"Our results provide the first quantification of the effects of extreme heat stress on global crop yields," said Delphine Deryng of the Tyndall Centre for Climate Change Research, UK. "Instances of extreme temperatures, brought about by a large increase in global mean temperature, can be detrimental to crops at any stage of their development, but in particular around anthesis-the flowering period of the plant. At this stage, extreme temperatures can lead to increased pollen sterility and reduced seed set, greatly reducing the crop yield."

Unlike maize, for spring wheat and soybean the positive effects of carbon fertilisation on yield would be likely to outweigh the negative effects of extreme heat, the team found. This would lead to overall yield increases on a global scale as carbon dioxide concentrations rise, despite possible yield losses in parts of the tropics and sub-tropics. So far, field studies of carbon fertilization have taken place in temperate countries, the team points out; tropical plants could respond much less positively, perhaps because their growth is limited by poor soil.

Taking carbon fertilization effects into account, under the "business-as-usual" or RCP8.5 scenario, extreme heat stress during the flowering period could double global losses of maize yield by the 2080s, reduce projected gains in spring wheat yield by half and cut projected gains in soybean yield by a quarter, the study revealed.

Deryng and colleagues also looked at how heat stress could affect crop yields in key agricultural producing countries. The US and Brazil could experience up to 15-50% reductions in average soybean yield, respectively, and 3-15% reductions in average maize yield, they discovered.

"Given the important role of these two countries in global maize and soybean productions, these negative impacts on yields could affect global food prices and jeopardise food security," said Deryng. "We also found substantial beneficial effects on yield for spring wheat and soybean in some countries. However, in most cases by incorporating the effects of extreme heat stress these increases are reduced."

Writing in Environmental Research Letters (ERL), the team says that the wide range of impacts across regions underscores the need for carefully targeted adaptation responses including breeding and technology programmes for greater crop heat tolerance.

Mitigating climate change to a radiative forcing of 2.6 W per sq. m by 2100 would remove most of the harmful effects of heat extremes, the study indicated. "Early reduction in greenhouse gas emissions leads to a greater than 80% reduction in yield losses when assuming carbon dioxide fertilisation effects to be negligible," said Deryng. Such mitigation would also reduce much of the uncertainty in projected impacts, making the job of adaptation planners easier.

To come up with the results, the team used outputs from 72 climate change scenarios in the PEGASUS global crop model. "Our study has used climate change projections originating from a wide range of climate model experiments, more than in any previous study, allowing us to provide much better estimates of uncertainties in future impacts on crops," said Deryng. "We are able to provide a range of uncertainties linked to uncertainties in climate modelling projections, which shows that business as usual, for instance, triggers a much wider range of simulated impacts on crops."

The team reported their results in Environmental Research Letters (ERL).

Related links

• Global crop yield response to extreme heat stress under multiple climate change futures Delphine Deryng, Declan Conway, Navin Ramankutty, Jeff Price and Rachel Warren 2014 Environ. Res. Lett. 9 034011

• ERL

•  Delphine Deryng, Tyndall Centre for Climate Change Research

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