The average American drives his car 8,322 miles each year, emitting 1.9 to 4.7 tons of carbon dioxide (depending on vehicle model and fuel efficiency). He also consumes 3,774 calories each day. (Yikes!) So, what do these statistics have in common?

Americans’ habits are hazardous to their health—and the planet’s, according to Drs. Gidon Eshel (right) and Pamela Martin of the University of Chicago. (See yesterday’s blog entry, Vegan Diet Is Earth-Friendly.)

In 2002, energy used for food production accounted for 17% of all fossil-fuel use in the United States, and the burning of these fossil fuels emitted three-quarters of a ton of carbon dioxide per person. This alone amounts to approximately one-third the average greenhouse-gas emissions of personal transportation. But livestock production and its animal waste also emit greenhouse gases not associated with fossil-fuel combustion—primarily methane and nitrous oxide.

“An example would be manure lagoons that are associated with large-scale pork production,” Dr. Eshel says. “Those emit a lot of nitrous oxide into the atmosphere.”

While methane and nitrous oxide are relatively rare compared with carbon dioxide, they are, molecule for molecule, far more powerful greenhouse gases than carbon dioxide. A single pound of methane, for example, has the same greenhouse effect as approximately 50 lbs. of carbon dioxide.

In their study published last month in Earth Interactions, Drs. Eshel and Martin compared the energy consumption and greenhouse-gas emissions underlying five diets: the average American, red meat, fish, poultry and vegetarian (including eggs and dairy)—each of which equaled 3,774 calories per day. The vegetarian diet turned out to be the most energy-efficient, followed by poultry and the average American diet. Fish and red meat virtually tied as the least efficient.

The impact of producing fish came as the study’s biggest surprise to Dr. Martin, an assistant professor of geophysical sciences.

“Fish can be from one extreme to the other,” she says. Sardines and anchovies flourish near coastal areas and can be harvested with minimal energy expenditure. But swordfish and other large predatory species required energy-intensive long-distance voyages.

As for red meat, “the adverse effects of dietary animal fat intake on cardiovascular diseases are by now well established,” the researchers write. “Similar effects are also seen when meat, rather than fat, intake is considered. To our knowledge, there is currently no credible evidence that plant-based diets actually undermine health; the balance of available evidence suggests that plant-based diets are at the very least just as safe as mixed ones, and most likely safer.”

Drs. Eshel and Martin now plan to examine the energy expenditures associated with small organic farms to see whether they offer a healthier planetary alternative to large agribusiness companies. They know a 5- to 10-acre plot on an organic farm typically provides enough vegetables to support 200–300 families—and “we’re starting to investigate whether you can downscale food production and be efficient that way,” Dr. Martin says.

Photo by Lloyd DeGrane