Most of us are aware that our cars, our coal- or gas-generated electric power and even our cement factories adversely affect the environment. Until recently, however, the foods we eat had gotten a pass in the discussion. Yet according to a 2013 report by the United Nations Food and Agriculture Organization (FAO), the livestock supply chain that produces meat and milk for our diets causes more greenhouse gases—carbon dioxide (CO2), methane, nitrous oxide and the like—to spew into the atmosphere than does either transportation or industry. (Greenhouse gases trap solar energy, thereby warming the earth's surface. Because gases vary in greenhouse potency, scientists tally them according to the amount of CO2 that would have the same warming effect over decades.)