Cartels can be very powerful, but they usually contain the seeds of their own destruction — often the result of market power being pushed so far that it invites a countervailing response. That is Opec’s history in a sentence.

The responses to high prices, which nudged $150 in 2008, came from both supply and demand. On the supply side, the shale revolution has added 7.6m barrels a day to US oil production with more to come. At the same time, efficiency gains have cut oil demand in the developed world by 4.4m b/d since 2000.

Opec is reeling. Last week’s Vienna meeting of the cartel, which announced a modest cut in production of 1.2m barrels a day, produced a small surge in prices. But a price of $63 per barrel for Brent crude still represents a fall of 26 per cent since the beginning of October.