It is hard to get a grip on food. The UN’s World Health Organisation worries about diminishing supplies and increased prices in poor countries; recent riots and near-riots in Haiti, Bangladesh and Egypt were sparked by the growing cost of wheat and rice. But, as Paul Roberts observes in "The End of Food", the developed world has lived through "a near miraculous period during which the things we ate seemed to grow only more plentiful, more secure, more nutritious, and simply better. " 46． In the second half of the 20th century, world output of corn, wheat and cereal crops more than tripled. Yet there is not enough to feed the rich, the aspirational and the poor in the world. A golden age has been transformed quite suddenly into a global crisis.Mr Roberts insists that modern agribusiness is unsustainable and becoming more so. "Precisely at the moment in history when we need to shift our system of food production into overdrive, our agricultural engine is breaking down," he says. The industry has taken cheap oil for granted. Oil fuels transportation and farm machinery, and natural gas is the basis of synthetic nitrogen production ( prices have tripled since 2002). Agriculture accounts for three- quarters of freshwater use, and water is becoming an increasingly scarce and expensive resource. Climate change makes some old assumptions about farming redundant. 47．A combination of these factors, he says, will ultimately force a complete rethinking of the way we make food.For years government subsidies held down grain prices, making food cheaper. 48．Water was also plentiful-it takes 1,000 tonnes of water to produce a tonne of grain-and an ingenious process known as Haber-Bosch makes synthetic nitrogen fertiliser easily available to grain farmers. Ruthless price-cutting at supermarkets means consumers have grown accustomed to eating too much. (In the late 19th century, Europeans already thought Americans ate three or four times more than was necessary. ) The most damaging consequence is that by 2000 31% of American adults were obese, with another 16% defined as overweight. American airlines spend $ 275 million a year more on fuel simply to lift the heavier passengers. Mr Roberts claims that every year obesity causes 400,000 premature deaths in America. Food has become as deadly as tobacco.A fruitful start would be to halve the size of portions in all American restaurants, but most consumers are reluctant rethinkers. 49．Eating organic product could be a partial solution, although one study suggests that the cost of avoiding intensive farm chemicals would mean a 31% increase in food prices. Government scientists believe that genetically modified crops might be the only way out of the crisis, but a majority of consumers are reluctant to listen.Is there a model for the future 50．Fashionably, Mr. Roberts believes that a local system based on easily obtainable seasonal foods that do not need to be transported huge distances would form part of a solution. The economics and greenery of this are far from proven. Mr Roberts can find only one country that has made "serious efforts" in this direction: Cuba, hardly a comforting example. The coming food crisis, warns the author, is as intractable as global warming, and no less urgent. 47