The eurozone crisis is hotting up once again. The unwinding of the post-summit euphoria saw Spanish 10-year bond yields briefly pass the critical 7 per cent level yesterday. The flood of cash seeking safety left German two-year yields negative for only the second time. Over in the US the 10-year Treasury yield has been lower only once. But the people really feeling the heat are US farmers, scorched by the most serious drought in almost a quarter of a century.

Soyabean prices set a new record, wheat is back to the levels that sparked the Arab revolts, and US maize (corn) futures for December delivery – the current crop – have soared 44 per cent in three weeks.

This matters, and not only to those on the breadline.

For the US, consumer food prices are likely to be offset by cheaper petrol and natural gas, as Phil Isherwood at Absolute Strategy Research points out. But in the developing world food has a big impact on inflation, so higher prices could limit central banks’ ability to continue slashing rates.

Leaving aside the risk of revolution, changes in food prices offer a handy proxy for inflation risks. Industrial metals prices are a guide to growth hopes, so the pair tell us a lot about expectations for growth and inflation.

At the moment the message from commodities is deeply gloomy: the ratio of food to metals prices is the worst since early 2009, with metals falling for most of this year.

Equities are on a different wavelength. US shares are up 9 per cent this year, with the 10-year Treasury returning only 5 per cent. Germany has almost as big a gap, while the mining-heavy UK equity market is even with gilts.

Maybe commodities have decoupled from other assets, as investors realise the decade-long bull market is over. There are two other interpretations: shares still have a long way to fall; or shareholders have spotted an imminent pick-up in real growth that commodities markets have missed.

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