An international scientific panel has found that climate change will pose great risks to the world's food supply in coming decades, potentially reducing output and raising prices at a time when global food demand is expected to soar.
The finding is by far the starkest warning the UN-appointed group, the Intergovernmental Panel on Climate Change, has ever issued about the food supply. Its last report, in 2007, was more sanguine, essentially finding that climatic warming and the rising carbon dioxide would boost agricultural production in large areas, though that report cited some risks.
The warning is contained in a draft report leaked on Friday.
The draft report warns that effects of climate change already sweeping the world are likely to intensify as human emissions of greenhouse gases continue to rise.
Echoing past findings, the draft points out that land ice is melting, leading to a rise in sea levels and putting coastal communities at risk of flood. It describes a natural world in turmoil as plants and animals try to migrate to escape rising temperatures, and warns that many could become extinct.
Saving a significant part of the world's biological diversity may require far more aggressive human management of natural systems, the report states.
Efforts to adapt to climate change have begun in many countries, the report finds. President Barack Obama on Friday signed an executive order to step up such efforts in the United States. But these efforts remain inadequate compared with the risks, the report says.
It finds that it is not too late for cuts in emissions to strongly affect future risks of climate change, though the costs would be incurred in the next few decades and the main benefits would likely be seen in the late 21st century and beyond.
The Intergovernmental Panel on Climate Change is the main scientific body charged with assessing climate science and reporting on the risks to the world's governments. Its main reports come out every five to six years.
The group won the Nobel peace prize, along with Al Gore, in 2007 for its efforts to alert the world to the risks.
Hundreds of billions of dollars are being spent every year to reduce emissions, in direct response to past findings from the group, though many analysts have said these efforts are inadequate to head off drastic climatic changes later in the century.
On the food supply, the new report finds benefits of global warming may be seen in areas such as northern lands that are now marginal for food production. But overall, climate change could reduce agricultural output as much as 2 per cent a decade for the rest of the century.
During that period, demand is expected to rise as much as 14 per cent a decade, the report found, as billions of people in developing countries acquire the money for richer diets.
Rising food prices would be likely to hit the world's poor hardest, as has occurred in recent years, with spikes caused to a large extent by certain types of weather extremes, such as severe heat waves, linked to climate change.
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.
If the report proves correct, global food demand might have to be met - if it can be met - by farming new land. That could entail chopping down large areas of forest.
The report was leaked on a blog hostile to the scientific panel.
''It's a work in progress,'' said Jonathan Lynn, a spokesman for the intergovernmental panel. ''We don't have anything to say about the contents. It's likely to change.''
But in a brief interview, Lynn did not dispute the authenticity of the document.