Drought has already spurred significant changes in Australia’s agricultural heartland. Some farmers are abandoning rice, which requires large amounts of water, to plant less water-intensive crops like wheat or, especially here in southeastern Australia, wine grapes. Other rice farmers have sold fields or water rights, usually to grape growers.

Image Shane Ryan heading out for a day of driving. Credit... Justin Mott for The New York Times

Scientists and economists worry that the reallocation of scarce water resources  away from rice and other grains and toward more lucrative crops and livestock  threatens poor countries that import rice as a dietary staple.

The global agricultural crisis is threatening to become political, pitting the United States and other developed countries against the developing world over the need for affordable food versus the need for renewable energy. Many poorer nations worry that subsidies from rich countries to support biofuels, which turn food, like corn, into fuel, are pushing up the price of staples. The World Bank and the United Nations Educational, Scientific and Cultural Organization called on major agricultural nations to overhaul policies to avoid a social explosion from rising food prices.

With rice, which is not used to make biofuel, the problem is availability. Even in normal times, little of the world’s rice is actually exported  more than 90 percent is consumed in the countries where it is grown. In the last quarter-century, rice consumption has outpaced production, with global reserves plunging by half just since 2000. A plant disease is hurting harvests in Vietnam, reducing supply. And economic uncertainty has led producers to hoard rice and speculators and investors to see it as a lucrative or at least safe bet.

All these factors have made countries that buy rice on the global market vulnerable to extreme price swings.

Senegal and Haiti each import four-fifths of their rice, and both have faced mounting unrest as prices have increased. Police suppressed violent demonstrations in Dakar on March 30, and unrest has spread to other rice-dependent nations in West Africa, notably Ivory Coast. The Haitian president, René Préval, after a week of riots, announced subsidies for rice buyers on Saturday.

Scientists expect the problem to worsen. The Intergovernmental Panel on Climate Change, set up by the United Nations, predicted last year that even slight warming would lower agricultural output in the tropics and subtropics.