What does the future hold for crop yields?

Models generally agree that climate change could lead to a disaster for global agriculture. Cathal O'Connell and Elizabeth Finkel look at the evidence.

A farmer inspects an ear of corn. Different crop models provide different predictions of future yields. – Smith Collection/Getty Images

No matter what we do now, the planet’s climate will change. One of the crucial questions is what will happen to our crop yields. If temperatures increase by 2ºC by the middle of the century, and our population hits 9 billion or more, will our agricultural production be up to it?

Different crop models have come up with different predictions. Partly that’s because on the one hand, plants benefit from higher CO2 concentrations that drive their photosynthetic engines to produce more sugars. On the other hand, crops are highly adapted to specific conditions and when the patterns of temperature and rainfall change, that may hammer yields. Then factor in that farms around the world vary greatly, with very different abilities to adapt to change. Compare a maize farmer tilling her small plot in South Africa, to a farmer driving his combine harvester over thousands of hectares in America’s Midwest and you begin to get an inkling of why models produce different predictions.

But prediction is important. Crop yields affect food prices. Robust forecasts not only influence agronomic decisions but can also affect the market, potentially reducing price volatility and mass food shortages.

Two recent papers don’t exactly resolve the disagreements between the models, but they at least help define some areas of agreement.

One paper published in Nature Climate Change in March, led by Andrew Challinor from the University of Leeds, amassed 1,700 simulations of how the three food staples – wheat, corn and rice – are likely to perform over the coming century. The research represents “the largest pool of data from diverse modelling studies ever used for a global synthesis of this kind”, wrote Reimund Rotter from MTT Agrifood Research, Finland, in an accompanying editorial. These data, twice as much as had been available for the fourth report by the Intergovernmental Panel on Climate Change (IPCC) in 2007, were used to inform the fifth IPCC report released at the end of March this year.

The fourth IPCC report predicted that a moderate degree of global warming would lead to crop yield rises, particularly for cool climates such as Scandinavia or Russia, if famers can adapt their farming practices by planting new varieties, planting earlier to avoid heat stress, or using different farming techniques.

But the new findings are more worrisome. They show that losses kick in earlier, especially in the tropics. Beyond a single degree of temperature rise, wheat, rice and maize start showing declining yields. In temperate regions like Europe, the yields don’t start plummeting until after a 2ºC rise.

'Climate change is likely to increase the existing inequities between
developing and developed countries'

Rice and wheat farmers who can adapt their crops might reduce those losses, but maize famers are less likely to.

After the middle of the century if temperatures rise by more than 2ºC, two-thirds of the studies agree that the falls in yield across staple crops will be more than 10%.

CSIRO Australia scientist and co-author Mark Howden says that for wheat alone, the average decline will be 2% per decade. In temperate regions that translates to a 2% drop for every degree of temperature rise; in tropical regions, an 8% decline.

Clearly, on that scenario, the biggest losers will be developing countries in tropical zones.

“Climate change is likely to increase the existing inequities between developing and developed countries,” says Howden but he points to a flip side. “Farmers in developing countries operate at a relatively low level of efficiency. There is more room to move.”

Another paper published in Science in May agrees with the poor outlook for maize. Led by David Lobell at Stanford University, the multi-nation team used a large data set for yields in individual farms across America’s Midwest corn belt for the period 1995-2012. They found that while overall yields were rising each year, those farms that experienced drier weather did not show those annual improvements. In other words, plants exposed to drier conditions did not gain from higher CO2 levels.

It’s probably a harbinger of things to come. As co-author Graeme Hammer at the University of Queensland explains, the drier air – a product of warmer temperatures – is working against yields by increasing crop water stress. With a drier atmosphere expected, this data set predicts that by 2060 the yields will drop by between 15% to 30%.

The hope is that plant breeders can deliver new highly-adapted species of wheat, rice and maize before the crunch comes. But given breeding cycles can take a decade, there is not time to lose. For Australia at least, says Howden, “at the moment we have no menu of plants we can pick from to slot in”.

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