It has become a distressingly familiar question. With the price of agricultural staples such as corn, soyabeans and wheat soaring for the third summer in five years, the prospect of another price shock is once again becoming a prominent concern for investors and politicians alike.

The debate marks a dramatic shift from just a few weeks ago, when traders were expecting bumper crops and policy makers were comforting themselves that – if nothing else – falling commodity prices would offer some relief to the troubled global economy.

But since then, scorching heat and a paucity of rain across the US has withered the country’s corn and soyabean crops, with the US Department of Agriculture this week making the largest downward revision to its estimate for a corn crop in a quarter of a century. ...