That’s about 8 percent of each country’s total greenhouse gas emissions.

Methane is increasingly drawing attention from researchers because its worldwide levels have doubled in less than 10 years after remaining flat since the 1990s. The reasons behind the increase are not well understood.

In the United States, 42 percent of agricultural emissions come from animal agriculture. Two-thirds of those gases are directly emitted by ruminants: animals like cows, buffalo and sheep that use bacteria in their stomachs to ferment food. That allows them to eat foods, like grasses, that humans can’t.

It also creates a lot of methane when they burp and pass gas. Some non-ruminants — like pigs, horses and mules — also release methane, but not as much.

These calculations of livestock emissions exclude the fertilizers used to grow food for livestock and they typically don’t look at alternative-use scenarios, like what would happen if we removed cows from grasslands and let wild ruminants like bison and deer take over.

Worldwide, livestock accounts for between 14.5 percent and 18 percent of human-induced greenhouse gas emissions. The percentage is lower in the United States in part because our overall greenhouse gas emissions are so much higher than other countries: In the United States we emit 16.5 metric tons per person per year compared to a worldwide average of about five metric tons. Most of America’s emissions come from power plants and transportation, with each accounting for a third of the total.