Shortages of freshwater used for irrigation may double the detrimental effects of climate change on agriculture, according to a new analysis that combines climate, agricultural, and hydrological models.
Researchers say a warmer world may have severe consequences for global agriculture and food supply, reducing yields of major crops even as population and demand increases.
Given the present trajectory of greenhouse gas emissions, agricultural models estimate that climate change will directly reduce food production from maize, soybeans, wheat, and rice by as much as 43 percent by the end of the 21st century.
But hydrological models looking at the effect of warming climate on freshwater supplies project further agricultural losses, due to the reversion of 20 to 60 million hectares of currently irrigated fields back to rain-fed crops.
“It’s a huge effect, and an effect that’s basically on the same order of magnitude as the direct effect of climate change,” says Joshua Elliott, a research scientist with the Computation Institute’s Center for Robust Decision Making on Climate and Energy Policy (RDCEP), Argonne National Laboratory, and lead author of the paper.
“So the effect of limited irrigation availability in some regions could end up doubling the effect of climate change.”
Dramatically different predictions
Agricultural models and hydrological models both incorporate the influence of climate, but are designed by different scientific communities for different purposes. While agricultural models simulate how temperature, precipitation, and other climate factors may alter the yield for various crops, hydrological models seek to estimate water-related characteristics such as stream flow, water availability, and storm runoff.
The two types of models overlap in estimating the amount of water used for agricultural irrigation, by far the largest human use of freshwater in the world. But when researchers fed each type of model with the same climate model forecasts, the models produced dramatically different predictions about the future demand for freshwater irrigation.
The researchers discovered discrepancies in how hydrological models incorporate processes such as the carbon cycle and crop water productivity when compared to agricultural models—a finding that will help make existing models more accurate.
“This is absolutely the first study in which a multi-model ensemble of hydrological models was compared to a multi-model ensemble of crop models,” Elliott says. “Several modeling groups have already changed the way that they are modeling the hydrological cycle with respect to crops because of the results of this paper.”
Not a global loss
Published in the Proceedings of the National Academy of Sciences, the comparison also produced new insight about the potential agricultural consequences of climate change. Due to climate change alone, the models predicted a loss of between 400 and 2,600 petacalories of food supply, 8 to 43 percent of present day levels.
But due to the decline in freshwater availability—and the associated conversion of irrigated cropland to rain-fed—the models predict an additional loss of 600 to 2,900 petacalories, researchers say.
However, while the models predict freshwater shortages in some areas of the world, such as the western United States, India, and China, other regions may end up with a surplus of freshwater. Redistributing that excess water to restore or add irrigation to rain-fed crop areas could dampen some of the consequences of climate change upon irrigation and agriculture, Elliott says.
“We found that maximal usage of available surplus freshwater could end up ameliorating between 12 and 57 percent of the negative direct effects of climate change on food production,” Elliott says. “However, there are lots of different political, economic, and infrastructural reasons why you would consider that to be overly optimistic.”
Source: University of Chicago