Which foods are the worst for the environment?

(Photo: Reuters, Paolo Whitaker)

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)

But there's another way to look at the same information. If you stop eating beef, you can't replace a kilogram of it, which has 2,280 calories, with a kilogram of broccoli, at 340 calories. You have to replace it with 6.7 kilograms of broccoli. Calories are the great equalizer, and it makes sense to use them as the basis of the calculation.

When you reorder the chart to look at climate impact by calorie, the landscape looks different. The ruminants still top the chart, but the monogastrics look a whole lot better. Low-calorie crops like broccoli don't do so well. Although beef still looks bad and beans still look good, pork and poultry are on a par with green vegetables. (Which means that a beef-and-leaf paleo diet is the worst choice going, environmentally speaking.)

The claim that vegetarianism is kinder to the planet also fails to consider a couple of kinds of meat that aren't on the Environmental Working Group's chart. Deer and Canada geese do active damage in the areas where they're overpopulated, and wild pigs leave destruction in their path wherever they go. Eat one of those, and do the planet a favor.

Most people, though, are most likely to get their food from the farm, and it's important to note that, although the chart attaches one number to each kind of food, farming styles vary widely and not all pork chops — or tomatoes, or eggs — are created equal. Unfortunately, it's all but impossible for us consumers to figure out the climate impact of the particular specimens on our dinner table, whether they're animal or vegetable.

According to the Food and Agriculture Organization of the United Nations, organic agriculture's CO2 emissions per acre are significantly less than those of conventional agriculture. But yields per acre are also generally lower, and that mitigates the savings. Counterintuitively, the strawberry you buy from the farmer down the road might have a bigger environmental footprint than the strawberry you buy from far away, where a large farm in an ideal climate may grow it more efficiently. But it might not. You can't know. It's maddening.

When it comes to meat, trying to eat responsibly presents a genuine conundrum: What's best for the planet is often what's worst for the animal. The efficiencies of modern conventional livestock farming do indeed decrease greenhouse gases, but they also require the confinement and high density that draw the ire of animal welfare advocates.

Growing an animal as quickly as possible decreases climate impact because it's that many fewer days (or weeks or months) the animal is here to pollute. Increasing feed efficiency likewise decreases the acreage devoted to growing the animal's food. Rich Pirog, senior associate director of the Center for Regional Food Systems at Michigan State University, has studied the environmental impact of various ways of raising livestock; he has co-authored studies of Iowa cattle and pigs. For beef, he found that feedlots, where cattle are kept at high densities and fed grain, beat pastures, where animals are allowed to graze, in the tally of environmental impact. (A study just published in the Proceedings of the National Academies of Science reached a similar conclusion.)

For pigs, there was some overlap in conventional farming and "niche" systems, in which pigs have deep bedding and outdoor access. Pirog says that "the most efficient niche producers were pretty comparable to the average conventional producers." There's less research on poultry, but what has been done indicates that chickens raised in confinement also use fewer resources.

Confinement equals efficiency, but confinement also equals, well, confinement. Although no farmers I've ever spoken with believe their animals are unhappy, many welfare-minded meat consumers (including me) prefer to support a system in which animals have elbow room and outdoor access; where cages aren't used, tails aren't docked and antibiotics aren't routinely administered.

There are other arguments, on both sides — so many that it's easy to pick the ones that make the case for whichever kind of agriculture you're inclined to support. Grass-fed cows don't compete for plants humans can eat, and animals grazing on non-irrigated pasture don't compete for water that could be used to grow food (true!), but grass digestion creates more methane than grain digestion (also true!). Grazing cattle on grasslands can sequester carbon in the soil, but improperly managed grazing can make things worse rather than better. Pollution from manure reservoirs on conventional farms can threaten water and crops, but manure in reservoirs, from animals in confinement, can be converted to energy by methane digesters. Then there's the price of meat, inevitably higher in less efficient systems.

The meat-vs.-other-meat debate is irrelevant to the committed vegetarian, but there are issues other than greenhouse gases in the meat-vs.-plant debate, too. The case for meat includes the ability of an animal to contribute constructively on an integrated farm (chickens help with pest control), the potential for turning food waste (spent grain, whey, expired dairy) into high-quality protein, and the ability to use grasslands, inappropriate for row crops, to produce human food (with grazing cows or goats).

The case for plants has to include their nutritional value. Carbon aside, broccoli beats pork, hands down. And it has to consider killing, which many plant eaters find unacceptable. While the moral implications are beyond the brief of a column devoted to matters of fact, we all have to acknowledge that agriculture is an animal-killing enterprise. Does the rat, poisoned because it's a threat to the grain stores, count for less than the pig, raised and slaughtered with care?

But let's go back to where we began, with greenhouse gases. Even if climate impact is your top priority, it's important to look at the food data in the context of other lifestyle factors. Eating beans is definitely better than eating beef. Driving a Prius is better than driving a Hummer. But one decision trumps every other — potentially by orders of magnitude — and that's how many children you have. No amount of bean-eating or Prius-driving will compensate for reproducing, and it's the childless, not the vegetarians, who are more likely to save the planet. Which doesn't mean that we should ignore the benefits of beans and Prii — or that we shouldn't have kids — it just means that we should acknowledge that human survival takes a climatic toll. Our obligation isn't to minimize our carbon footprint at the expense of all other considerations; it's to try to be prudent, taking those considerations into account.

There are many ways to do that, but no one label — vegetarian, local, organic — has the corner on responsibility. For me, animal welfare is important, and my take on meat is that we should eat less of it, pay more for it, use all of it, and know where it's from. But that's not the last word. There isn't a last word, which means there's not a lot of room for sanctimony. While I think we all need to pay attention, vegetarians shouldn't tell omnivores to eat quinoa instead of pork any more than omnivores should tell vegetarians to eat venison instead of quinoa.