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A Warming Climate Could Cause Dramatic Reduction in U.S. Crop Yields, Expert Says

The current global trend of rising average temperatures could bring worrisome reductions in crop yields within a decade, a leading agricultural economist told a Capitol Hill briefing co-sponsored by AAAS.

Michael J. Roberts, assistant professor of agricultural and resource economics at North Carolina State University, said computer models suggest that average global temperatures will rise enough to cause severe weather extremes that could cut yields by 20% for major crops such as corn and soybeans for the period 2020-2049.

That could happen even with marked reductions in emissions of heat-trapping carbon dioxide (CO2), he said. If those emissions remain on a “business as usual” path, Roberts said, the yield reductions could be as much as 80% for the period 2070-2099.

Michael J. Roberts

Roberts discussed his findings at a 25 March briefing on Capitol Hill called “Weather, Climate, and Food,” the latest in a series of climate briefings organized by the American Meteorological Society. The event was co-sponsored by AAAS, the American Geophysical Union, and the American Statistical Association. It focused on how future predictions for weather and climate change could affect the world’s food supply.

Donald A. Wilhite, director of the School of Natural Resources at the University of Nebraska-Lincoln, also spoke at the briefing and discussed how improving drought management could soften the economic blow felt during periods of severe drought. Both Wilhite and Roberts emphasized that climate change will have a noticeable impact on agriculture in the decades ahead. Weather extremes and rising temperatures will change the geographic areas, such as the Midwest, we now think of as the primary sources of staple food and likely lower their yields.

Despite some claims that increased CO2 levels might actually stimulate crop growth, Roberts said such effects are uncertain and recent research suggests these effects could be much less beneficial than previously believed. Both he and Wilhite argued that with enough planning and preparation, the impact of climate change on our staple food supply could be reduced.

Roberts said severe drought, increased temperatures, and other weather extremes in the United States—and the fluctuations in crop yields that they bring—can affect corn, rice, wheat, and soybean prices all over the globe. “Our production really drives the world’s prices,” Roberts said. “This is why the U.S. is so important.”

If the price of corn increases in the United States, it will likely increase everywhere else. “Small quantity changes have big price effects,” Roberts said. In 2010, the September forecast for October’s corn yield was 4% lower than expected. That prediction resulted in the price of corn rising 8% the same day the prediction announcement was released, he said.

The projected direct impacts from climate change on U.S .crop yields was startling, according to the data and models Roberts presented. He discussed data from Indiana weather records dating back more than 100 years showing that the number of degree days (a temperature measure) per year above 86 degrees directly correlated to crop yield. This correlation was prominent in the records from the 1930s Dust Bowl. If the number of very hot days per year were to increase substantially, Roberts said, crop yields of key staples such as corn, soybeans, and cotton would likely decline more than 20% in the years 2020-2049. Roberts said these numbers reflect a scenario in which there is relatively slow growth in the total amount of heat-trapping carbon dioxide emitted into the atmosphere. The crop yields could fall even more—as much as 80% for the period of 2070-2099—if CO2 emissions continue on a “business as usual” path, he said.

Early research on the benefits of CO2 fertilization, conducted in greenhouses, showed that the crops could initially benefit from higher concentrations of the gas in the atmosphere, increasing the yield. However, Roberts said, when applying the same methodology in the open field, the researchers were “a lot less confident that [CO2 fertilization] will be much of a boost at all.”

Roberts acknowledged that the computer projections he discussed do not take into account any effects of CO2 fertilization or shifts in growing areas that might offset some of the impact of temperature extremes in traditional growing regions. But he said the models do suggest the very distinct possibility that global food prices could rise during times of uncertainty and transition in farming practices because of lower crop yields.

According to Roberts, a temporary increase in world food prices probably would not hurt the United States economy, since it produces so much of the world’s corn and soybeans. “Big price increases could make agriculture a larger share of GDP,” he said, “and it could probably be a benefit to the United States.” He also noted that this scenario also has a significant negative side. “Impacts here would have a big influence on world prices,” he said. “For the 2 billion people living on two dollars a day, or less, it’s probably a really big deal.”

Donald A. Wilhite

Wilhite underscored the need to make the necessary preparations for drought. He is the founding director of the National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln and spoke of the importance of being aware of drought before it becomes a crisis. He said advance steps to mitigate the impact of drought include conserving water at appropriate times; shoring up reservoir infrastructure before a drought crisis emerges; drought contingency planning at all levels of government that emphasizes proactive, risk-based management; and improved drought early warning, prediction, and information delivery systems.

Those actions, combined with policy changes at the state and federal level, can lessen a drought’s severity. According to the NDMC website, policy actions taken by states to mitigate drought in the past include passing legislation to protect instream flows, providing low-interest loans to farmers, and imposing limits on urban development.

Wilhite shed light on a problem he calls the “Hydro-illogical Cycle,” a frame of mind that he says most people use when they think of drought. They don’t realize there is a drought until they are in the middle of one and have to go directly into a crisis-management mode, he said.

“Over the past two decades,” said Wilhite, “severe drought conditions have affected nearly all portions of the nation and resulted in serious economic, social, and environmental impacts.” The recent drought in the West contributed to the depletion of aquifers and other natural water resources in an effort to fulfill the needs of agriculture and population, Wilhite said. With more people migrating to areas of the country with fewer water resources, he said, the impact of future droughts is expected to increase.

Because of this potential and the public psychology of drought, Wilhite stressed the need for a transition from drought crisis-management plans to drought mitigation plans. Drought mitigation will be an increasingly important tool, he said, as the world’s mean temperature continues to rise.


Visit the AAAS Climate Resources page.


Ryan Rexroth

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