April 19, 2006 -- -- Your personal impact on global warming may be influenced as much by what you eat as by what you drive.
That surprising conclusion comes from a couple of scientists who have taken an unusual look at the production of greenhouse gases from an angle that not many folks have even thought about. Gidon Eshel and Pamela Martin, assistant professors of geophysics at the University of Chicago, have found that our consumption of red meat may be as bad for the planet as it is for our bodies.
If you want to help lower greenhouse gas emissions, they conclude in a report to be published in the journal Earth Interactions, become a vegetarian.
In the interest of full disclosure, it should be noted that both researchers are vegetarians, although they admit to cheating a little with an occasional sardine. They say their conclusions are backed up by hard data.
Eshel and Martin collected that data from a wide range of sources, and they examined the amount of fossil-fuel energy -- and thus the level of production of greenhouse gases -- required for five different diets. The vegetarian diet turned out to be the most energy efficient, followed by poultry, and what they call the "mean American diet," which consists of a little bit of everything.
There was a surprising tie for last place. In terms of energy required for harvesting and processing, fish and red meat ended up in a "virtual tie," but that's just in terms of energy consumed. When you toss in all those other factors, such as bovine flatulence and gas released by manure, red meat comes in dead last. Fish remains in fourth place, some distance behind poultry and the mean American diet, chiefly because the type of fish preferred by Americans requires a lot of energy to catch.
Can changing your diet really have much of an impact?
"It is comparable to the difference between driving an SUV and driving a reasonable sedan," said Eshel, who drives a Honda Civic, and only when he has to.
Eshel, who grew up on a farm, has always been interested in ecology and the impact we have on the planet. He got into this research, he says, because "now that I'm a professor of geophysics, I have tools in my tool kit that I can apply much more quantitatively and rigorously to evaluate what we do."
It's probably safe to say that both he and Martin figured the vegetarian diet would come out on top, but demonstrating that wasn't easy.
The first hurdle, Eshel says, was coming up "with those semirealistic diets." We don't all eat the same way, of course, so how do you figure out the accumulative impact of our widely varied diets?
The researchers began with data from the U.S. Department of Energy that quantifies the "food disappearance" rate.
"What they are referring to is the rate at which food disappears from supermarket shelves," Eshel said. On the basis of that data, they were able to construct the five semirealistic diets.
Then they collected data from a wide range of sources, mostly available to anyone on the Internet, concerning the amount of energy required to grow, harvest and prepare the foods that make up those five diets.
The centerpiece is the "mean American diet." About 72 percent of the calories from that diet are plant-based.
"Of course, most of it is tomatoes and ketchup and potatoes and french fries, but none the less it is plant-based," Eshel said.
Of the remaining 28 percent, about half comes from meat, and the rest from dairy and eggs.
When they looked at only carbon dioxide emissions associated directly with energy consumption, they came up with the vegetarian diet far less damaging to the planet than the others.
However, the researchers admit that their findings can't be considered exact. Take fish, for instance.
"The seafood portion of American diets is heavily skewed toward what is called charismatic predator fish," Eshel said, which are harder to catch. "Sword, shark and tuna and so on require long-distance ocean journeys, and those efforts are not efficient. They require a lot of labor and a lot of fossil fuel."
Switching from red meat to fish won't necessarily help the planet, unless you can develop a taste for anchovies, which prefer to remain closer to shore and travel in large schools that can be easily caught and harvested.
Considering the fact that so much of our energy goes into such areas as transportation, is it realistic to think of lowering greenhouse gases by changing the typical American diet?
The researchers insist it is. The United States, Eshel says, accounts for about 28 percent of the world's total greenhouse gas emissions.
"The U.S. has five sectors of the economy that are large emitters," he said. "Those are transportation, industrial, commercial, residential and agriculture."
Energy used in agriculture has grown substantially in recent years, he says, and now stands at around 18 percent or 19 percent of the nation's energy use.
The researchers say their findings show that at least 6 percent of that use comes from the production of foods that are not energy efficient, like cattle and the food to feed them. Considering that the "mean American diet is responsible for an additional ton-and-a-half of greenhouse gas emissions a year from each of us," the savings could be substantial.
"It's a huge deal," Eshel said.
So do these two vegetarians want us to give up our hamburgers and tuna and eat cauliflower?
"I hate to sound preachy," Eshel said, but it wouldn't hurt to give a little.
"I say eat whatever works for you, but just keep in mind that the less animal-based food you eat, and the more you replace those calories with plant-based food, the better off you are, in terms of your health as well as your contributions to the health of the planet."
Bring on the anchovies!