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 890 litres 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 kilograms of feed to make one kilogram of beef, but only 3.5 kilograms for pork and two kilograms 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. 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 such as broccoli don't do so well. Although beef still looks bad and beans still look good, pork and poultry are on par with green vegetables. (Which means 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 favour.
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.
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.
There are other arguments, on both sides -- so many 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.
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. We all have to acknowledge agriculture is an animal-killing enterprise. But does the rat, poisoned because it's a threat to the grain stores, count for less than the pig, raised and slaughtered with care?
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 we should ignore the benefits of beans and hybrid vehicles -- or that we shouldn't have kids -- it just means we should acknowledge 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.
Haspel farms oysters on Cape Cod and writes about food and science.
-- Washington Post-Bloomberg