There’s less to that food mile than you might think
Over the last decade, the locavore community has embraced the idea of “food miles,” touting that local food travels a very short distance to get to your plate. Some grocery stores even post the proximity of farms they buy from, and several authors have written books about a "100-mile" or "250-mile" diet that encompasses eating food produced within a specific radius.
Advocates say local food is fresher, more nutrient-rich, and many have suggested it's "greener," too, because less fuel is required to ship it.
That last argument has an inherent logic to it, considering that some food travels tens of thousands of miles to reach your plate.
But when you look at the energy used to grow and process food through its whole life cycle — from seed to plate — the trip to the grocery store contributes only a tiny fraction of its greenhouse gas emissions, according to environmental and civil engineer Scott Matthews.
“If you're someone who cares about the greenhouse gas impact of food, you should be thinking about diet changes, not localization," Matthews said.
Matthews is a professor at Carnegie Mellon University in Pittsburg, Penn., and he usually studies the life cycle assessment of things like cars and buildings. But Matthews, along with coauthor Chris Weber, got funding from an Environmental Protection Agency fellowship and the National Science Foundation to look into the "food miles" idea.
They found that transporting food from the farm to the grocery store on average is just 4 percent of the food's emissions over its life cycle. The majority of food's carbon footprint, 83 percent in their estimate, actually comes from growing it — from inputs like tractor fuel, fertilizer, growing grain to feed animals.
Even when you just consider transportation, that last trip from farm to grocery store is actually only one quarter of its total "food miles," Matthew said. You also have to consider delivery of fertilizer and seeds to the farm and shipping the food to be washed and processed and boxed.
"Most of the people who were talking about food miles were really implying that the food miles is really from 'farm' to 'store,' meaning the last particular set of miles for the finished product, the food product you were actually going to consume," Matthews said.
The study found that even if you went totally local, and magically got all your food from zero miles away, you'd likely only reduce your contribution to greenhouse gas emissions by about 5 percent.
A meaty issue
You get a lot more mileage, so to speak, by changing what you eat.
"Most of the message really is thinking about cows vs. non-cows, in terms of reducing impact," Matthews said.
Red meat is by far the biggest greenhouse gas emitter. Matthews’ data show if the average American cut just one day a week of red meat out of their diet, it would have the same effect as if that person bought all their food locally for an entire year.
Why does red meat has such a big environmental footprint?
"Both ends of the cow," Matthews said.
From farm to grocery
Manure and burping, as Matthews politely described it, emit significant amounts of methane, which is a greenhouse gas 25 times more warming than carbon dioxide. And another third of red meat's emissions come from fuel used on the farm and to grow the corn feed, and lastly:
"About a third are effectively farm or soil management practices. So while we didn't actually study the benefits you might get from different farming practices, the potential is there to be about a one-third reduction for red meat,” he said.
Jude Capper is a professor of dairy sciences at Washington State University. She studies the carbon footprint of dairy and red meat; her research is funded by her university and the beef industry.
Her red meat numbers don't quite match Matthews’ study, but Capper said there is a lot of nuance in how beef is raised. For example, grass-fed beef actually has a higher carbon footprint than feedlot cattle, she said. This is partly because grass-fed cows take longer to beef up (and thus emit more methane over their lifetime) and partly because eating grass for them is sort of like eating beans for humans.
Capper also points to her research which found that over the last 30 years the cattle industry has improved the growth rate of cattle and the beef yield per animal.
"So we've effectively saved 125-odd days of feed, fertilizers, fuel, and obviously the methane from those animals as well. So those two improvements, in total have cut carbon footprint by 18 percent," she said.
But Capper conceded that cattle still probably are the highest greenhouse gas emitters. And she said that data in Matthew's study — which found red meat with the highest emissions by far, followed by dairy, then cereals and carbs, then fruits and vegetables, then chicken and fish and eggs— seem about right.
But back to the Matthews’ study and the contention that buying local alone does little to reduce your carbon footprint.
Matthews said when the study came out in 2008, some people in the locavore community reacted negatively.
"We wrote a paper scientifically saying that food miles are a small part of the carbon footprint of someone. And the way it was getting spun, was that if you're making decisions on food miles you're being dumb," Matthews said.
Not so, he said, noting that he and coauthor Weber both buy local food "but we don't do it because we're trying to reduce our carbon footprint. We do it because we're trying to support local agriculture, because of freshness and all these other things."
Rich Pirog, director of the Center for Sustainable Food Systems at Michigan State University and an author of a paper on food miles, pointed to many other benefits to local food that are hard to measure. Local food is often fresh and healthy, and many people really enjoy the social aspects of the farmers market.
And he notes it does have environmental benefits:
"The local food systems continue to preserve the genetic diversity that we have. You go to grocery store and you look at apples or tomatoes, it's pretty much just a few varieties, cultivars. But many farmers at farmers markets are using heirloom varieties, and so they're helping to preserve a wide genetic diversity, and that is an environmental benefit."