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Helen Harwatt is a researcher trained in environmental nutrition, a field focused on developing food systems that balance human health and sustainability. She’s interested in policy, but realistic about how much progress can be expected under the aforementioned leadership. So she and colleagues have done research on maximizing the impacts of individuals. As with so many things in life and health, that tends to come down to food.

Recently Harwatt and a team of scientists from Oregon State University, Bard College, and Loma Linda University calculated just what would happen if every American made one dietary change: substituting beans for beef. They found that if everyone were willing and able to do that—hypothetically—the U.S. could still come close to meeting its 2020 greenhouse-gas emission goals, pledged by President Barack Obama in 2009.

That is, even if nothing about our energy infrastructure or transportation system changed—and even if people kept eating chicken and pork and eggs and cheese—this one dietary change could achieve somewhere between 46 and 74 percent of the reductions needed to meet the target.

“I think there’s genuinely a lack of awareness about how much impact this sort of change can have,” Harwatt told me. There have been analyses in the past about the environmental impacts of veganism and vegetarianism, but this study is novel for the idea that a person’s dedication to the cause doesn’t have to be complete in order to matter. 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.

Read: The economic case for worldwide vegetarianism

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.

This inefficient process happens on a massive scale. Brazil, the world’s largest exporter of red meat, holds around 212 million cattle. (In June, the U.S. temporarily suspended imports of beef from Brazil due to abscesses, collections of pus, in the meat.) According to the United Nations, 33 percent of arable land on Earth is used to grow feed for livestock. Even more, 26 percent of the ice-free terrestrial surface of Earth is used for grazing livestock. In all, almost a third of the land on Earth is used to produce meat and animal products.