The wild events in the Chinese domestic stock market in recent days have shown signs of broadening to other financial markets. Weakness in metal and oil prices, and in commodity currencies such as the Australian and New Zealand dollars, are normally reliable indicators that China is a growing global concern.

But this is more surprising on this occasion, because recent activity data suggest that the hard landing risk in China’s economy has abated. Investors have clearly become concerned that the iron control normally exerted by the Chinese authorities over the financial system is wobbling.

Both Paul Krugman and John Cochrane, from very different analytical positions, have argued that government intervention in stock markets is unlikely to succeed. In any rational world, this would be a far bigger threat to global financial stability than the Greek crisis. Yet investors with long memories will recall that a dramatic intervention in the stock market by the Hong Kong Monetary Authority in August 1998 forcefully reversed the bear market in the Hang Seng index, despite being ridiculed at the time by almost all “informed” international financial opinion.