Ben Bernanke, Federal Reserve chairman, produced a brief dollar rally by expressing concern that a falling exchange rate could bolster inflation. Mr Bernanke is correct to be concerned about a weak dollar because it is part of a larger pattern of price developments that may also define his role in US monetary history. He appears likely to be remembered as the first Fed chairman to preside over a recession without any coincident decline in commodity prices. Instead, prices have risen to record levels since the US economy began to slow sharply last year.

The main reason commodity prices have been so resilient is the changing composition of global output. Developing countries have become the world's dominant growth leaders over the past five years and their share of global resources has risen dramatically. They accounted for more than 59 per cent of global copper consumption in 2007 compared with 36 per cent in 1998. Their share of zinc consumption was 63 per cent, against 43 per cent in 1998. Their share of aluminium consumption was 58 per cent up from 34 per cent in 1998.