High temperatures cause crops like corn, wheat and rice to grow faster, but reduce plant fertility and grain production. With average growing-season temperatures expected to rise more than 6 degrees Fahrenheit, crop yields will fall 20 to 40 percent, the report estimates. This is the prediction of scientists at the University of Washington and Stanford University.

"We are headed for a completely out-of-bounds situation for growing food crops in the future," said report co-author Rosamond Naylor, director of Stanford's Program on Food Security and the Environment. Even in the United States, where warmer temperatures are projected to initially increase some crop yields through the middle of this century, total harvests are projected to fall by 2100. But worldwide, the impacts will fall most heavily on impoverished subsistence farmers, UW atmospheric sciences professor David Battisti pointed out.

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