Natural gas now produces more electricity in the U.S. than coal, which as recently as a decade ago was responsible for more than half of the nation’s energy. The shifting economics are a major reason that more than 290 U.S. coal plants have either closed or been scheduled for retirement during the past eight years, according to a tally by the Sierra Club, a nonprofit advocacy group that turned up the pressure on power companies with a series of Clean Air Act lawsuits.