Most of us are now aware of the damaging effect of greenhouse gas emissions from rising global temperatures. As temperatures rise, glaciers melt and sea levels rise. Climate change is also exacerbating water scarcity worldwide.
Water scarcity has a major impact on agricultural productivity and food scarcity. These effects will be most felt in arid regions, where agriculture depends on irrigation, humanity’s greatest diversion of fresh water.
For the most part, we view rising levels of carbon dioxide as an environmental problem. But atmospheric CO2 can also increase agricultural productivity by helping plants grow. How do these potential problems balance each other? In a recently published study in Nature climate changescientists have looked at the global implications of carbon dioxide’s ability to improve agricultural productivity.
Elevated levels of CO2 can improve photosynthesis and reduce foliar-level transpiration, the process by which some of the water plants take from the soil is returned to the atmosphere. These changes can reduce growing seasons and water loss. The result could be an increase in what is called “crop water productivity”, ie the amount of food produced for each unit of water consumed.
If elevated CO2 levels increase crop yields and reduce water consumption on a large scale could help ensure water and food security despite climate disruptions. Combining data from a vast network of field experiments and global crop models, the scientists obtained a spatially defined global perspective on crop water productivity under elevated CO2 levels and associated projected climate change. They assessed several staple crops: wheat, corn, rice and soybeans.
Depending on the crop type, global crop water productivity has increased by 10 to 27 percent by the 2080s. Dry regions showed large increases based on crop type. The greatest increase in crop water productivity was observed for rain-fed wheat at 48 percent.
The team also examined the contribution of CO2 on crop productivity in different regions, comparing the impact of elevated temperatures with and without the extra CO2. Greater effects were seen in maize grown in semi-arid regions, including southern Africa, the Middle East, parts of Central Asia, the western US and the Iberian Peninsula.
The effects of climate change on crops are complex, involving water availability, average temperature, frequency of extreme temperatures and CO22 fertilization, all of which affect overall productivity. To find out the ultimate impact, all these influences must be taken into account. The new study provides some numbers on how some of the various factors interact. It suggests that elevated CO2 concentrations could partially offset global yield losses and reduce agricultural water use by 4 to 17 percent.
Nature climate change2016. DOI 10.1038/NCLIMATE2995 (About DOIs).