Who are the biggest U.S. methane emitters?

• Methane is a potent greenhouse gas, with roughly 30 times the warming power of carbon dioxide.
  
• About 60% of global methane is emitted by human activity, with a large portion linked to America's natural gas boom.
• More than 8,000 facilities in the U.S. emit large amounts of methane, not counting emissions from agriculture.

President Donald Trump's move to loosen methane emission standards is premised on the notion that the oil and gas industry produces relatively little of the pollutant. In fact, although methane has many sources, fossil fuel production is the No. 1 emitter in the U.S.

Methane is a potent greenhouse gas, with roughly 30 times the warming power of carbon dioxide. It accounts for roughly one-fifth of the human-induced warming the planet has experienced so far. Given its disparate origins, however, methane is hard to regulate and even to track. Global data show that methane emissions, which had declined starting in 1990, began rising again around 2007. Many attribute the spike to the U.S. shale gas boom, with started around the same time.

"[S]hale-gas production in North America over the past decade may have contributed more than half of all of the increased emissions from fossil fuels globally," a study concluded earlier this month. The U.S. this year became the world's largest oil and gas producer. 

"The increase in methane has contributed significantly to the accelerated global warming and climate disruption the Earth has experienced in recent years," Robert Howarth, a professor of ecology at Cornell University and the study's author, told the Associated Press.

Not all scientists agree with those conclusions. Earlier studies have shown that most of the methane in the atmosphere came from bacterial sources. Gas-emitting bacteria naturally live in places low on oxygen, like wetlands and the stomachs of cattle, sheep and goats. Here, too, human activities — the creation of rice paddies, raising of huge numbers of sheep and goats, and landfilling organic material that then rots — have helped the methane-belching microbes proliferate.

According to data from the Environmental Protection Agency, nearly one-third of U.S. methane emissions come from oil and gas development. 

However, the EPA has proved to be less than thorough when it comes to tracking methane emissions. One study published in Science last year found that the U.S. is producing about 60% more methane than the agency estimates, largely due to leaks along the supply chain. 

"Methane is the most significant component of natural gas, and anytime natural gas is leaked that results in methane emissions," said Peter Zalzal, lead attorney at the Environmental Defense Fund, an advocacy group. Leaks happen in the processing, transmission and storage of natural gas, he said.

So where do those emissions take place? The EPA tracks, though imperfectly, facilities that belch large amounts of greenhouse gases. This Greenhouse Gas Reporting Program accounts for about 85 percent of all emitters in the U.S., but omits the agriculture sector. 

The facilities topping the list are those focused on coal. Warrior Met Coal, a metallurgical coal mine in Brookwood, Alabama, released more methane than any other fossil-fuel producer in 2017, emitting the equivalent of 3.7 million metric tons of CO2. A pair of mines in southwest Pennsylvania, owned by Consol Energy, were No. 2 and No. 3 on the EPA's list, together emitting the methane equivalent of 4.7 million metric tons of CO2. (A third mine was No. 11 on the list.) 

Other big methane emitters include a trio of West Virginia mines owned by Ohio Valley Resources, a subsidiary of the coal giant Murray Energy. The Marion County Mine, Marshall County Mine and Monongalia County Mine each emitted 2.6 million metric tons of CO2 equivalent in 2017, the EPA data show, a total of 7.8 million tons. That's equivalent to the energy used by 934,000 households in one year.

Another property from the same owner, the Harrison County Mine, is No. 10, emitting an additional 1.5 million metric tons. (Harrison is one of several West Virginia mines being sued by environmental groups for allegedly polluting nearby waterways.)

When these emissions are mapped out, higher-than-normal concentrations of methane emissions are visible in states that produce a lot of natural gas, including California, Pennsylvania, West Virginia and Colorado.

But Texas — the epicenter of the U.S. oil and gas boom — tops the list list, spewing as much as 20% of the methane in the EPA's database of large emitters. Of the more than 8,000 sites on that list, roughly 1,180 of them are to be found in the Lone Star state; some 350 sites are in the Houston area alone. 

The concentration of methane in relatively few locations around the U.S. presents an opportunity, environmentalists say, making it possible to significantly reduce emissions with a handful of fixes. Experts also say cutting back on methane would do much to limit global warming to 2 degrees Celsius because the gas doesn't persist as long as carbon dioxide. Whereas CO2 sticks around in the atmosphere for hundreds of years, methane dissipates after about a decade.

In theory, fossil-fuel extractors should have an incentive to limit how much methane they release into the air, since any leakage is effectively natural gas that's not sold to a paying customer. 

That could be one reason that a few major energy companies, including Shell, BP America and ExxonMobil, have asked the U.S. government to tighten — not loosen — federal methane rules. 

BP America chief Susan Dio has said that there is a "clear business case" for lowering methane emissions. That's one area where the environmental movement and extraction industry agree. But it remains to be seen if companies will voluntarily cut methane pollution when there isn't a legal requirement to do so.

"We certainly have the ability to substantially reduce those [methane] emissions with existing technology," said the EDF's Zalzal. "We can get very close to having no emissions."