Initially many observers thought the market divergence would be shortlived. Some believed the US bonanza would evaporate, that once the easy gains had been made the marginal costs would rise, new production would be harder to bring on, and prices would return to higher levels. So far this has not happened: the typical marginal costs of new shale gas production are now around $3 per million BTU. In 2011 shale gas accounted for a quarter of US natural gas production.

The impact on reserves has been equally dramatic. In 2000, US dry natural gas resources were estimated by the Energy Information Administration at 1,500 trillion cubic feet, almost entirely in conventional reservoirs. By 2012 estimated reserves had risen by 67 per cent to more than 2,500 trillion cubic feet, of which a third was accounted for by gas in shale formations.

There is a longer-term prospect that North America will begin to export liquefied natural gas (LNG) to the rest of the world, helping to restore a global pricing equilibrium. To do this, American LNG import terminals which were constructed a few years ago to cope with an anticipated domestic shortage of gas are being reconfigured to handle exports. However, no gas has yet been exported, and there are already rumblings within the US that the country should not allow its sudden good fortune to be dissipated by sharing it with foreigners.

Great bulwark of free enterprise it may be, but the US has a long history of energy protectionism, from oil import quotas in the 1950s, wellhead price controls in the 1970s and a refusal to allow exports of Alaskan oil in the 1980s. But even if the US refuses to licence LNG exports, Canada, whose own traditional export markets south of the border have been hammered by the shale revival, will certainly look to sell LNG to the world.

Meanwhile the US has begun to benefit from significantly lower energy costs. Industrial output looks like growing at 4-5 per cent  this year and next. Power generators are switching to gas from more expensive and dirty coal, and this trend is forecast to increase dramatically over the next quarter-century. Americans' perennial sense of insecurity, most recently voiced by President Obama, about over-reliance on energy imports, looks like evaporating. The technological advances that have given the US its abundant shale gas will also provide crude oil from shale.