Most of us are by now aware of the harmful effect greenhouse gas emissions exert through rising global temperatures. As temperatures go up, glaciers melt and ocean levels rise. Climate change also exacerbates water scarcity worldwide.

Water scarcity significantly impacts agricultural productivity and food scarcity. These impacts will be felt the most in arid regions, where agriculture depends on irrigation, which represents humanity's largest diversion of fresh water.

For the most part, we think of rising levels of carbon dioxide as an environmental problem. But atmospheric CO 2 can also boost agricultural productivity by helping plants grow. How do these potential issues balance out? In an investigation recently published in Nature Climate Change, scientists have looked into the global implications of carbon dioxide's ability to enhance agricultural productivity.

Increased levels of CO 2 can enhance photosynthesis and reduce leaf-level transpiration, the process by which some of the water that plants draw from the ground gets released back into the atmosphere. These changes can reduce growing seasons and water loss. The result could be an increase in what's called "crop water productivity," i.e. the amount of food produced for each unit of water expended.

If elevated CO 2 levels increase crop yield and reduce water consumption at large scales, this could help ensure water and food security despite the climate disruptions. By combining data from a massive network of field experiments and global crop models, the scientists obtained a spatially defined global perspective on crop water productivity under elevated CO 2 levels and associated projected climate change. They assessed several staple crops—wheat, maize, rice, and soybean.

Depending on the crop type, global crop water productivity increased by 10 to 27 percent by the 2080s. Arid regions exhibited large increases that were based on crop type. The largest increase in crop water productivity was observed for rain-fed wheat at 48 percent.

The team also examined the contribution of CO 2 on crop productivity in different regions, comparing the impact of elevated temperatures with and without the additional CO 2 . Larger effects were seen on maize grown in semi-arid regions including southern Africa, the Middle East, parts of central Asia, the western US, and the Iberian Peninsula.

Climate change's effects on crops are complex, with water availability, average temperature, frequency of temperature extremes, and CO 2 fertilization all influencing overall productivity. Figuring out the ultimate impact will require accounting for all these influences. The new study provides some numbers on how some of the different factors interact. It suggests that elevated CO 2 concentrations could partially offset global yield losses and reduce agricultural water usage by 4 to 17 percent.

Nature Climate Change, 2016. DOI 10.1038/NCLIMATE2995 (About DOIs).