Environment - Wheat will age prematurely in a warmer world

Extreme temperatures are already cutting wheat yields in India (Image: Narinder Nanu/AFP/Getty Images)

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's more, models used to predict the effects of global warming on food supply may have underestimated the problem by a third.

In India's breadbasket, the Ganges plain, winter wheat is planted in November and harvested as temperatures rise in spring. David Lobell of Stanford University in California used nine years of images from the MODIS Earth-observation satellite 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 ºC having a particularly strong effect. He then inferred yield loss, using previous field studies as a guide.

This revealed a much stronger effect of temperatures on yield than previous studies. Lobell's data predicted that yield losses in the Ganges plain will be around 50 per cent greater from an average warming of 2 ºC than existing models.

"It surprised me a little how much crop models underestimate the observed effects," says Lobell. They might have especially underestimated the impact of hot spells.

Wheat is the world's second-biggest crop, and provides a fifth of the world's protein, according to CIMMYT, 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 ºC.

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 yield drops 5 per cent for every 1 ºC the average temperature climbs above 14 ºC.

Lobell's work suggests losses could be sooner and greater. "This is an early indication that a situation that was already bad could be even worse," says Andy Challinor of the University of Leeds, UK.

Two-thirds of wheat in poor countries, and 23 per cent in rich countries – nearly half the world's total crop – is at risk from warming, says Hans-Joachim Braun of CIMMYT. Previous estimates suggested that by 2050, warming could cut wheat yields by 30 per cent in places like India – 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 looking for answers. "We are selecting for more extensive roots so that plants can reduce both water and heat stress," 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's sensitivity to temperature and light can be used to breed a slower-growing wheat that doesn't become spindly when planted early.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate1356

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