The Aliso Canyon natural gas well blowout, which lasted for months and sickened scores of nearby residents, has been confirmed as the largest methane leak in history.

According to a peer-reviewed study published Thursday in the journal Science, the nearly four-month leak released roughly 100,000 tons of methane—effectively doubling the methane emissions rate of the entire Los Angeles Basin. Southern California Gas Co. said it stopped the leak earlier this month. State Division of Oil, Gas and Geothermal Resources engineers confirmed the leak was halted last week.

"Aliso Canyon will be, certainly, the biggest single [methane] source of the year," said co-lead study author Stephen Conley of UC Davis and Scientific Aviation. "It's definitely a monster."

Beyond that, he told Inside Climate News, "It's the biggest leak in U.S. history."

As the Los Angeles Times reports, "Conley piloted a single-engine plane rigged with methane and ethane sensors through the plume and analyzed it during 13 different research flights between Nov. 7 and Feb. 13, with the last readings taken just two days after the well was temporarily halted."

The newspaper continues:

With each flight, he would start very low—perhaps a couple hundred feet off the ground—fly through the plume, turn around and go back through at about 100 feet higher. He would do this until he reached the top of the plume, which could take anywhere from about 16 to 35 passes. Adding these slices up would give him the total emissions from the plume at that time. Even after the first flight, the methane readings alarmed him.
“It was 20 times larger than anything else we'd ever measured—it was just kind of a shock," said Conley, who recalled thinking, “What in the world was this big?"

With the leak's pollution equivalent to annual emissions from half a million cars, the disaster will substantially affect California's ability to meet greenhouse gas emission targets for the year, added Tom Ryerson of the National Oceanic and Atmospheric Association (NOAA). "Our results show how failures of natural gas infrastructure can significantly impact greenhouse gas control efforts," he said.

In a blog post Thursday, Environmental Defense Fund chief scientist Steven Hamburg noted that Conley's results were "much higher than those provided by the gas company."

But that's not surprising, he continued, explaining that "accurately measuring these large sources is difficult using ground-based approaches like those used by the gas company to estimate the size of emissions from the facility."