In the United States, cattle are far from the largest source of greenhouse gases, which include carbon dioxide, methane and others. Their total contribution is dwarfed by the burning of fossil fuels for electricity, transportation and industry. But livestock are among the largest sources of methane, which can have 80 times the heat-trapping power of carbon dioxide although it persists for less time.

Estimates vary, in part because animal emissions are more difficult to quantify than, say, flue gases at a power plant. But enteric fermentation by beef cattle accounts for nearly 2 percent of total emissions in the United States, according to the Environmental Protection Agency.