Q: What is the impact of a person becoming a vegetarian on global warming?

Asked by Virginia Troyer, ’85, Santa Cruz, Calif.

I would bet that when most people think about reducing their global warming impact, they think about things like biking to work and turning down the thermostat, not changing their what they put on their plate. But according to a 2006 report by the United Nations Food and Agriculture Organization (FAO), that won't be enough to get the job done. The meat in our diets causes more carbon dioxide (CO2) and other greenhouse gases (GHGs) to be emitted worldwide than the either transportation or industry.

I hate to pick on T-bone steaks and meatballs, but beef is by far the worst climate offender. One reason is that it takes 20 pounds of feed to produce one pound of edible beef. The farming, processing and transportation of that feed requires gas-guzzling machinery, pesticides, fertilizers and trucking over long distances.

But all that human activity pales in comparison to an unavoidable fact of biology: the more you feed cattle, the more they belch and fart. Factor in the equally prodigious piles of manure, and it adds up to vast quantities of methane, a powerful greenhouse gas. All told, the methane produced by cattle nearly triples the Global Warming Potential of their feed production.

Talk about bad gas. But there's more! We raise more cattle every year, and the expansion of livestock is a key factor in tropical deforestation, especially in the Amazon. The FAO report estimates that 70 percent of the deforested land in the Amazon is occupied by pastures. That's obviously bad news for the plants, animals and people pushed aside, but it's bad news for us, too—deforestation pumps still more GHGs into the atmosphere, and dead trees can't suck the CO2 back out.

So what difference does a person make if they switch from the average American diet to a plant-based one? It depends how you calculate it. According to a conservative estimate by Gidon Eshel and Pamela Martin, geophysicists at the University of Chicago, it is the difference of more than 3,000 pounds of "CO2-equivalent" greenhouse gases each year (roughly equivalent to a 4000-mile road trip in a Toyota Camry). By this estimate the country's greenhouse gas emissions would drop by six percent if everyone switched to a vegetarian diet.

But a more recent study conducted by the Worldwatch Institute estimates that the difference would be even greater. By their estimations, livestock and their byproducts account for 51 percent of global greenhouse gas emissions. While many vegetarian diets still include livestock byproducts such as dairy, their study suggests that reducing meat products may have an even greater impact than previously thought.

As it is, Americans consume far more meat than people of any other nation—upwards of 270 pounds a year, per capita. That's about 60 percent more than the average meat consumption in the rest of the developed world. We could start a whole new column on the health effects of that kind of diet, But let's stick with climate change for now. The bottom line, says Stanford global food expert Rosamond Naylor, is that "we could do a lot in this country just by reducing the amount of meat we eat."

What to eat is a personal choice, of course. But I've found that the choice isn't as hard as you might think—though there are always some bumps along the way.

I have been mostly vegetarian for almost three years now. I say "mostly" because it took me months to admit my meat-free decision to my family. You see, I grew up in Alberta, Canada, in a city lovingly known as "Cowtown." It took me several trips home before I was ready to tell my cattle-raising relatives in "the Texas of the North" that I've become a granola-munching Californian. This resulted in vegetarian-except-for-holidays syndrome. I'm over it now.

I realize that going veggie is not something we're all going to do tomorrow, so let me make a couple of reasonable suggestions. For starters, consider choosing poultry or pork over beef. Birds and pigs are more efficient converters of feed to edible weight, and raising them creates less greenhouse gas at every step of the process. And if you currently eat meat every day, why not consider switching to veggie fare one day a week? "Meatless Mondays" could help you trim your personal carbon footprint—and who knows, you might just find you like it as much as I do.

Jess McNally plans to receive her bachelor's and master's degrees in earth systems in 2010.