"You are talking about hundreds of millions of additional people looking for food because they won't be able to find it where they find it now," said David Battisti of the University of Washington, US. "You can let it happen and painfully adapt or you can plan for it. You could also mitigate it and not let it happen in the first place, but we’re not doing a very good job of that."

Adaptation options include developing crop varieties that are tolerant to heat and heat-induced water stress, and new irrigation systems. Because the effects will be so widespread, potentially even reaching mid-latitudes, buying in food from other areas is unlikely to be an option.

"The impact of increasing greenhouse gases, even under emission scenarios that are much less than business as usual, will make it very likely that the growing season average temperature will be out-of-bounds compared to even the hottest summer observed to date, throughout much of the tropics and subtropics," Battisti told environmentalresearchweb. "Since this is a region where the summer temperature already exceeds the optimum temperature for crops, crop yields will be reduced significantly from central Asia to southern Europe, from the southern US to northern Argentina and for all of Australia and Africa."

To obtain this result, Battisti and Rosamond Naylor of Stanford University, US, examined growing season temperatures produced by the 23 global climate models used in the 2007 IPCC report.

"Unlike extreme events and some other features of the impact of increasing greenhouse gases, climate models do a pretty good job at simulating seasonally averaged temperature," said Battisti. "Summer average temperature is a quantity that matters to crops."

The researchers also looked at the effects of high temperatures on crop yields in the past. Record high summer temperatures in France in the summer of 2003 reduced maize and fodder production by 30%, fruit harvests by 25% and wheat harvests by 21%. Similarly, extremely high, summer-averaged temperature in the former Soviet Union in 1972 led to the price of wheat rising from $60 to $208 per metric ton in international markets between 1972 and 1974.

Now the researchers, who reported their work in Science, plan to examine the impact of climate change on the distribution of pests and pathogens that affect agriculture in the tropics, together with colleagues at the University of Washington and Cornell University.