The benefits of eating locally grown food may not extend to curbing global warming, according to a comprehensive study of greenhouse gas emissions from U.S. food.
On a typical spring day, lunch for Seattle-based writer Sage Van Wing includes pasta with pork sausage from a small local farm. The peppers, cheese, and shallots on top come from the nearby farmers market. Van Wing is a locavore-she tries to eat only locally grown foods whenever possible.
Red meat and dairy are responsible for nearly half of all greenhouse gas emissions from food for an average U.S. household.
Van Wing, who coined the term locavore with a friend 3 years ago, says curbing global warming is one of many social and environmental reasons for eating locally. And for many people, "food miles", the distance food travels from farm to plate, are a simple way to gauge food's impact on climate change.
But it's how food is produced, not how far it is transported, that matters most for global warming, according to new research published in ES&T . In fact, eating less red meat and dairy can be a more effective way to lower an average U.S. household's food-related climate footprint than buying local food, says lead author Christopher Weber of Carnegie Mellon University.
Weber and colleague Scott Matthews, also of Carnegie Mellon, conducted a life-cycle assessment of greenhouse gases emitted during all stages of growing and transporting food consumed in the U.S. They found that transportation creates only 11% of the 8.1 metric tons (t) of greenhouse gases (in CO2 equivalents) that an average U.S. household generates annually as a result of food consumption. The agricultural and industrial practices that go into growing and harvesting food are responsible for most (83%) of its greenhouse gas emissions.
For perspective, food accounts for 13% of every U.S. household's 60 t share of total U.S. emissions; this includes industrial and other emissions outside the home. By comparison, driving a car that gets 25 miles per gallon of gasoline for 12,000 miles per year (the U.S. average) produces about 4.4 t of CO2. Switching to a totally local diet is equivalent to driving about 1000 miles less per year, Weber says.
A relatively small dietary shift can accomplish about the same greenhouse gas reduction as eating locally, Weber adds. Replacing red meat and dairy with chicken, fish, or eggs for one day per week reduces emissions equal to 760 miles per year of driving. And switching to vegetables one day per week cuts the equivalent of driving 1160 miles per year.
Several other recent studies have analyzed particular foods and poked holes in the food mile concept. For example, it can be more energy efficient for a British household to buy tomatoes or lettuce from Spain than from heated greenhouses in the U.K.
The new work expands on those studies by providing a comprehensive look at the U.S. food supply. Weber used an input-output life-cycle assessment, which counts not only the CO2 produced when food is shipped but also all greenhouse gases, including methane (CH4) and nitrous oxide (N2O), emitted from farm production. This means counting all the way back to the fossil fuels used to manufacture fertilizer and tractors.
"There is more [total] greenhouse gas impact from methane and nitrous oxide than from all the CO2 in the supply chain," Weber says. In large part, he adds, this is because N2O and CH4 emission in the production of red meat "blows away CO2". Cows burp CH4, and growing their feed uses large amounts of fertilizers that are converted to N2O by soil bacteria.
Edgar Hertwich, an expert on life-cycle analysis who is at the Norwegian University of Science and Technology, calls the results "quite convincing" but notes that consumers should still keep an eye on food flown on airplanes, which have very high greenhouse gas emissions. "Food miles are a very good idea, but not for the faint of heart," adds Gidon Eshel, a Bard Center Fellow at Bard College. "Counting transport alone won't do the trick; you need a full life-cycle analysis."
"It's still useful to think about transport," says David Pimentel of Cornell University, an ecologist who has conducted life-cycle analyses of food's energy use. He recently calculated that if a typical American drives home with a 1 pound can of corn, 311 calories of fossil fuel energy are used to transport the 375-calorie corn in the can.
Van Wing read Weber's paper and found it a "holistic and helpful" look at food miles. But the research doesn't change her outlook on food, she says. She will continue to buy from local growers, whose production practices she can see firsthand.