China's foreign exchange reserves are so vast that the numbers barely register. Yet in just six months the reserve pile has jumped another $318bn (€213bn, ￡195bn), a sum nearly equal to the annual gross domestic product of Argentina.

With a grand total of $2,270bn, that is a lot of insurance against financial crises but, as China has discovered this year, such reserves are also a trap, especially when two-thirds are in vulnerable US dollar assets.

What to do with all these reserves has been the subject of intense domestic debate. When the crisis hit last year, there were plenty of calls to reduce Chinese exposure and punish US profligacy by dumping dollar assets. But the inevitable plunge in the dollar and Treasury bonds from such a move would make China a big loser too. The conversation was dominated for a while by the central bank boss Zhou Xiaochuan's call for the dollar to be replaced as the global reserve currency. Yet this is a project for decades, not years.