Which is responsible for more global warming: your BMW or your Big Mac? Believe it or not, it's the burger. The international meat industry generates roughly 18% of the world's greenhouse-gas emissions—even more than transportation—according to a report last year from the U.N.'s Food and Agriculture Organization.
Much of that comes from the nitrous oxide in manure and the methane that is, as the New York Times delicately put it, "the natural result of bovine digestion." Methane has a warming effect that is 23 times as great as that of carbon, while nitrous oxide is 296 times as great.
There are 1.5 billion cattle and buffalo on the planet, along with 1.7 billion sheep and goats. Their populations are rising fast, especially in the developing world. Global meat production is expected to double between 2001 and 2050. Given the amount of energy consumed raising, shipping and selling livestock, a 16-oz.T-bone is like a Hummer on a plate.
If you switch to vegetarianism, you can shrink your carbon footprint by up to 1.5 tons of carbon dioxide a year, according to research by the University of Chicago. Trading a standard car for a hybrid cuts only about one ton—and isn't as tasty.