(Image credit: Wellcome Library, London “A Man Walking”, Image V0048616, Collotype after Eadweard Muybridge, 1887.)
The idea that driving a car is better for the environment than walking sounds like the raving of an antediluvian climate change denier. Of course there is personal benefit in walking, and all of us should walk more, not less. But the sad truth is that producing, processing, and preparing the food needed to power walking consumes almost as much fossil fuel energy as driving the same distance in a car. How can that be? It turns out that our food industry today is terribly energy-consumptive. It wasn’t always; it has changed dramatically over the past couple of decades, and not for the better. The title should really be something like “How our wasteful food industry is helping to drive our fossil fuel energy consumption.”
Food in, energy and carbon dioxide out
We humans survive by taking in food and using the energy that’s produced by metabolizing it. The amount of food we take in depends on what we are: a 120-pound young adult male needs about 2,200 Calories a day to maintain body weight; a woman of the same size would need about 2,100. (I’m using the term ‘Calorie’ because it’s the one we usually see in our everyday lives; it’s the same as the ‘kcal’, the term preferred by scientists.) A young male weighing 170 pounds needs more, about 2,800 Calories a day (similar size and age female would need about 2,400). A rider in the Tour de France burns around 8,000. And an old man, weighing the same 170 pounds, has shrunk his needs back down to just over 2,400; a similarly-sized and aged woman would need 2,100. The average caloric intake in the USA in 2010 was about 2,700 Calories a day, according to the US Department of Agricultue (for a graphic illustration of how diets have changed since 1970, check this).
We self-regulate reasonably well: For an average adult, eating just 8% more a day than they need to maintain weight will lead to a weight gain of about 20 pounds a year. (However, the USDA figures show an increase of 23% in Calories per capita between 1970 and 2010. This correlates directly with an increase in obesity and Type II diabetes.)
Some of the energy we get from food goes to activities such as moving our muscles or feeding our brains. (The brain uses more energy than you might think — about 20% of the total.) Some energy goes to keeping us warm. And essentially all of the food burned in energy metabolism is converted to carbon dioxide. (Some of it gets ‘eaten’ by the bacteria in our gut to produce methane.) So, even just staying alive adds to greenhouse gases. But how much?
A diet that provides equal numbers of calories from carbohydrate, fat and protein produces about 1,026 grams (2 and a quarter pounds) of carbon dioxide for 2,700 Calories. That amounts to over 375 kilograms (824 pounds) of CO2 a year, about 29% of the weight of a Toyota Corolla! But that’s the unavoidable energy and CO2 cost of sustaining human life; that’s not the problem.
And now, the real cost of food
Over the past few decades, the food industry has changed dramatically in terms of its energy use. The numbers are staggering (sources and calculations are at the bottom of the post). In 2007, the Cornell ecologist David Pimentel calculated that the ratio of fossil fuel burned by the food industry to the calories we actually eat is 14:1 (1). The food writer Eric Garza provides the same number. Back in 2002 the USDA estimated that the ratio was 12:1, and everyone agrees that it’s increasing. Pimentel and colleagues also calculated that 17% of the total energy used in the United States each year goes to food production, distribution and consumption, a number in keeping with what other analysts have concluded. Energy consumption in 2013 was 189,500 Calories per capita per day (this number actually declined slightly between 2007 and 2013). If 17% went into food production, distribution, and consumption, that would be 32,200 Calories. The fossil fuel contribution to total energy production in the United States is 84%. Using these numbers, the ratio of fossil fuel energy in/food energy consumed would be about 10:1, but this doesn’t include the cost of research and development, water supply, wastewater treatment and healthcare for people on poor diets. Considering all of these numbers, 13 is probably a reasonable estimate for the Calories of fossil fuel energy needed to provide one Calorie of consumed food.
The amount of fossil fuel burned daily to provide our daily ration of 2,700 Calories is then 35,100 Calories, an amount that will generate 8.1 kilograms of CO2, corresponding to just under 3,000 kilograms (just over 6,550 pounds) of CO2 a year. Other sources (2) estimate the amount of CO2 generated for food production per person per year to be 3.1 – 3.2 metric tonnes (about 6,930 pounds).
To summarise: for each person consuming 2,700 Calories of food a day, about 35,100 Calories of fossil fuel energy is burned each day, and around 3,000 kilograms of CO2 is produced each year. It’s the ratio of fossil fuel energy required for each Calorie of food that we eat that kills the efficiency of walking.
If we add the 375 kilograms (824 pounds) of CO2 that we generate from food in our bodies each year to the CO2 generated by the production, distribution and preparation of that food, the annual production of CO2 related to food is 3,300 kilograms (7,260 pounds) per person. That’s the weight of 2.5 Toyota Corollas.
What is the energy cost of driving?
The energy required to drive your car is . . . well, it depends on your car. Let’s pick something fairly middle of the road: a Toyota Corolla. According to the EPA, a middle-of-the-pack 2009 Corolla with a 4-cylinder engine gets 30 miles per American gallon, combined highway and city driving. That corresponds to about 640 Calories/kilometers (3).
The car has to be manufactured, and that takes energy too. MacKay, in his book “Sustainable Energy Without the Hot Air”, provides some guiding numbers, leading to my estimate of about 26.2 million Calories to build a Corolla (4). Amortizing that number over a 300,000 kilometer life of the car, it contributes 87 Calories per kilometer to the energy cost of driving the car (4). The total energy cost of the car is then 727 Calories per kilometer, let’s say 730.
So, what is the energy cost of walking?
The best way to measure how much energy a person uses while walking, running, or doing any other activity is to measure how much oxygen is consumed and CO2 is produced. This is a little cumbersome, since it requires a mask that captures the gases, but it’s quite accurate. It’s been done many times, resulting in ever more refined estimates. Earlier estimations, from the United States Army Medical Research and Development Command at Natick, Mass. (5), haven’t been changed much by more recent results. Here are some numbers (rounded off) that reflect the energy consumed by a person walking 5 kilometers per hour (3.1 mph), our average walking speed:
Person weighing 50 kilograms (110 pounds): 25 Calories per kilometer (40 Calories per mile)
Person weighing 70 kilograms (154 pounds): 35 Calories per kilometer (56 Calories per mile)
Person weighing 90 kilograms (198 pounds): 45 Calories per kilometer (72 Calories per mile)
Those numbers reflect the energy need created by the walking itself; in addition, each individual also has a basal rate of metabolism that goes on regardless of any activity. For the middle weight individual, that’s about 100 Calories an hour (a little less, a little more, for the lighter and heavier person, respectively). But to be clear, walking creates an energy deficit that must be met by consuming food; it’s wrong to think that we’re consuming calories anyway, so the walking is just part of that basal rate. If you don’t believe that, go on a 20 kilometer hike in the mountains.
The energy costs of walking are small numbers, compared to the energy cost of driving the car. But remember that the fossil-fuel energy cost of each of those Calories consumed by walking is 13 Calories. So, for our middleweight walker, the fossil fuel energy cost is actually 455 Calories per kilometer (35 x 13). The Corollas consumed 730. More, but not that much more. An anecdotal report has the new Prius going 59 miles to the American gallon, which comes to 4 liters of gas per 100 kilometers, or 327 Calories per kilometer driven. Add to that the energy cost of construction, say 87 Calories per kilometer (the Prius is smaller than the Corolla, but the hybrid technology requires more energy to produce), and the driving cost is just 414 Calories of fossil fuel per kilometer. Our human walker required 455. So if your car is a 2016 Prius, the cost ratio is inverted; walking takes more energy than driving the car (of course, if there’s more than one person in the car, it’s going to be even less efficient to walk).
Good for the automobile industry and our legislators for getting automobile fuel consumption down. But no cheers for the food industry for the shocking amount of fossil-fuel energy required to produce, ship, and prepare our food. And none either for us, the consumers, for insisting that the dollar cost of food must be as low as possible, leading to ever increasing mechanization and its accompanying fossil fuel energy cost.
We spend about as much fossil fuel energy on food as on driving
We hear a lot about the carbon dioxide produced by driving our cars and SUVs. The energy cost of personal driving in 2010 in the USA was roughly 33,300 Calories per person per day (6). In other words, it consumed less fossil-fuel energy than the production of food. (Commercial driving, mainly big trucks and buses, adds about 20% to the total.) The trend is toward increasing fuel efficiency standards for new vehicles (in 2014, the EPA standard for new cars was 36.4 and for light trucks, which includes SUVs, 26.3 mpg). Meanwhile, the energy cost of food production continues to climb. According to Shelly K. Schwartz of CNBC.com, in 2010 the Department of Agriculture found energy consumption per capita fell by 1% between 2002 and 2007, but food-related energy use grew nearly 8%. The energy cost, and the cost in terms of carbon dioxide produced, by the food industry in the USA is only going up.
Back in 1900, it took about 1 Calorie of energy (in addition to sunlight) to produce 1 Calorie of food. In 1940 that number was about 2.3. Today, we’re looking at 13, or higher — some experts think that the ratio is now up to 20:1. Increasing mechanization of the food industry has meant lower costs of production, but has required hugely increased energy consumption.
Things that make a difference
Different foods require different amounts of energy to produce. Meat, particularly from cattle raised in feedlots, is very expensive — the energy needed to produce one pound of beef has been estimated to be 24 times greater than the energy needed for one pound of corn. And the pound of corn provides almost 50% more calories; the energy cost of producing a calorie of beef, compared to a calorie of corn, is 35! Other meat-vegetable comparisons are less extreme (chicken costs a lot less to produce, per Calorie, than beef, for example). Fortunately for the environment, beef consumption is on the way down in the USA, while chicken is shooting up. But meat still provides around 25% of our calories.
Although the energy cost of producing meat on the farm is much higher than, say, grain, a strictly vegetarian (or vegan) diet wouldn’t change the overall energy cost of food production drastically: the portion of the energy used in the production phase on the farm is only about 24% of the total (7). And most omnivores already eat a significant amount of non-meat.
Many fruits and vegetables also require a lot of energy to produce per Calorie contained because they have a lower energy content per unit of weight. Of course, their role in our diets is different from that of meat, or grains; they provide essential micronutrients (vitamins and minerals), and fibre.
Something that makes a huge difference to the energy (and dollar) cost of food production is loss: according to the US Department of Agriculture, 31% of the food produced is wasted at one stage or another (8). We insist on perfect fruit, so imperfect fruit is left behind, either in the field or at the grocery store; at home, people often throw away food instead of eating leftovers.
Energy source matters to food efficiency. Areas, mainly in the western part of North America, that have access to hydroelectric power can reduce the fossil fuel consumption, and CO2 emission, associated with food production. Solar and wind power, if they can be brought to significant levels, will do the same. But the time when fossil fuel consumption for food production starts decreasing is not here yet.
There’s a growing interest in getting food from the producer to the consumer more directly, and this certainly diminishes the energy cost. Farmers’ markets and other local sources, locavore eating, can help.
To really reduce the fossil fuel energy cost of getting around, ride a bike. It takes about half as much energy as walking to cover a given distance.
By now you will realize that the title of this piece should have been something like “Stop wasting energy in the food industry.” Walking is obviously of benefit. It helps maintain health, lowering medical costs, and driving has all kinds of hidden costs, such as building and maintaining roads, the harmful effects of pollution, and the lack of direct human interaction when we are seated in our cars. And if you walk for an hour each day, which provides a pretty good health benefit, your total consumption of fossil fuels, if everything else stays exactly the same, goes up by about 1%. (The total energy consumption of the average North American is an astounding 200,000 Calories per day. That’s total energy consumption for all purposes, including manufacture, transport, and personal use. Another example of waste — Europeans consume half as much total energy, and live pretty well.)
Michael Pollan, considered an expert on food production and its costs, claims that food production, and all the related activities, contribute as much as 37% of the greenhouse gases we produce. The associated activities are more diverse than you might think — chemical fertilizer production, land clearing, the conversion of waste food to methane, which has about thirty times more greenhouse impact than CO2 — the list is long and growing.
Back to the front page
Background information, and the numbers
- In 1989, Pimentel and colleagues estimated that 1500 liters of gasoline was used, per capita, to produce, distribute and consume food in the USA (Pimentel et al., “Interdependence of Food and Natural Resources” in Food and Natural Resources, ed. by David Pimentel and Carl Hall. San Diego, CA: Academic Press, Inc.) Every liter of gasoline burned produces 2.35 kilograms of CO2 and 8,174 Calories of energy. So that gas consumption produced, per capita, (1500 x 8,174/365 =) 33,600 Calories of energy per day, and (1500×2.35 =) 3,500 kilograms of CO2 a year. 33,600 is 14 times 2,400, the energy content of the food actually eaten by people per day in 1989.
- In 2007, total CO2 emission in the USA was 5,800 Million tons; this works out to 18.1 tons per person per year. Food production, at 17% of the total, then creates 3.1 tons of CO2 per year per person. Christopher Weber and H. Scott Matthews of the Department of Civil and Environmental Engineering and Department of Engineering and Public Policy at Carnegie Mellon University in Pittsburgh concluded that the CO2 emission related to food in America was 8.1 tons per year per household. The average household is 2.56 people, so the emission per person is 3.2 tons, according to them. (C. L. Weber and H. S. Matthews, “Food-Miles and the Relative Climate Impacts of Food Choices in the United States”, Environ. Sci. Technol., 2008, 42 (10), pp 3508–3513.)
- Gasoline mileage: 30 mpg (US) is 7.85 L/100 km. A liter of gasoline contains 8,174 kcal. of energy. So the Corolla needs about 7.85 * 8,174/100, or 642 kcal to go one kilometer. Call it 640.
- Mackay quotes two widely different values for the cost of actually making a car: a high figure of 65 and a low figure of 26 million kWh (the smaller number assumes that the material in the car is recycled, which lowers the cost). I use the smaller number for the Corolla, which is fairly small as cars go.
- Pandolf, K., B. Givoni, and R. Goldman. Predicting energy expenditure with loads while standing or walking very slowly. J. Appl. Physiol. 43:577–581, 1978.
- According to this website the average driver in America logged 13,500 miles in 2010. There were 210 million drivers out of a total population of 309 million that year. So the average person drove (210/309)x13,500 = 9,170 miles, or 14,700 km. According to a US Department of Transportation website, the average fuel efficiency for all light duty vehicles (cars, SUVs, vans, pickup trucks) in 2014 was 23.3 mpg. So, if the average miles driven was about the same in 2010 and 2014, 9,170 miles at 23.3 mpg represents 394 US gallons, or 1,491 liters of gasoline. This comes to 4.08 liters per day per person, representing 33,300 Calories of energy.
- Steinhart, J. S. and C. E. Steinhart “Energy Use in the US Food System”, Science 184:307 (1974).
- The USDA estimates that 3,900 Calories of food are produced per capita each day. If the actual consumption per capita is 2,700 Calories, that’s a 31% loss.