The calculations are based on data and a basic ecological concept that have been around for decades, but no one had actually done the math.

"It's just never been done," said climate researcher Gidon Eshel of the University of Chicago. "The data is simply there to mine."

Eshel and colleague Pamela Martin have published their study in the current issue of the scientific journal Earth Interactions.
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The ecological concept has been taught in biology classes for decades: As energy moves up a food web — from plants to grazing animals to predators — only about 10 percent survives each step. In other words, 100 calories worth of beef patty require about 1,000 calories of grain which, in turn, require 10,000 calories of sunlight.

So if you choose to cut out the middleman (the cow) and get your 100 calories directly from the grain, you only have to grow one-tenth as much grain.

Eshel and Martin gathered U.S. food statistics, along with other data on fossil fuel use by agricultural and personal transportation. Then they looked at how much greenhouse gas was generated by the production of food.