The argument that a vegetarian diet is more planet-friendly than a carnivorous one is straightforward: If we feed plants to animals, and then eat the animals, we use more resources and produce more greenhouse gases than if we simply eat the plants. As with most arguments about our food supply, though, it's not that simple. Although beef is always climatically costly, pork or chicken can be a better choice than broccoli, calorie for calorie.
Much of the focus on the climate impact of meat has been on cattle, and with good reason. Any way you slice it, beef has the highest environmental cost of just about any food going, and the cow's digestive system is to blame. Ruminants - cows, sheep, goats and also yaks and giraffes - have a four-chambered stomach that digests plants by fermentation. A byproduct of that fermentation is methane, a greenhouse gas with some 20 times the heat-trapping ability of carbon. One cow's annual output of methane - about 100 kilograms - is equivalent to the emissions generated by a car burning 235 gallons of gasoline.
Methane isn't the only strike against ruminants. There's also fertility. Cows can have one calf per year, which means the carbon cost of every cow destined for beef includes the cost of maintaining an adult for a year. Pigs, by contrast, can have two litters a year, with 10 or more pigs per litter.
Then there's feed conversion. It takes six pounds of feed to make one pound of beef, but only 3.5 pounds for pork and two pounds for chicken. Considering the methane, the babies and the feed, it's clear that the ruminants do more damage than their one-stomached barnyard compatriots (monogastrics, they're called).
Comparing cows with pigs, and meat with plants, is often done using data from the Environmental Working Group, which produced a report in 2011 that detailed the environmental cost of meat. The report includes a chart that ranks various foods according to the amount of emissions generated in the course of production. Ruminants are the worst offenders, with lamb generating 39 kilograms of carbon dioxide (or its equivalent) for each kilogram of meat, and beef generating 27. Then come pork (12), turkey (11) and chicken (7). Plants are all lower, ranging from potatoes (3) to lentils (1).