The price of staple foods such as corn, already high, could more than double in the next 20 years, and climate change is responsible for up to half of this expected increase, warns a report Tuesday by the international aid organization Oxfam.

The world's poorest people will be hardest hit as the demand for food rises 70% by 2050 while the world's capacity to increase food production declines, according to the "Growing a Better Future" report. Oxfam released the report as part its new global GROW campaign, launching on June 1, to address pressures on the global food system amid a population boom.

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"We are fighting both sides of the war on hunger," Raymond C. Offenheiser, president of Oxfam America, said in a statement. "U.S. policies are making it more difficult for the small farmers, who grow much of the world's food, to have enough to feed their own families."

The report attributes the problems to several factors:

Traders: Four global companies control the movement of most of the world's food. Three companies -- Archer Daniels Midland, Bunge and Cargill -- control an estimated 90% of the world's grain trade. Their activities help drive volatile food prices and they profit from them.

India: Despite doubling the size of it economy between 1990 and 2005, the number of hungry people in India increased by 65 million -- more than the population of France -- because economic development excluded the rural poor and social protection schemes failed to reach them. Today, one in four of the world's hungry people live in India.

United States: U.S. policy ensures 15% of the world's corn crop is diverted to engines, even at times of severe food crisis. The grain required to fill the gas tank of an SUV with biofuels is enough to feed one person for a full year.

Oxfam America urges President Obama, the U.S. Congress and the private sector to take immediate steps to invest in small-scale food production, stop subsidies for the corn-ethanol industry, modernize food aid and end "excessive speculation" in agricultural commodities.