It could be much more difficult than we thought to feed everyone in a warmer world. Satellite images of northern India have revealed that extreme temperatures are cutting wheat yields. What鈥檚 more, models used to predict the effects of global warming on food supply may have underestimated the problem by a third.
In India鈥檚 breadbasket, the Ganges plain, winter wheat is planted in November and harvested as temperatures rise in spring. of Stanford University in California used nine years of images from the to track when wheat in this region turned from green to brown, a sign that the grain is no longer growing.
He found that the wheat turned brown earlier when average temperatures were higher, with spells over 34 潞C having a particularly strong effect. He then inferred yield loss, using previous field studies as a guide.
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This revealed a much stronger effect of temperatures on yield than previous studies. Lobell鈥檚 data predicted that yield losses in the Ganges plain will be around 50 per cent greater from an average warming of 2 潞C than existing models.
鈥淚t surprised me a little how much crop models underestimate the observed effects,鈥� says Lobell. They might have especially underestimated the impact of hot spells.
Wheat is the world鈥檚 second-biggest crop, and provides a fifth of the world鈥檚 protein, according to , a major international wheat lab based in Mexico. Loss of wheat yields is a major threat to food security.
Ageing wheat
Wheat evolved in cool uplands and has few defences against heat. Crop scientists have long known that its photosynthetic machinery can be damaged by night-time temperatures over 34 潞C.
Such damage to mature wheat triggers premature ageing of the kind Lobell saw: the plant stops photosynthesising, turns from green to gold, and prepares to drop its seeds. If this happens while the wheat is still funnelling nutrients into the seeds, the result is anaemic grain. Models based on smaller-scale studies in warm wheat-growing areas like Australia suggest that .
Lobell鈥檚 work suggests losses could be sooner and greater. 鈥淭his is an early indication that a situation that was already bad could be even worse,鈥� says of the University of Leeds, UK.
Two-thirds of wheat in poor countries, and 23 per cent in rich countries 鈥� nearly half the world鈥檚 total crop 鈥� is at risk from warming, says of CIMMYT. Previous estimates suggested that by 2050, warming could cut wheat yields by 30 per cent in places like India 鈥� a figure that may now be optimistic. Yet global yields need to rise 50 per cent by then to feed the growing population.
Wheat 2.0
Wheat breeders are already . 鈥淲e are selecting for more extensive roots so that plants can reduce both water and heat stress,鈥� says Matthew Reynolds of CIMMYT. But he says boosting yields will likely require deep genetic changes, perhaps to the enzyme that powers photosynthesis, rubisco.
Indian farmers might also plant wheat earlier, so it is harvested before spring temperatures rise. However, this means planting at higher temperatures, says Braun, producing fast-growing, spindly plants. CIMMYT has just started a project to find out whether genes governing wheat鈥檚 sensitivity to temperature and light can be used to breed a slower-growing wheat that doesn鈥檛 become spindly when planted early.
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