Protests are erupting and governments are responding with often counterproductive price and export controls making food scarcity the 21st century's political hot potato. .

Severe weather in producing countries and a boom in demand from fast-developing nations have left world wheat stocks are at a 30-year low. Grain prices have been on the rise for five years, ending decades of cheap food.

Drought, a declining dollar, a shift of investment into commodities and use of farm land to grow fuel have all put the squeeze on food supplies.

But population and the growing wealth of China and other emerging countries are likely to be more enduring factors.

World population is set to hit 9 billion by 2050, and most of the extra 2.5 billion souls will live in the developing world, driving up demand for dairy and meat products.

Global food prices rose 35 percent in the year to the end of January, according to the United Nations, accelerating an upturn that began in 2002. Since then, prices have risen 65 percent.

"The recent rise in global food commodity prices is more than just a short-term blip," British think- tank Chatham House said in January. "Society will have to decide the value to be placed on food and how ... market forces can be reconciled with domestic policy objectives."

In Mexico, the European Union and Africa, governments are considering lifting bans on genetically modified crops to allow its farmers to compete with the United States.

Several governments, including China, have imposed restrictions to limit grain exports and keep more of their food at home.

But this knee-jerk response to food emergencies can result in farmers producing less food and threatens to undermine years of effort to open up trade.

"If one country after the other adopts a `starve-your-neighbor' policy, then eventually you trade smaller shares of total world production of agricultural products, and that in turn makes the prices more volatile," said Joachim von Braun, director general of Washington's International Food Policy Research Institute.

Waves of discontent are already starting to be felt. Protesters rallied in Indonesia recently and in the Philippines fast-food chains were urged to cut rice portions to counter a surge in prices.

And history is not repeating itself. Real commodity prices remained flat or even fell during the rapid industrialization of the United States and Germany in the early 20th century. But China's rapid industrialization, with 1.3 billion people, is on a totally different scale, according to the central bank of Australia.

The Chinese, who ate just 20 kilograms of meat per capita in 1985, now eat 50kg a year. Each kilogram of beef takes about 7kg of grain to produce, which means land that could be used to grow food for humans is being diverted to growing animal feed.

As the West seeks to tackle global warming, a drive towards greener fuels is also compounding the problem. It is estimated that one in four bushels of corn from this year's US crop will be diverted to make fuel ethanol.

Palm oil is also at record prices because of demand to use it for biofuel, causing pain for low-income families in Indonesia and Malaysia, where it is a staple.

But despite the rising criticism of biofuels, the US corn-fed ethanol industry enjoys wide political support because it helps farmers.

John Bruton, the European Union's ambassador to the United States, predicts that the world faces 10 to 15 years of steep rises in food costs. And it is the poor in Africa and, increasingly, Southeast Asia, who will be most vulnerable. But responding with aid and other policy options to help the hungry distorts markets.

Others hope that better fertilizers and higher-yielding crops - some of them genetically modified - will keep production in line with demand.

The 19th-century British political economist Thomas Malthus said population had the potential to grow much faster than food supply, a prediction that efficient farming consistently proved wrong. Now, some are revisiting his predictions.

REUTERS