EPA boosts estimate of U.S. methane emissions

The U.S. Environmental Protection Agency, criticized for understating how much methane the United States spews into the atmosphere, has boosted its estimate of total U.S. methane emissions by 13 percent. That’s an increase of more than 3.4 million metric tons of the greenhouse gas and has the same long-term global warming impact as a year’s worth of emissions from about 20 million cars.

The new calculation, released in an EPA report April 15, revises the agency’s U.S. methane emission estimates for 2013 to 28.859 million metric tons, up from the agency’s previous estimate of 25.453 million metric tons. Two-thirds of that increase comes from the natural gas and petroleum sectors, with much of the rest coming from landfills. The report also provides the first estimate of methane emissions for 2014, a slight increase to 29.233 million metric tons.

Globally, methane emissions account for about a quarter of human-caused global warming. Several studies over the last few years have suggested that EPA significantly underestimated the U.S. share of those emissions (SN Online: 4/14/16).

While the new methane estimates are “a step in the right direction,” the agency still has a ways to go, says David Lyon, an environmental scientist at the Environmental Defense Fund. Even with the higher methane estimates, the agency is still undercounting U.S. emissions by about 20 to 60 percent, Lyon says. EPA’s reporting influences U.S. regulation of methane-producing industries such as agriculture and fossil fuel production.

A sizable portion of the still-at-large methane probably comes from “super emitters,” methane sources that contribute a disproportionate share of total emissions. These sources are typically malfunctioning equipment, making them difficult for EPA to account for.