CHICAGO - When it comes to global warming, hamburgers are the Hummers of food, scientists say.
Simply switching from steak to salad could cut as much carbon as leaving the car at home a couple days a week.
That's because beef is such an incredibly inefficient food to produce and cows release so much harmful methane into the atmosphere, said Nathan Pelletier of Dalhousie University in Canada.
Pelletier is one of a growing number of scientists studying the environmental costs of food from field to plate.
By looking at everything from how much grain a cow eats before it is ready for slaughter to the emissions released by manure, they are getting a clearer idea of the true costs of food.
The livestock sector is estimated to account for 18 percent of global greenhouse gas emissions and beef is the biggest culprit.
Even though beef only accounts for 30 percent of meat consumption in the developed world it's responsible for 78 percent of the emissions, Pelletier said Sunday at a meeting of the American Association for the Advancement of Science.
That's because a single kilogram of beef produces 16 kilograms carbon dioxide equivalent emissions: four times higher than pork and more than ten times as much as a kilogram of poultry, Pelletier said.
If people were to simply switch from beef to chicken, emissions would be cut by 70 percent, Pelletier said.
Another part of the problem is people are eating far more meat than they need to.
"Meat once was a luxury in our diet," Pelletier said. "We used to eat it once a week. Now we eat it every day."
If meat consumption in the developed world was cut from the current level of about 90 kilograms a year to the recommended level of 53 kilograms a year, livestock related emissions would fall by 44 percent.
"Given the projected doubling of (global) meat production by 2050, we're going to have to cut our emissions by half just to maintain current levels," Pelletier said.
"Technical improvements are not going to get us there."
That's why changing the kinds of food people eat is so important, said Chris Weber, a professor of civil and environmental engineering at Carnegie Mellon University in Pennsylvania.
Food is the third largest contributor to the average US household's carbon footprint after driving and utilities, and in Europe - where people drive less and have smaller homes - it has an even greater impact.
"Food is of particular importance to a consumer's impact because it's a daily choice that is, at least in theory, easy to change," Weber said.
"You make your choice every day about what to eat, but once you have a house and a car you're locked into that for a while."
The average US household contributes about five tons of carbon dioxide a year by driving and about 3.5 tons of equivalent emissions with what they eat, he said.
"Switching to no red meat and no dairy products is the equivalent of (cutting out) 8,100 miles driven in a car ... that gets 25 miles to the gallon," Weber said in an interview following the symposium.
Buying local meat and produce will not have nearly the same effect, he cautioned.
That's because only five percent of the emissions related to food come from transporting food to market.
"You can have a much bigger impact by shifting just one day a week from meat and dairy to anything else than going local every day of the year," Weber said.
For more information on how to eat a low carbon diet, visit www.eatlowcarbon.org.