Estimating The Environmental Impact Of Southern California’s Great Methane Leak

Over the past three months, more than 87,000 metric tons of methane, a potent greenhouse gas, have spewed into the atmosphere from a leak in a failed natural gas well in Southern Calif­ornia. Southern Calif­ornia Gas Co., the well’s owner, announced Jan. 18 that it hopes to stop emissions from the Aliso Canyon Storage Facility in northern Los Angeles by late February.

But the leak, one of the largest of its kind in U.S. history, will have a lasting environmental impact on local, state, and perhaps federal levels.

More than 2,500 households in the Porter Ranch neighborhood near the facility have been forced to relocate because of noxious odors from pollutants such as mercaptan, which is added to odorless natural gas for safety purposes. Some recent reports have also suggested that SoCalGas may have underestimated the amount of benzene, a carcinogen, which has been released during the leak. Timothy O’Connor, director of the Calif­ornia oil and gas program at the Environmental Defense Fund (EDF), an environmental advocacy group, says it may be difficult to know how much benzene exposure has occurred because of a variable emissions rate and a lack of extensive testing when the leak first started. Although both benzene and mercaptan are harmful in high doses, little is known about the health effects of long-term, low-dose exposures.

On Jan. 6, Calif­ornia Gov. Jerry Brown (D) declared a state of emergency and ordered increased regulations and inspections on gas facilities in the state. The leak could thwart Calif­ornia’s ambitious goals to cut greenhouse emissions statewide.

In addition to footing the bill for relocating people and addressing the leak, SoCalGas faces lawsuits from the city and local residents claiming that the company failed to properly maintain its facility.

Starting in mid-November, SoCalGas tried to plug the well but failed. So the gas company began drilling a relief well on Dec. 4, expecting to reach its target, 2,600 meters below the surface, in February. The company also scuttled a risky proposal to divert and incinerate some of the methane.

SoCalGas did not respond to C&EN’s requests for comment.

Assessing the environmental impact of a leak like this is complicated because of the varying ways to evaluate a greenhouse gas’s global warming potential, says David T. Allen, a chemical engineering professor at the University of Texas, Austin, and former chair of the Environmental Protection Agency’s scientific advisory board.

The Earth-warming effects of greenhouse gases depend, in part, on the window of time being considered, Allen explains. For example, methane, at the time of its release, is 120 times as potent as the same amount of carbon dioxide in terms of radiative forcing—the capacity to trap heat in the atmosphere, leading to temperature increases.

But over time, methane is oxidized to CO₂, so a specific release of the hydrocarbon will lose its warming potency. Many scientists calculate greenhouse gas effects in terms of a 20-year time horizon, over which a methane release will have more than 80 times the radiative forcing as the same amount of CO₂. Others calculate the impact over 100 years, when methane’s warming potency drops to 28–35 times that of CO₂.

Because of its potency, methane is a grave concern for climate scientists. In the U.S., leaks from natural gas systems account for 29% of all methane emissions.

EDF has been assessing the Aliso Canyon leak. At its peak in November, the leak was emitting 58,000 kg of methane per hour, or 25% of the daily emissions in Calif­ornia. To put that in perspective, the potential effect of radiative forcing from the leak’s daily emissions, calculated over a 20-year time horizon, was equivalent to that of the daily CO₂ emissions of 7 million cars. Calif­ornia, with its population of 39 million, has about 30 million cars.

“It’s an enormous-sized leak no matter how you compare it,” says Steven Hamburg, chief scientist at EDF.

“No single source is going to be enormous on a global scale of all greenhouse gases,” he points out, “but a single leak that equates to a quarter of the emissions of methane of a population of 39 million—that’s a big deal.”

As of Jan. 12, the leak had slowed by 60% to an emission rate of 21,500 kg of methane per hour.

Although the leak may slow Calif­ornia’s progress toward reaching its goal of reducing greenhouse gas emissions by 40% by 2030, “I don’t think we’ll be unable to meet it,” EDF’s O’Connor says.

On a federal level, environmental scientists say the event is a textbook example of a worst-case scenario that they’ve been trying to prevent, by calling for expedient upgrades of aging gas supply infrastructures in big cities across the U.S.

The long-term importance of the leak, Hamburg says, “is that it’s representative of a larger problem—which is making sure we have regulations and monitoring to make sure our natural gas supply chain is not leaking.”