depends on several factors. For instance, engine temperature can greatly affect gas mileage, as short trips with cold engines can burn twice as much fuel as the same car would once warmed up.30 Applying this to Tierney’s example of a 24-mpg car could yield an actual fuel economy of only 12 mpg over the 1.5-mile trip if the engine is cold. In recognition of these factors, we generously assume the car in this assessment gets 17.1 mpg,31 considerably higher than the typical U.S. vehicle in these conditions. Incorporating a well-to-pump analysis that found extracting, refining, and transporting fuel effectively consumes 20% of the gasoline before it reaches the pump,32 and discounting the life-cycle GHG emissions generated by the manufacture and maintenance of the car,33 the car trip would generate slightly more than 1 kg of CO2-equivalent at 17.1 mpg over thePacific