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.)

The FAO report found that livestock production contributes 7.1 billion tons of “CO2-equivalent” greenhouse gases, which amounts to 14.5 percent of the global emissions of these gases. Producing a half-pound hamburger—a patty of meat the size of two decks of cards—releases as much greenhouse gas into the atmosphere as driving a typical passenger vehicle for 37 miles.

In truth, every food we consume, vegetables and fruits included, incurs hidden environmental costs: transportation, refrigeration and fuel for farming, as well as methane emissions from plants and animals, all lead to a buildup of atmospheric greenhouse gases. Take asparagus: in a report prepared for the city of Seattle, Daniel J. Morgan, now at Vanderbilt University, and his co-workers found that growing just half a pound of the vegetable in Peru emits greenhouse gases equivalent to 1.2 ounces of CO2—as a result of applying insecticide and fertilizer, pumping water and running heavy, gas-guzzling farm equipment. To refrigerate and transport the vegetable to an American dinner table generates another two ounces of CO2-equivalent greenhouse gases, for a total CO2 equivalent of 3.2 ounces.

That is nothing compared to beef. The 2013 FAO study estimated that, while the details depend on the production method, specialized beef cattle emit on average 1.25 pounds of methane for each pound of beef they produce. Because methane has roughly 23 times the global-warming potential of CO2, those emissions are the equivalent of releasing roughly 29 pounds of CO2 into the atmosphere for each pound of beef produced.

Raising animals also requires a large amount of feed or grazing land per unit of body weight. Changes in land cover from forest or meadow to pasture exact a carbon price, as does nitrous oxide emitted by fertilizer and feed crop residues. These sources add another 22.2 pounds of CO2-equivalent greenhouse gases for every pound of food obtained from the cattle. Finally, farms for raising animals produce copious manure and other wastes that give rise to greenhouse gases.

Taking such factors into account, the FAO figured that in beef production, each pound of carcass weight generates the equivalent of 67.6 pounds of CO2—pound for pound, more than 300 times the amount emitted by producing asparagus.

That number lumps the less desirable parts of the carcass—used for bone meal and dog food—along with the high-quality meat, and it reflects a global average. The emissions intensity is higher still for beef meat, and it varies widely across the world. In the least efficient operations, a pound of protein from beef may come at the cost of 500 pounds of CO2-equivalent emissions. A feedlot, or concentrated animal feeding operation (CAFO), is not only cheaper to run but also has a far lower environmental cost. Yet even the best of these operations still generate the equivalent of 100-plus pounds of CO2 for every pound of beef protein.

Other common meats have much lower impact on the atmosphere; the FAO estimate that the average per-pound environmental cost of pork protein is about 50 pounds of CO2-equivalent greenhouse gases, and forchicken protein that cost is about 20 percent lower still.

What can be done? Improving waste management and farming practices would certainly reduce the carbon footprint of beef production. Methane-capturing systems can, in principle, turn the gas into electricity. But those systems remain too costly to be commercially viable.

The dietary choices we make as individuals can also make a difference. Eating locally produced food, for instance, can reduce shipping—although trucking food in small batches from nearby farms saves surprisingly little in greenhouse emissions. The most effective change, for people in the U.S. and the rest of the rich world, would be simply to eat less beef.

The following graphics quantify the links between beef production and greenhouse gases in sobering detail. The take-home lesson is clear: we ought to give careful thought to diet and its consequences for the planet if we are serious about limiting the emissions of greenhouse gases. Click on the graphics to enlarge.