Higher CO2 levels could offset some crop losses from climate change

As climate change drives higher temperatures and more frequent droughts around the world, many researchers predict severe threats to agriculture and food security. But a new study aggregating several climate and crop models suggests that the primary driver of climate change—rising levels of carbon dioxide—may also prove beneficial to crops, mitigating a portion of the damage.

The study, published in Nature Climate Change and led by Delphine Deryng, an environmental scientist at the University of Chicago’s Computation Institute, Columbia University’s Center for Climate Systems Research and the NASA Goddard Institute for Space Studies, finds that higher atmospheric CO2 will increase the water productivity of staple crops, including wheat, maize, rice and soybean. Modeling data found that in 2080 the elevated CO2 would improve agricultural water efficiency and increase crop yield—but not enough to completely offset the expected decline from climate change.

“We find that there is a huge increase in crop water productivity, but if you look at the net effect on yield and production, there’s still a negative impact due to extreme heat and water stress conditions,” Deryng said. “But increases in biomass yield with less water have crucial implications for food and water security, as well as agricultural policy.”

The new study finds large regional differences in the effect of high CO2 on crop water productivity, as well as significant differences in how it affects the four different crops studied. For instance, they projected that farms in arid regions, particularly those using primarily rainwater, will benefit more from the positive effect of elevated CO2.

“It’s important to look at where the CO2 effect will occur on each cropping system and how much it will relieve water stress,” Deryng said. “Then you can inform long-term adaptation strategies to adequately take advantage of the beneficial effects and mitigate the negative impact of climate change.”

The study used data from a major international collaboration, jointly led by the Agricultural Model Intercomparison and Improvement Project and the Inter-Sectoral Impact Model Intercomparison Project. These scientific efforts aim to capture the uncertainty related to climate impact projections—the range of outcomes possible for the effect of climate change on areas such as agriculture, economics, health, land use and energy.

Deryng and colleagues from institutions in the United Kingdom, Sweden, Switzerland, Germany, Austria and the United States produced data to estimate crop water productivity in 2080, when carbon dioxide levels are projected to reach as high as double the levels of 2000. They compared crop yield and water use under two scenarios: the expected changes in climate and elevated CO2, and the expected changes in climate but with current-day CO2 levels.

Changes in climate conditions such as higher temperatures and more frequent drought produced severe global declines in crop yield, ranging from 21 to 35 percent for staple foods maize, rice, soybean and wheat. But when climate change and elevated CO2 were modeled together, the effect on agriculture was much weaker, showing large declines for maize and a modest increase for wheat. For soybeans, half of the simulations found an increase in crop yield, while the other half found a decrease.

The mitigating effect of CO2 is explained by increased crop water productivity, reflecting higher crop yield and less evapotranspiration, the loss of water to the air. Agriculturally, that means farmers may be able to maintain or increase crop yield with less water (assuming no other damage like extreme heat stress for instance), which may be critical as global competition for freshwater resources intensifies.

However, the study identified important gaps in scientific knowledge about how CO2 affects crops under different environmental conditions. While these simulations used results from studies of crops experimentally grown in high CO2, these experiments have only been performed in limited geographic and climate circumstances. Based on results of this study, agronomists should run large field-scale experiments in understudied regions in Africa, South America and India, which already experience high food and water insecurity.

Another important caveat—and avenue of research—is whether high-CO2 conditions reduce the nutritional content of staple food crops, Deryng said. Recent studies have found reduced zinc, iron and protein in crops grown under elevated carbon dioxide, which could make the benefits on crop yield less meaningful for public health and world hunger.

Citation: “Regional disparities in the beneficial effects of rising CO2 concentrations on crop water productivity,” by Delphine Deryng, Joshua Elliott, Christian Folberth, Christoph Müller, Thomas A. M. Pugh, Kenneth J. Boote, Declan Conway, Alex C. Ruane, Dieter Gerten, James W. Jones, Nikolay Khabarov, Stefan Olin, Sibyll Schaphoff, Erwin Schmid, Hong Yang, and Cynthia Rosenzweig, published online April 18, 2016, DOI: 10.1038/nclimate2995.