The debate over climate change has reached a rarefied level of policy abstraction in recent months. Carbon tax or cap-and-trade? Upstream or downstream? Should we auction permits? In general, these policies aim to do a simple thing, in a simple way: persuade us to undertake fewer activities that are bad for the atmosphere by making those activities more expensive. Driving an SUV would become pricier. So would heating a giant house with coal and buying electricity from an inefficient power plant. But there's one activity that's not on the list and should be: eating a hamburger.

According to a 2006 United Nations report, livestock accounts for 18 percent of worldwide greenhouse gas emissions. Some of meat's contribution to climate change is intuitive. It's more energy efficient to grow grain and feed it to people than it is to grow grain and turn it into feed that we give to calves until they become adults that we then slaughter to feed to people. Some of the contribution is gross. "Manure lagoons," for instance, is the oddly evocative name for the acres of animal excrement that sit in the sun steaming nitrous oxide into the atmosphere.

Two researchers at the University of Chicago estimated that switching to a vegan diet would have a bigger impact than trading in your gas guzzler for a more fuel-efficient car. A study out of Carnegie Mellon University found that the average American would do less for the planet by switching to a totally local diet than by going vegetarian one day a week. That prompted a prominent environmentalist to recommend that people give up meat one day a week to take pressure off the atmosphere.

In comparison with cars or appliances or heating your house, eating pasta on a night when you'd otherwise have made fajitas is easy. It doesn't require a long commute on the bus or the disposable income to trade up to a Prius. It doesn't mean you have to scrounge for change to buy a carbon offset. In fact, it saves money. It's healthful. And it can be done immediately. A Montanan who drives 40 miles to work might not have the option to take public transportation. But he or she can probably pull off a veggie stew. A cash-strapped family might not be able buy a new dishwasher. But it might be able to replace meatballs with macaroni andcheese. That is the whole point behind the cheery PB&J Campaign, which reminds that "you can fight global warming by having a PB&J for lunch." Given that PB&J is delicious, it's not the world's most onerous commitment.

Going vegetarian might not be as effective as going vegan, but it's better than eating meat, and eating meat less is better than eating meat more. It would be a whole lot better for the planet if everyone eliminated one meat meal a week than if a small core of die-hards developed perfectly virtuous diets. So, if we're going to take global warming seriously, if we're going to make crude oil more expensive and tank-size cars less practical, there's no reason to ignore the impact of what we put on our plates.