Why Walking to Work Can be More Polluting Than Driving to Work
By Richard B. McKenzie
“Walking can be 1.5 times more polluting than driving.”
Which is more polluting—driving a mile to work or walking that mile? The easy answer is, of course, driving. Cars have tailpipes; people don’t. Far more energy is needed to push a 3,000-pound car along the road than is needed to move a 150- to 250-pound body along a sidewalk. Walking seems like the green thing to do.
But appearances can be deceiving, making easy answers dead wrong. That’s the case here when the calories expended in walking are replaced.
Counting the Ways Energy is Consumed in the Food-Supply Chain
The primary reason that walking to work can be more polluting than driving is that growing crops and raising animals so that they can be consumed and digested by humans involves a food-supply chain that now extends to all corners of the Earth and uses a lot of energy. An unavoidable byproduct of this energy use is greenhouse gas emissions. How can this be? Let us count the ways:
- 1. Many categories of farm equipment—such as tractors, mowers, trucks, cars, balers, and combines—can be as gas-guzzling and polluting as the eighteen-wheelers on the nation’s highways.
- 2. Farms use a lot of electricity—generated by distant and often coal-burning power plants—to run irrigation equipment and heating/cooling systems for cattle barns, pig or poultry pens, and animal waste disposal plants.
- 3. The nation’s entire food industry—ranging from the production of fertilizer and pesticides to crops and livestock to food processing, packaging, and transportation and then on to food preparation by consumers—uses nearly a fifth of the fossil energy burned annually in the United States.1
- 4. Field hands who actually pick crops hunched over rows need to eat extra-large energy-dense meals to replace the 5,000 (or more) calories they can burn daily, and the calories they down are also produced in energy-intensive ways.2
- 5. A major input in agriculture is natural gas, and the cost of natural gas can be as much as 90 percent of the total production cost of fertilizers and pesticides.3
- 6. When humans eat animals to get their replacement calories for walking to work, they can tap into a lot of stored energy, which has substantial associated greenhouse gases. It takes about sixteen pounds of grain and 2,500 gallons of water to produce a pound of beef.4
- 7. Beef cattle and dairy cows may not release large quantities of CO2. However, they are able to digest the grains and grasses they consume only by allowing them to ferment in their several stomachs, and the fermentation produces, through belching and flatulence, over a hundred gases (with three of the four major detrimental gases being ammonia, hydrogen sulfide, and nitrous oxide). The most environmentally detrimental gas released by cattle is methane, which, per cubic foot, has up to twenty-three times as much global-warming impact on the higher atmosphere as carbon dioxide.5 Methane represents a fifth of all greenhouse gases that contribute to global warming, and, according to climate scientists, increases with humans’ meat consumption. Farm animals produce a lot of manure. Each of the country’s 13 million dairy cows drops an average of 21 tons of manure a year.6 As manure from cattle (and other farm animals) decomposes, it releases annually 5.5 million tons of methane gas (20 percent of all methane gas released in the United States).7 Although a portion of the animal-based methane is recovered to produce electricity, one head of cattle can easily be more polluting than a single car (and there are 50 percent more cattle in the world than cars).8
- 8. Carbon-based energy goes into the production of food regardless of whether it is harvested, transported, shelved, consumed—or thrown away, with half to two-thirds of the food produced on the farm making it to people’s stomachs.9 Food that is thrown away in consumers’ trash bins represents the single largest form of waste that goes into landfills, constituting, according to the Environmental Protection Agency (EPA), about 12 percent of all municipal landfill waste from households and costing governments at several levels over a billion dollars in disposal costs. According to the EPA, Europeans and North Americans throw away an annual average of between 620 and 660 pounds of food per person.10 And 10 percent of the food bought by restaurants goes out in the trash, with discarded restaurant food waste representing 1 percent of all waste in landfills.11 Food thrown away inevitably decomposes, releasing methane gas into the environment, only a minor portion of which is captured for commercial use. Pollution from food waste is a form of collateral damage from people walking and replacing the calories they expend and must be included in the total pollution associated with walking.
Food and Energy Consumption
The food-supply chain in the United States burns a total of 10.3 quads of fossil-fuel-based energy. (A “quad” is a very large measure of energy: 1×1015 BTU; a BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.) The basic problem is that food energy actually produced equals only 1.4 quads, which is about 13.5 percent of the energy absorbed in production. Then, between a third and a half of that potential food energy is wasted at one stage of production or another.12
Moreover, the human body is also not very efficient at converting the potential energy in the food it consumes into useful work13: Only about 15 percent of the potential energy in food eaten goes into activities such as walking, as well as maintaining all bodily functions. This means that the energy that the human body actually converts into work is meager percentage-wise—something on the order of 1.3 percent of the fossil fuel energy that is used along the entire length of the food-supply chain.14
By way of contrast, although the gasoline-power engine is not a paragon of greenness, its energy efficiency is substantially higher in moving people and things from one point to another, with 14 to 30 percent of the potential energy in gasoline actually moving cars. The rest of the potential, 70 to 86 percent, is released as heat and gases into the atmosphere.15 And remember that the limited energy efficiency of many combustion engines, which is what makes many people think walking is more efficient than driving, is itself a factor in making the food-supply chain, and, therefore, walking, energy-intensive.
Derek Dunn-Rankin, a professor of engineering at the University of California, Irvine and an avid environmentalist, computes that a 180-pound person walking one mile to and from work at a pace of two miles per hour will burn 200 calories above the 2,000 calories burned each day to maintain the body’s basic metabolism. However, the production of those 200 calories in food takes fifteen to twenty times as much energy in the form of fossil fuels. This means that driving a high fuel economy car (40 miles per gallon) will use, in fossil fuel energy, only about two-thirds to one half the energy that the person uses in replacing the calories expended on walks. (Heavier walkers use even more energy when they walk and when they replace the greater calories they expend in moving their weight.) Energy use and pollution do not have a one-to-one correspondence, which causes Dunn-Rankin to conclude, “My bottom line would be that walking can be 1.5 to 2 times more polluting than driving (if you use a high mileage car). If you use a monster car, you are better off walking always.”16
For more on these topics, see Energy by Jerry Taylor and Peter Van Doren in the Concise Encyclopedia of Economics.
Well, maybe all-electric (and, to a lesser extent, hybrid) cars can make driving all the more compelling for commutes to the office and grocery store. Electric cars do not have tail pipes because they don’t have exhaust. They are really green, right? No, not really. Electric cars can make driving appear relatively greener. Bjorn Lomborg, who views himself as a reluctant and “skeptical environmentalist,” and who now believes global warming is real and a product of human activity, has pointed out that roughly half of all the electricity generated in the United States is produced from coal. Research shows that all-electric cars that charge their batteries off coal-fired generators cause electric power plants to emit 6.5 ounces of carbon per mile, half the emissions of gasoline-powered cars.
Lomborg also notes that when electric cars arrive at their showrooms, “the production of the electric car has already resulted in sizeable emissions—the equivalent of 80,000 miles of travel in the vehicle.” He adds, “If a typical electric car is driven 50,000 miles over its lifetime,17 the huge initial emissions from its manufacture means the car will actually have put more carbon dioxide in the atmosphere than a similar-size gasoline-powered car driven the same number of miles. Similarly, if the energy used to recharge the electric car comes mostly from coal-fired power plants, it will be responsible for the emission of almost 15 ounces of carbon dioxide for every one of the 50,000 miles it is driven—three ounces more than a similar gas-powered car.” In short, electric-car owners have to put a lot of miles on their cars before the reduction in emissions from driving more than offsets the added emissions from the car’s production. Given the limited driving ranges of electric cars, accomplishing that feat can take years.18
When it comes to energy use and greenhouse gases emitted, appearances can be grossly deceiving. Granted, people who drive everywhere are energy users and polluters. But walkers also use fossil fuels through the food they eat to replace the calories burned while walking. Of course, driving can be more polluting under some circumstances, such as when large SUVs are the preferred vehicles or when drivers insist on doing wheelies at every stoplight. Bicycling the distance can also be less polluting than driving. Dunn-Rankin sums up the central, largely counterintuitive, point of this commentary: “Driving a small [or moderate-size] car and not having to replace burned calories saves more energy (and greenhouse gases) than walking when the extra calories expended are replaced.”19
See Pimentel, David. 2006. “Impacts of Organic Farming on the Efficiency of Energy Use in Agriculture.” Organic Center State of Science Review. Ithaca, N.Y.: Organic Center, Cornell University, August, p. 1.
See the table for calories burned in various occupations, and on different tasks on farms, provided by NutriStrategy, accessed December 22, 2012 from http://www.nutristrategy.com/caloriesburnedwork.htm.
Schnepf, Randy. 2004. Energy Use in Agriculture: Background and Issues, order code RL32677. Washington, D.C.: Library of Congress, Congressional Research Service, Agricultural Policy Resources, Science, and Industry Division, November 19, p. 3.
As reported by North Carolina State University and AT&T University Cooperative Extension (October 7, 2008), accessed December 29, 2012 from http://www.extension.org/pages/35850/on-average-how-many-pounds-of-corn-make-one-pound-of-beef-assuming-an-all-grain-diet-from-backgroundi.
As estimated by the U.S. Environmental Agency (n.d.), accessed on December 29, 2012 from http://www.epa.gov/rlep/faq.html.
See U.S. Environmental Protection Agency. n.d. “Common Manure Handling Systems,” as accessed on October 30, 2013 from http://www.epa.gov/agriculture/ag101/dairymanure.html. See also Procon.org. 2011. “Milk: State by State Dairy Cow Emissions: The Fart Chart, January 17, as accessed October 30, 2013 from http://milk.procon.org/view.resource.php?resourceID=001154.
Environmental Protection Agency. n.d. Ruminant Livestock: Frequently Asked Questions, accessed October 26, 2013 from http://www.epa.gov/rlep/faq.html. See also Forgarty, David. 2007. Potent Methane Is an Overlooked Greenhouse Gas. USA Today, April 30, accessed October 26, 2013 from http://usatoday30.usatoday.com/weather/climate/2007-04-30-methane_N.htm.
As reported by the U.S. Environmental Protection Agency, “Livestock Manure Managements,” September 1999, accessed January 9, 2013 from http://www.epa.gov/methane/reports/05-manure.pdf. As reported by Silverman for How Stuff Works (n.d.), accessed January 2, 2013 from http://science.howstuffworks.com/zoology/mammals/methane-cow.htm.
As reported by the Society of St. Andrews, as accessed December 22, 2013 from http://endhunger.org/food_waste.htm.
As reported by Galbraith, Kate. 2012. “The Battle Against Food Waste.” New York Times, January 15, accessed December 28, 2012 from http://www.nytimes.com/2012/01/16/business/global/the-battle-against-food-waste.html?_r=0.
As reported by Barclay, Eliza. 2012. “For Restaurants, Food Waste Is Seen As Low Priority.” NPR, November 27, accessed on December 28, 2012 from http://www.npr.org/blogs/thesalt/2012/11/27/165907972/for-restaurants-food-waste-is-seen-as-low-priority.
As reported by Dunn-Rankin, Derek. 2010. “Energy and Personal Power,” a paper presented at the Sustainable Energy Technology Club, November 2. Dunn-Rankin cites the Center for Sustainable Systems, University of Michigan, accessed from http://css.snre.umich.edu/facts/.
The human body is approximately twice as efficient in converting the energy in food for maintaining bodily functions, like keeping up the body’s internal temperature, as for work (including walking) (Dunn-Rankin (in private communication, October 6, 2013)
The 1.3 percent was obtain by reducing the energy produced (1.4 quads) by a third, or to .91 quads and then multiplying .91 quads by the energy efficiency of the human body, 15 percent, which gives .137 quads, the amount of energy extracted from food. The .137 quads is then divided by the total energy that goes into food production, 10.4 quads.
The U.S. Department of Energy puts the energy efficiency of cars at 14-26 percent, as accessed December 28, 2012 from http://www.fueleconomy.gov/feg/atv.shtml. Wikipedia reports that the energy efficiency of gasoline engines is between 25 and 30 percent, with diesel engines reaching as high as 40 percent, as accessed December 28, 2012 from http://en.wikipedia.org/wiki/Engine_efficiency#Gasoline_.28petrol.29_Engines.
Personal correspondence October 30, 2013.
Why just 50,000 miles? Because the typical electric car is driven short distances.
Lomborg, Bjorn. 2013. “Green Cars Have a Dirty Little Secret: Producing and Charging Electric Cars Means Heavy Carbon-Dioxide Emissions. Wall street Journal, March 11, as accessed September 25, 2013 from http://online.wsj.com/article/SB10001424127887324128504578346913994914472.html.
Even if the electric car is driven for 90,000 miles and the owner stays away from coal-powered electricity, the car will cause just 24% less carbon-dioxide emission than its gas-powered cousin. This is a far cry from “zero emissions.” Over its entire lifetime, the electric car will be responsible for 8.7 tons of carbon dioxide less than the average conventional car.
Those 8.7 tons may sound like a considerable amount, but it’s not. The current best estimate of the global warming damage of an extra ton of carbon-dioxide is about $5. This means an optimistic assessment of the avoided carbon-dioxide associated with an electric car will allow the owner to spare the world about $44 in climate damage. On the European emissions market, credit for 8.7 tons of carbon-dioxide costs $48.
Dunn-Rankin (2010, slide 15).
*Richard McKenzie is Walter B. Gerken Professor Emeritus in Economics and Management in the Merage School of Business at the University of California, Irvine and author of Why Popcorn Costs So Much at the Movies, And Other Pricing Puzzles (2008) and Heavy! The Surprising Reasons America Is the Land of the Free—And the Home of the Fat (Springer, 2011)
For more articles by Richard B. McKenzie, see the Archive.