The first time Stephen Conley flew through the plume of natural gas hovering above Aliso Canyon, California, he knew the situation was bad. He couldn’t see the methane or ethane pouring out from the old well, but he could smell the rotten-egg odor of the mercaptan added to natural gas to warn people of leaks. “It was nasty,” he recalls.

And then there were the readings from his plane’s scientific instrumentation. Conley has flown his specialized research plane over the sites of many oil and gas leaks in the past. In normal, leak-free air, he usually detects about 2 parts per million (ppm) of methane. Over a leak, that might go up to 4 or 5 ppm. But the air over California in November had levels of 50 ppm a mile from the leak site.

“That’s when I first got this idea that, holy crap, this is a big leak,” says Conley, an atmospheric scientist at the University of California, Davis, and Scientific Aviation.

Now, analysis of Conley's data reveals that by the time the leak had been plugged, just over 107,000 tons of methane and 8,000 tons of ethane had been released from Aliso Canyon. That's the equivalent of the greenhouse gas emissions from half a million cars, spewed into the air near Los Angeles over the span of 16 weeks.

“On the scale of the control efforts that have been put in place to minimize greenhouse gas emissions, it rolls that back years,” says study co-author Thomas Ryerson, a research chemist at the National Oceanic and Atmospheric Administration.

Though methane is a powerful greenhouse gas and the Aliso Canyon event was a “monster” emitter, the event highlights an even bigger problem, Conley says. There are hundreds of natural gas storage facilities like this one around the country, and there’s nothing in place to monitor these facilities for leaks or respond to them quickly.

“Even if each one is leaking [a little bit], that’s a big number,” Conley warns.

The Aliso Canyon leak came from a natural gas storage facility that had started out its life in 1954 as an oil well. In 1973, that well was converted into natural gas storage, a common practice for U.S. energy companies that need a place to store the fuel near towns and cities.

On October 23, residents of the nearby town of Porter Ranch reported smelling a gas leak, and Southern California Gas Company discovered the leak at Aliso Canyon. Two weeks later, Conley was tasked by the California Energy Commission, for whom he had been working under contract, to fly through the plume above the leak and map out where and how much methane and ethane were being emitted.

Conley and his team made 13 flights through the plume between November 7, two weeks after the leak began, and February 13, two days after the leak was plugged.

Because the natural gas had been stored in an old oil well, it also contained small amounts of substances, such as benzene and toluene, that wouldn’t normally be found in a natural gas pipeline, says Ryerson. Other scientists led by Donald Blake of the University of California, Irvine, collected samples of the gas down on the ground and analyzed it back in the lab. Combining that data with Conley's measurements of methane and ethane gave the researchers “the DNA of the leak,” Ryerson says.

The team confirmed that efforts to stop the leak had been successful, though 3 percent of the natural gas stored in the facility had been lost by that time. The data also showed that the Aliso Canyon event released enough methane to make this the largest leak in history in terms of climate impact, Conley and his colleagues report this week in Science. Only one previous event, at Moss Bluff, Texas in 2004, released more natural gas, but most of that burned off in a huge fireball.

The leak also released some 2.5 tons of benzene, a carcinogen, into the atmosphere, they found. That sounds like a lot, but cars and other sources emit about a thousand times more every year, Ryerson says. Individuals who were in the way of the plume may have been exposed to more worrying amounts of the substance, but for now there’s no way to know.

Southern California Gas Company has stated that it will mitigate the greenhouse gas emissions caused by the leak. Francesca Hopkins, an Earth systems scientist at NASA’s Jet Propulsion Laboratory, has some ideas about how they can do that.

While at UC Irvine, she led a study that mapped out methane emissions across the LA Basin using a white Ford Transit van equipped with a snorkel and a host of scientific equipment. As they report in the Journal of Geophysical Research: Atmospheres, her team found methane leaking from compressed natural gas fueling stations, gas-fired power plants, landfills—even ones that had been closed for 50 years—and, of course, cows.

Plugging up those “fugitive leaks” could be part of the gas company's mitigation efforts, Hopkins says. And targeting methane leaks could bring a far bigger bang for the buck than carbon dioxide emissions, she says. While methane has a shorter lifespan in the atmosphere, it is also a far more potent greenhouse gas and one that has an economic value, since lost methane is essentially wasted fuel. Luckily, methane is also a lot easier to get rid of because it can be burned.

Conley notes that this one leak’s overall contribution to climate change is just a drop in the bucket. That’s because there’s already so much carbon dioxide, methane and other greenhouse gases being released around the globe. For his team, the real issue is how to prevent such massive leaks from happening more often and becoming a bigger climate threat.

“Nobody really knows yet what caused Aliso to happen,” Ryerson says. If it had happened in a spot more distant from where people live, it might not have been noticed for a lot longer. Even then, the team was only able to measure the magnitude of the event because Conley was already under contract to the state.

Scientists were also available to map the plumes from two previous oil and gas disasters—the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 and a natural gas leak in the North Sea in 2013—and provide key information for stopping the events. But Ryerson and Conley both note that the three situations were largely due to luck.

“There is no standing capability for a quick-response airborne chemical measurement” of a disaster, Ryerson says. They argue that some sort of “grab-and-go package” should be developed to get scientists to a site within hours rather than weeks or months.

“We’ve been lucky three times in a row,” Ryerson says. “We should do something to be ready for the fourth.”