How hot is too hot? A new University of Florida study shows that global warming could dramatically reduce peanut production in places where temperatures are already high.

The finding is bad news for parts of India and Africa, where peanuts are an important source of protein for millions of people. And it could eventually mean permanent change for agriculture in parts of the Southeastern United States, where peanuts have long been a major crop.

Will move northward

“In the U.S., peanut production will start moving northward toward more temperate regions,” said P.V. Vara Prasad, a research scientist at UF's Institute of Food and Agricultural Sciences. “In Africa and India, where temperatures are already above the optimum for growing peanuts, production will drop drastically.”

Prasad is part of a team of researchers who grew peanut plants under a range of temperatures and atmospheric conditions in small, greenhouse-like “growth chambers” on the UF campus. Their goal was to measure the effects of global warming — the increase in average temperatures caused by rising levels of man-made carbon dioxide — a heat-trapping “greenhouse gas” in the atmosphere.

“We know that human activities such as deforestation and the burning of fossil fuels have caused atmospheric carbon dioxide levels to increase by one-third since 1750,” said Hartwell Allen, a researcher with UF and the U.S. Department of Agriculture and an author of the study, which was published in the December 2003 issue of the journal Global Change Biology.

“It's generally accepted that if current trends continue, we'll see a doubling of carbon dioxide levels by the end of this century,” he said.

That could increase average temperatures worldwide by 4 to 10 degrees Fahrenheit in the next 100 years, Allen said.

Studies at UF and other institutions have shown those temperature increases could have disastrous effects on the yield of staple crops such as rice and soybeans. But since plants need to absorb carbon dioxide in order to grow, researchers have held out hope that higher levels of the greenhouse gas in the atmosphere would encourage plant growth, offsetting some of those losses.

The UF researchers grew peanut plants under conditions typically found in the heart of peanut-growing country in the U.S. Southeast, where daily highs during the growing season average 90 degrees Fahrenheit. They grew peanuts in three other greenhouses set at higher temperatures. They also grew a second set of plants under the same set of temperatures, and exposed those plants to twice the amount of carbon dioxide currently found in the atmosphere.

The UF team found that a doubling of carbon dioxide would offer just enough boost to keep up peanut yields if daily highs rose to about 97 degrees. But at higher temperatures, yields dropped by about six percent per degree of increase.

Pollination problems

The problem, Prasad said, is that heat affects a plant's ability to pollinate, lowering production of the seeds that people harvest from the plant. Carbon dioxide encourages the plant to grow larger and more lush, with more flowers to offset the decline in fertility cause by higher temperatures. But the extra flowers can only do so much.

“If you continued the progression of increasing temperatures, you'd be left with a plant that grows big and green, but produces no seed at all,” Prasad said. “And we've experienced the same effect in other crop plants, like rice, soybeans and dry beans.”

The results could spell trouble for peanut farmers in places such as India and West Africa, where temperatures during the growing season area already well into the 90s.

“These places could lose six percent of their production for every degree of increase, which is scary,” Allen said. “What's still scarier is that in these regions the crops are mostly rain-fed. If global warming also leads to drought in these areas, yields could be even lower.

Crop shifts

Allen said global warming would likely force Third World peanut growers to shift to other crops, such as millet or sorghum. But it's possible that yields of those crops could also be affected by higher temperatures, he said.

The United States will not go unaffected, Prasad said. While scientists often speak of climate change in terms of increases in average temperature, few growing seasons turn out to be “average,” he said.

“We could see a heat wave that would push temperatures much higher than average in any given year, and in any given year, there are pockets of higher temperature,” he said. “The result is that growing peanuts is going to become much riskier as the planet gets warmer.”

Prasad said those risks could lead farmers in American's southernmost peanut-producing areas, including Florida and southern Georgia and Alabama, to get out of the business.

“We are all concerned about changes in the earth's atmosphere, and I think models such as this one will help us prepare for changes in the future,” said Howard Valentine, executive director of the Peanut Foundation in Alexandria, Va.

Time is now

The UF researchers say it's time to begin developing plants that can deal with rising global temperatures.

“We should be conserving fossil fuel now and searching for heat-tolerant cultivars, if the global temperature does rise as theory is suggesting,” said UF agronomy professor Ken Boote, one of the study's authors. “Understanding heat stress effects will help us understand and manage for present warm, dry peanut-cropping season.”