Crews work on a relief well at the Aliso Canyon facility above the Porter Ranch area of Los Angeles. Dean Musgrove/Los Angeles Daily News via AP, Pool, File

The L.A. gas spew was “equivalent to the annual energy sector methane emissions from medium-sized E.U. nations.”

Scientists had already been saying it, and now it’s a research-backed fact: The nearly four-month-long gas leak in the Los Angeles neighborhood of Porter Ranch was the largest in U.S. history.

In a paper published Thursday in Science, researchers report that the cracked well buried deep underground in the Southern California Gas Company’s San Aliso Canyon storage facility released some 97,100 tons of methane between October 23, when the leak was first detected, to February 18, when the well was sealed. That amount is the equivalent of about a quarter of the amount of methane emitted annually across the entire L.A. Basin.

The leak represents a significant setback for California’s efforts to reduce its emissions, particularly because methane is so potent as a greenhouse gas—much more so than carbon dioxide.

“The total release from Aliso Canyon … is equivalent to the annual energy sector CH4 emissions from medium-sized EU nations,” the researchers write. “The radiative forcing from this amount of [methane], integrated over the next 100 years, is equal to that from the annual GHG emissions from 572,000 passenger cars in the U.S.”

The leak was also a public health hazard while in progress. The putrid odor of mercaptan, a chemical commonly added to methane to give it a smell, sickened and displaced thousands of Porter Ranch residents, who are only just beginning to move back home.

The scale of the leak, and the attention it attracted, was unusually dramatic. But leaking natural gas pipes are a common feature all over the country, reflecting the industry’s lax monitoring of aging infrastructure and a lack of strong regulations, according to Mark Brownstein, the vice president of climate and energy at the Environmental Defense Fund. Evidence suggests that these small leaks in the transmission and storage of gas add up to a significant source of the total emissions coming from the natural gas industry.

“Aliso Canyon is a dramatic example of something that happens on a daily basis,” Brownstein tells CityLab.

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