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.
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.