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 have found.
In a departure from an earlier assessment, the scientists concluded 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.
The scientists say they already see the harmful effects in some regions.
The warnings come in a leaked draft of a report 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 sweeping effects from climate change occurring across the planet and warns that these 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 attempt to migrate to escape rising temperatures, and warn that many could become extinct.
The panel's previous report, in 2007, was more hopeful. While it did warn of risks and potential losses in output, particularly in the tropics, 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 recent work also challenges previous assumptions about how much food production could increase in coming decades because of higher carbon dioxide levels in the atmosphere. The gas, though it is the main reason for global warming, also acts as a kind of fertilizer for plants.
The IPCC 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.
Hundreds of billions of dollars are being spent every year to reduce emissions in response to past findings from the group, though many analysts have said these efforts are inadequate to head off drastic climatic changes this century.
On the food supply, the new report finds that benefits from global warming may be seen in some areas, like northern lands that are now marginal for food production. But it adds that overall, global warming could reduce agricultural production by as much as 2 percent each decade for the rest of this century.
During that period, demand is expected to rise as much as 14 percent each decade, the report found, as the world population is projected to grow to 9.6 billion in 2050, from 7.2 billion today, and many of those people in developing countries acquire the money to eat richer diets.
Any shortfall would lead to rising food prices that would hit the world's poor hardest, as has occurred from price increases of recent years. Research has found that climate change, particularly severe heat waves, was a factor in those price spikes.
The agricultural risks “are greatest for tropical countries, given projected impacts that exceed adaptive capacity and higher poverty rates compared with temperate regions,” the draft report finds.
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