The world faces a "perpetual food crisis" because global warming will likely lead to massive and simultaneous crop failures in many regions, possibly as early as the period from 2040 to 2060, a new study says.
The finding, appearing in the journal Science, is based on climate models that suggest the worst heat waves of the past - such as the one in Europe in 2003 that killed at least 30,000 people - are likely to become the new normal summertime temperatures.
Although the trend to extreme heat becoming the new normal could start in some parts of the world by mid-century, well within the lifetimes of many people now alive, the researchers are confident it will become a global phenomenon between 2080 and 2100.
Rising temperatures will wither crops that are heat-sensitive, including staples such as wheat, possibly cutting yields by 20 per cent to 40 per cent, according to the study, conducted by scientists at two U.S. universities. The impact will not be as pronounced on some crops, such as millet, that are more heat tolerant but not exactly palate pleasers.
The principal author of the study, University of Washington climate researcher David Battisti, says the reduction in yields of some of the world's most important food crops will have dire results, particularly in the tropics and subtropics, where many people are already malnourished.
"For me, this is the strongest argument that either you have to do something about global warming or you need to actually figure out how you're going to deal with these kind of permanent reductions in yield," Dr. Battisti said.
In an interview, Dr. Battisti contended that global warming's effect on agriculture is likely to be a larger threat to humanity than the submerging of coastal cities due to melting ice sheets in Greenland and Antarctica.
"This is going to unfold in the next 100 years," he said, "whereas the sea-level changes are going to probably, most likely, unfold over the next 300 to 400 years."
Models run to predict climate conditions due to higher atmospheric concentrations of greenhouse gases have typically focused on the threat of droughts in subtropical areas and not on the problems that accompany rising temperatures, as was done in this study.
But extreme heat can be as damaging to crops as insufficient moisture. Plants have an optimum temperature for growing and making seeds. Once it is exceeded, there is a reduction in leaf development and the size of kernels, key factors in determining yields.
During recent heat waves, such as the one in France, the poor harvests were mitigated because the rest of the global food system was still functioning well. The study said this may not be possible in the future because many areas could suffer extreme temperatures simultaneously.
"It will be extremely difficult to balance food deficits in one part of the world with food surpluses in another" unless major investments are made soon to develop heat-tolerant crops and better irrigation, the study states.
The effect on agriculture due to global warming is a controversial topic, with several factors suggested that might ease the damage.
Higher temperatures will extend the growing season in northern countries such as Russia and Canada. Plants also grow better in an atmosphere richer in carbon dioxide, the main greenhouse gas.
But Dr. Battisti said higher temperatures will benefit only northern agricultural areas over the next few decades, and by the end of the century, they too could suffer from heat stress. Even farther north, in Arctic areas, soil isn't suitable for farming. "You can't move that far north because all you end up with is pretty infertile tundra," he said.
