Previous studies have suggested a “nonlinear behavior of U.S. [crop] yields,” said Bernhard Schauberger, a PhD student and researcher at the Potsdam Institute for Climate Impact Research. One study suggested that in temperatures above 86 degrees, crops suddenly experience strong declines, he noted.
Now a new study, led by Schauberger along with colleagues from institutes around the world, reaffirms the idea that high temperatures could seriously harm the production of some of the world’s most important food crops, including corn, soybeans and wheat. And that could have big implications for the world’s food supply — as the paper notes, these three crops alone account for about a third of total harvested land worldwide.
It’s a tricky research area — plants can react to the changing climate in a lot of complicated and even contradictory ways, and their responses may differ from one region and one crop to the next. In cold climates for instance, warmer temperatures might be a boon for plant life — research has already revealed a greening effect in certain northern regions of the world. In other places, hotter growing seasons could be devastating.
Then there’s also the controversial subject of whether increased carbon dioxide levels in the atmosphere could actually be good for crops — after all, plants use carbon dioxide to make the sugar they turn into energy. This is a point that climate doubters have often seized upon in the past when arguing against the severity of future climate change. (One prominent member of this camp is William Happer, a Princeton physics professor rumored to be under consideration for a position in the Trump administration after a recent meeting with the president-elect.)
But those who question the consequences of climate change may have some reconsidering to do. The new study, published Thursday in the journal Nature Communications, indicates that high temperatures can have significant damaging impacts on crop yields — and while access to more water can do a good job of mitigating these effects, extra carbon dioxide may not be all that helpful after all.
The researchers used an ensemble of crop models — nine in all — to simulate the responses of maize, soybean and wheat crops in the United States to changes in temperature. The models relied on observational data collected from U.S. croplands, and when tested using historical climate conditions, they generally produced results that closely matched real-life observations.
The models indicated that high temperatures could have especially severe effects on maize and soybean crops — for every day in which temperatures exceed 30 degrees Celsius (86 Fahrenheit), their crop yields could decline by up to 6 percent. In simulations involving historical climate conditions, wheat didn’t experience any significant declines — although the researchers pointed out that the timing of wheat harvests means wheat crops have rarely been subjected to extreme high temperatures during the growing season.
In a substantially warmer future, though, all three crops could face severe temperature-related declines. Under a business-as-usual warming scenario, the models indicated that exposure to temperatures up to about 97 degrees could cause declines by 49 percent in maize crops, 40 percent in soybeans and 22 percent in wheat. Those declines could be even worse under higher temperatures.
On the other hand, the models also suggest that providing crops with extra water via irrigation could lessen some of these effects. Losses were substantially reduced at high temperatures when irrigation was increased. While extreme heat can have all kinds of physiological effects on plants, this finding indicates that water stress is a primary reason warming causes harm to crops.
“When temperature increases, the atmosphere demands more water,” Schauberger noted. This can cause more water to evaporate from the soil into the air, meaning less water is available for plants to drink up, and it can also cause more water to evaporate from the leaves of the plants, themselves. Past a certain point, plants can become stressed from lack of water.
When plants are short on water, they initiate a series of processes designed to help them survive — these may include closing the pores on the undersides of their leaves to prevent excess water loss. Since these pores are also responsible for gas exchange, this process also forces plants to take in less carbon dioxide, which can affect their ability to photosynthesize. Water-stressed plants may also devote more of their energy to spreading roots underground and less to growing body mass above ground. All of these processes can reduce their yield in crops.
“We can adapt to [water stress] in some sense by irrigating more, but only as there is enough water,” Schauberger said. In drought-stricken or otherwise water-stressed parts of the world, irrigation may not be a long-term option for protecting crops.
The study also suggests that we can’t necessarily count on rising carbon dioxide levels to balance out the effects of rising temperatures, either. In the simulations, extra carbon dioxide did not have any broad, significant impact on alleviating crop yield losses at high temperatures.
Schauberger noted that higher levels of carbon dioxide can, in fact, lead to an increase in plant growth, resulting in bigger plants. But bigger plants require more water, which can work against them in warm conditions — under high temperatures, the effect of having larger biomass can more or less cancel out the benefits of having access to more carbon dioxide. The result is that there are no significant differences in crop yields under elevated carbon dioxide conditions.
The researchers are careful to note that these findings shouldn’t be taken as concrete predictions for the future. For one thing, the study only examines one high-emissions future scenario, which may or may not reflect what actually happens in the coming years. There are also various forms of adaptation, aside from just increasing irrigation, that could help protect global food security in the future. Working to develop plants with greater heat tolerance is one strategy, Schauberger noted.
But, he added, preventing the problems caused by extreme temperatures in the first place is the best first line of defense.
“The best adaptation is mitigation,” Schauberger said. “The less climate change we actually cause, the less we have to deal with the impact.”