"I'm often asked whether climate change will threaten food supply, as if it's a simple yes or no answer," said David Lobell of Stanford University. "The truth is that over a 10- or 20-year period, it depends largely on how fast the Earth warms, and we can't predict the pace of warming very precisely. So the best we can do is try to determine the odds."

Population growth, rising food consumption per head and demand for crop-based biofuels are all set to boost global demand for crops in the near future, with the next 20 years likely to see the bulk of the growth. The UN's Food and Agriculture Organization estimates that global production of cereals will increase by 13% per decade, up to 2030.

In the past, yields have risen by about 10% across a 10-year period and 20% across 20 years, helping crop production keep up with demand without massive expansion of agricultural land. But factors such as lack of investment in R&D, new crop diseases and climate change could slow the rate at which yield increases.

"Climate change has substantially increased the prospect that crop production will fail to keep up with rising demand in the next 20 years," said Claudia Tebaldi of the US National Center for Atmospheric Research.

Lobell and Tebaldi examined the risks of climate change halving the crop-yield growth rate. Across a 10-year period, there's a one-in-four chance that this will occur for maize and a one-in-six chance for wheat, they found.

The outlook for maize is worse than for wheat because maize is grown across a smaller geographic range and is slightly more sensitive to temperature, according to the researchers. They calculated that maize yield decreases roughly 7% per degree of warming, while wheat yield decreases by 6%.

"We can't predict whether a major slowdown in crop growth will actually happen, and the odds are still fairly low," said Tebaldi. "But climate change has increased the odds to the point that organizations concerned with food security or global stability need to be aware of this risk."

To come up with their results, Lobell and Tebaldi used two approaches. The first ran lots of models, using the Coupled Model Intercomparison Project (CMIP5) for a 39-member multi-model ensemble under RCP8.5. The second approach, meanwhile, ran one model for lots of initial conditions – the team used the NCAR-DOE Community Climate System Model (CCSM3) for the A1B scenario under a 40-member ensemble with initial conditions from different days to represent internal climate variability.

The risks estimated by running CCSM3 with a range of initial conditions were higher than those from the CMIP5 multi-model ensemble.

Adaptation through measures such as moving crop growth to cooler areas or changing crop varieties could, in theory, reduce the impact of climate change on yield. "Although further study may prove otherwise, we do not anticipate adaptation being fast enough to significantly alter the near-term risks estimated in this paper," wrote Lobell and Tebaldi in Environmental Research Letters (ERL), in a paper entitled "Getting caught with our plants down: the risks of a global crop yield slowdown from climate trends in the next two decades".

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