The food that people eat is just as important as what kind of cars they drive when it comes to creating the greenhouse-gas emissions that many scientists have linked to global warming, according to a report accepted for publication in the journal Earth Interactions.

Both the burning of fossil fuels during food production and non-carbon dioxide emissions associated with livestock and animal waste contribute to the problem, the University of Chicago's Gidon Eshel and Pamela Martin wrote in the report.

The average American diet requires the production of an extra ton and a half of carbon dioxide-equivalent, in the form of actual carbon dioxide as well as methane and other greenhouse gases compared to a strictly vegetarian diet, according to Eshel and Martin. And with Earth Day approaching on April 22, cutting down on just a few eggs or hamburgers each week is an easy way to reduce greenhouse-gas emissions, they said. "We neither make a value judgment nor do we make a categorical statement," said Eshel, an Assistant Professor in Geophysical Sciences.

"We say that however close you can be to a vegan diet and further from the mean American diet, the better you are for the planet. It doesn't have to be all the way to the extreme end of vegan. If you simply cut down from two burgers a week to one, you've already made a substantial difference."

The average American drives 8,322 miles by car annually, emitting 1.9 to 4.7 tons of carbon dioxide, depending on the vehicle model and fuel efficiency. Meanwhile, Americans also consume an average of 3,774 calories of food each day.

In 2002, energy used for food production accounted for 17 percent 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. That alone amounts to approximately one-third the average greenhouse-gas emissions of personal transportation. But livestock production and associated 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," Eshel said. "Those emit a lot of nitrous oxide into the atmosphere."

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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 pounds of carbon dioxide.

In their study, Eshel and Martin compared the energy consumption and greenhouse-gas emissions that underlie five diets: average American, red meat, fish, poultry and vegetarian (including eggs and dairy), all equaling 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 Martin, an Assistant Professor in Geophysical Sciences. "Fish can be from one extreme to the other," Martin said. 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.

Martin and Eshel's research indicated that plant-based diets are healthier for people as well as for the planet.

"The adverse effects of dietary animal fat intake on cardiovascular diseases is by now well established. Similar effects are also seen when meat, rather than fat, intake is considered," Martin and Eshel wrote. "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."

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In their next phase of research, Eshel and Martin will examine the energy expenditures associated with small organic farms, to see if they offer a healthier planetary alternative to large agribusiness companies. Such farms typically provide the vegetables sufficient to support 200 to 300 families on plots of five to 10 acres.

"We're starting to investigate whether you can downscale food production and be efficient that way," Martin said.