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The simulations were shown to reproduce the observed strong reduction in past crop yields induced by high temperatures. The scientists found that increased irrigation can help reduce the negative effects of global warming on crops, but this only possible in regions where sufficient water is available.
“We know from observations that high temperatures can harm crops, but now we have a much better understanding of the processes,” said Bernhard Schauberger from the Potsdam Institute for Climate Impact Research, lead author of the study. “The computer simulations that we do are based on robust knowledge from physics, chemistry, biology; on a lot of data and elaborate algorithms. But they of course cannot represent the entire complexity of the crop system, hence we call them models. In our study they have passed a critical test.”
The scientists compared the model results to data from actual observations. This way, they can find out if they included the critical factors in their calculations, from temperature to CO2, from irrigation to fertilization.
For every single day above 30 degrees C (86 degrees F), maize and soybean plants can lose about 5% of their harvest. The simulations have shown that the models capture how rather small heat increases beyond this threshold can result in abrupt and substantial yield losses. Such temperatures will be more frequent under unabated climate change and can severely harm agricultural productivity, researchers said.
Harvest losses from elevated temperatures of 20% for wheat, 40% for soybeans and almost 50% for maize, relative to non-elevated temperatures, can be expected at the end of the century without efficient emission reductions, researchers said. These losses do not even consider extremely high temperatures above 36 degrees C (97 degrees F), which are expected to lower yields further.
The effects go far beyond the U.S., one of the largest crop exporters. World market crop prices might increase, which is an issue for food security in poor countries.
“The losses got substantially reduced when we increased irrigation of fields in the simulation, so water stress resulting from temperature increase seems to be a bigger factor than the heat itself,” said co-author Joshua Elliott from the University of Chicago. When water supply from the soil to the plant decreases, the small openings in the leaves gradually close to prevent water loss. They thereby preclude the diffusion of CO2 into the cells, which is an essential building material for the plants. Additionally, crops respond to water stress by increasing root growth at the expense of above-ground biomass and, eventually, yields.
“Irrigation therefore could be an important means of adaptation to dampen the most severe effects of warming. However, this is of course limited by the lack of water resources in some regions.”
Burning fossil fuels elevates the amount of CO2 in the air. This usually increases the water use efficiency of plants since they lose less water for each unit of CO2 taken up from the air. However, this cannot be confirmed as a safeguard of yields under high temperatures, the scientists argue. The additional CO2 fertilization in the simulations does not alleviate the drop in yields associated with high temperatures above about 30 degrees C.
The comparison of different computer simulations of climate change impacts is at the heart of the Inter-Sectoral Impacts Modelling Intercomparison Project (ISIMIP) comprising about 100 modeling groups worldwide. The simulations are generated in cooperation with AgMIP, the international Agricultural Model Intercomparison and Improvement Project.