April 18, 2008 -- Next time you find yourself standing in the grocery store, agonizing over whether your green conscience permits you to buy the garlic shipped in from China, relax. You'll do more to reduce the greenhouse gas impact of your diet by taking the ground beef out of your cart.
That's the finding of new analysis by Christopher Weber and H. Scott Matthews of Carnegie Mellon University in Pittsburgh, Pa., who compared the greenhouse gas emissions caused by producing and transporting various categories of food in the United States.
"If you're looking across everything that you're eating, the type of food that you're choosing matters a lot more than where it comes from," Weber said.
The pair found that transporting food from the farm or production site to the store contributed only 4 percent of the total greenhouse gas emissions in the U.S. food supply, while producing the food accounted for 83 percent.
Since production dominates, choosing categories of food whose growth and processing release less greenhouse gas can make a bigger difference than becoming a strict locovore, they report. Red meat produces more greenhouse gases than any other food category -- about 2.5 times more, on average, than chicken or fish.
As a byproduct of their digestion, cows naturally produce methane, a greenhouse gas 23 times more potent than carbon dioxide. Fertilizer and manure, meanwhile, create the even-more-potent greenhouse gas, nitrous oxide (N2O).
"A majority of food's climate impact is due to non-CO2 greenhouse gases," the team wrote.
Their findings appear online this week in the journal Environmental Science & Technology.
Dairy products come in second in greenhouse impact in most of the team's analyses, since they also come from cows. Cereals and carbohydrates; chicken, fish and eggs; and fruits and vegetables were similar to each other in their contributions.
Armed with this information, the team calculated that shifting roughly a quarter of the 240 red meat calories the average American eats a day to chicken, grains or fruits and vegetables would have the same effect as going completely local.
Of course, if you are already eating mostly veggies and grains, cutting out the transportation component by choosing local food won't hurt.
However, Weber cautioned, other studies have looked at the greenhouse gas impacts of individual foods like tomatoes and apples and found that long-term refrigerated storage or greenhouses may be required for even local supplies, which may create energy demands that balance or exceed that needed to transport the same foods from friendlier climes.
"There are other advantages to going local," said Pamela Martin of the University of Chicago, who has also studied the greenhouse gas footprint of the food supply. "There are also local economic benefits and food security issues."
Her preliminary research suggests that greenhouse gas emissions from fertilizers and pesticides outweigh transportation emissions.
The importance of transportation may be different in other countries, added Weber. In the U.K., a larger fraction of food arrives by plane, emitting more CO2. Emissions will also vary locally, depending on how far one is from coastal ports or train lines.
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