Natural gas, which is mostly methane, is viewed as a significant “bridge fuel” to help the world move away from the greenhouse gas emissions of fossil fuels, since burning natural gas for electricity produces about half as much carbon dioxide as burning coal. But methane is itself a potent greenhouse gas, and it currently leaks from production wells, storage tanks, pipelines, and urban distribution pipes for natural gas. Increasing its usage, as a strategy for decarbonizing the electricity supply, will also increase the potential for such “fugitive” methane emissions, although there is great uncertainty about how much to expect. Recent studies have documented the difficulty in even measuring today’s emissions levels.