In mid-October 2015 residents of the Porter Ranch neighborhood in northwestern Los Angeles started suffering from headaches, nausea, dizziness, and nosebleeds. About a week later an employee of Southern California Gas discovered the cause of their symptoms: a leak at the company’s nearby Aliso Canyon facility, a depleted oil field that serves as the second-largest natural gas storage system in the US. The leak’s full extent wasn’t verified until early November, when a scientist-pilot hired by the California Air Resources Board flew overhead with sensors that detected methane levels so high he thought the instruments were malfunctioning. By early January Governor Jerry Brown had declared a state of emergency. It was certainly a health emergency for the nearly 10,000 residents who were evacuated, but it was also a climate emergency: by the time it was finally plugged on February 18, about six billion cubic feet of natural gas—109,000 metric tons of methane—had escaped.…