SHALE gas has turned the American energy market on its head. Production has soared twelvefold since 2000, to 4.9 trillion cubic feet, or a quarter of the country’s total gas output. By 2035 the proportion could rise to half. As the shale gas flows, prices have come crashing down. Not long ago, America depended on imports of liquefied natural gas. Now it is likely to become a gas exporter.