Climate change will pose sharp risks to the world's food supply in coming decades, potentially undermining crop production and driving up prices at a time when the demand for food is expected to soar, scientists say.
In a departure from an earlier assessment, the scientists conclude that rising temperatures will have some beneficial effect on crops in some places, but that globally, they will make it harder for crops to thrive.
And, the scientists say, they are already seeing the harmful effects in some regions.
The warnings come in a leaked draft of a report under development by a U.N. panel, the Intergovernmental Panel on Climate Change. The document is not final and could change before it is released in March.
The report also finds other impacts from climate change occurring across the planet, and warns that they are likely to intensify as human emissions of greenhouse gases continue to rise. The scientists describe a natural world in turmoil as plants and animals try to migrate to escape rising temperatures, and warn that many could become extinct.
The warning on the food supply is the sharpest in tone that the panel has ever issued. Its previous report, in 2007, was more hopeful. While it did warn of risks and potential losses in output, that report found that gains in production at higher latitudes would likely offset the losses and ensure an adequate global supply.
The new tone reflects a large body of research in recent years that has shown how sensitive crops appear to be to heat waves.
The Intergovernmental Panel on Climate Change is the principal scientific body charged with reviewing and assessing climate science, then issuing reports about the risks to the world's governments. Its main reports come out every five to six years.