Some of the most important crops risk substantial damage from rising temperatures. To better assess how climate change will likely impact wheat, maize and soybean, an international team of scientists has run an unprecedented set of computer simulations of U.S. crop yields.
The simulations reproduced the observed strong reduction in past crop yields induced by high temperatures, confirming that they capture one main mechanism for future projections. The scientists, including Joshua Elliot and Delphine Deryng of the University of Chicago Computation Institute, found that increased irrigation can help to reduce the negative effects of global warming on crops, but only in regions where sufficient water is available. Eventually, limiting global warming is necessary to keep crop losses in check.
“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 and biology, and 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.”
Without efficient emission reductions, yield losses of 20 percent for wheat are possible by the year 2100.
For every day above 30 degrees Celsius, maize and soybean plants can lose about five percent of their harvest. The simulations showed how 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.
The elevated temperatures that can be expected at the end of the century without efficient emission reductions may cause harvest losses of 20 percent for wheat, 40 percent for soybean and almost 50 percent for maize, relative to non-elevated temperatures. These losses do not even consider extremely high temperatures above 36 degrees Celsius, 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.
Irrigation could be a means for adaptation, yet only in regions where there's sufficient water.
“The losses were 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 Elliott, researcher at the Center for Robust Decision-Making on Climate and Energy Policy.
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 carbon dioxide 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,” Elliott said. “However, this is of course limited by the lack of water resources in some regions.”
Burning fossil fuels elevates the amount of carbon dioxide in the air. This usually increases the water use efficiency of plants since they lose less water for each unit of carbon dioxide taken up from the air. However, this cannot be confirmed as a safeguard of yields under high temperatures, the scientists argued. The additional carbon dioxide fertilization in the simulations does not alleviate the drop in yields associated with high temperatures above about 30 degrees Celsius.
Citation: “Consistent negative response of U.S. crops to high temperatures in observations and crop models,” Nature Communications, Jan. 19, 2017