Livestock create a beefy portion of all greenhouse-gas emissions: 18 percent, according to the United Nations. Could the answer be as simple as two slices of bread and a slathering of peanut butter and jelly? Perhaps. Compared with a burger, this classic sandwich saves as much as 2.5 pounds of carbon dioxide, 280 gallons of water, and 50 square feet of land--even more if you wash it down with a glass of soy milk. "You don't have to change your whole diet to change the world," says the PB&J Campaign. "Just start with lunch."