Daniel Yergin writes:  American shale oil has become the decisive new factor in the world oil market in a way that could not have been imagined five years ago. It has proved to be a truly disruptive technology. But will that impact continue in a world of low prices?

Oil is now below $50 a barrel, a price too low for a good deal of the new shale oil development to make economic sense. Yet output is likely to continue to rise by another 500,000 barrels per day in the first half of 2015 because of sheer momentum and commitments already made.

Come the middle of the year, however, growth will flatten out. Producers will work hard to improve efficiency and lower costs, but in 2016, at these prices, American output could decline. Production elsewhere in the world will also be flattening.

But by then, the world economy might be doing better, stimulating oil demand. Prices could start rising again. If the gulf producers have their way, prices will not go back to $100 a barrel. Even at prices well below $100, American shale oil producers will find ways to drive down costs and output will start rising again. And the world’s new swing producer will find itself back in the swing of things.