Warming climate could cause reduction in U.S. crops

WASHINGTON — The current global trend of rising average temperatures could bring worrisome reductions in crop yields within a decade.

Computer models

According to Michael J. Roberts, assistant professor of agricultural and resource economics at North Carolina State University, computer models suggest that average global temperatures will rise enough to cause severe weather extremes that could cut yields by 20 percent 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 percent for the period 2070-2099.

Geographic areas changing

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. He argues that with enough planning and preparation, the impact of climate change on our staple food supply could be reduced.

Weather extremes

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 percent lower than expected. That prediction resulted in the price of corn rising 8 percent the same day the prediction announcement was released, he said.

Direct impacts

The projected direct impacts from climate change on U.S. crop yields was startling, according to Roberts’ data and models.

Using data from Indiana weather records dating back more than 100 years, he found 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 percent 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 percent 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.

Still unknown. 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.

Food prices

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 $2 a day, or less, it’s probably a really big deal.”

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