James Hamblin at The Atlantic examines the results of a study by Helen Harwatt at Oregon State University that examines how a single food substitution affects greenhouse gas production: beans for beef.
A relatively small, single-food substitution could be the most powerful change a person makes in terms of their lifetime environmental impact—more so than downsizing one’s car, or being vigilant about turning off light bulbs, and certainly more than quitting showering.
To understand why the climate impact of beef alone is so large, note that the image at the top of this story is a sea of soybeans in a silo in the Brazilian Amazon rainforest. The beans belong to a feed lot that holds 38,000 cattle, the growth and fattening of which means dispensing 900 metric tons of feed every day. Which is to say that these beans will be eaten by cows, and the cows will convert the beans to meat, and the humans will eat the meat. In the process, the cows will emit much greenhouse gas, and they will consume far more calories in beans than they will yield in meat, meaning far more clearcutting of forests to farm cattle feed than would be necessary if the beans above were simply eaten by people.
The kicker is that you don't even need to become a vegetarian or go vegan to accomplish this. Chicken, pork, fish, eggs and dairy are all consumable in this scenario. So don't have a cow, save the planet!