Risk of global food riots to surge within two years, expert says

The social tensions caused by rising food prices are already evident, von Braun said.

"The first sign was the tortilla riot in Mexico city, where 70,000 took to the streets. I think that was only the beginning -- there will be more," von Braun said. "For a year or two countries can stabilize with stocks. But the risk comes in the next 12 to 24 months. The countries that cannot afford to buy will be the losers, while those with huge foreign exchange reserves will bid up the world market."

Von Braun called on Europe to reconsider biofuel policies, provide more aid to poor nations, keep markets open and boost production.