Global Warming May Boost Crop Yields, Study Says

John Pickrell
for National Geographic News
November 25, 2002
Escalating greenhouse gas levels may significantly boost production of
fruits and seeds in crops such as wheat, rice, and soybeans, according
to a recent study.

But the effect may be a double-edged sword; the increase in yield appears to be linked to a decrease in the nutritional value of these crops.

"Crops have higher yields when more [carbon dioxide] is available, even if growing conditions aren't perfect," said Peter S. Curtis, an ecologist at Ohio State University and co-author of the study. "But there's a trade-off between quantity and quality. While crops may be more productive, the resulting produce will be of lower nutritional value."

The findings could impact scientific thinking.

"There is no doubt but that quality matters," said Robert Mendelsohn, an environmental economist at Yale University. "If scientists can demonstrate a distinct loss of quality, this would be important and could change our impression of the global impact of climate change on agriculture from benign to harmful."

Gaseous Fertilizer

Carbon dioxide is a natural element in the Earth's atmosphere, and plants use it as a kind of "gaseous fertilizer," said Curtis.

However, human activities such as burning fossil fuels for heat, electricity, transportation, industry, and other purposes, are increasing the levels of carbon dioxide in the atmosphere.

Carbon dioxide, methane, nitrous oxide, and other gases are known collectively as greenhouse gases because they trap heat in the atmosphere. Most scientists believe the increasing levels of carbon dioxide and other greenhouse gases is causing global climate changes that could melt ice caps, raise sea levels, create deserts, and intensify adverse and unpredictable weather conditions such as tropical storms.

To understand the role that rising carbon dioxide levels might have on plant growth, Curtis and his colleagues pulled together data from 159 studies published over the last 20 years. These studies detailed the effects of higher-than-usual carbon dioxide levels on 79 plant species. The species included domesticated crops such as corn, cotton, wheat, and rice, as well as wild plants such as heath plants (blueberry and cranberry, for instance), wild radish, plantains, and pasture grasses.

Other researchers have summarized the positive effect of elevated carbon dioxide on leaves, stems and roots, said Curtis, but few have examined the overall effects on the reproductive traits of plants—seed and fruit number, size, and nutritional quality.

"Reproductive traits are key characteristics for predicting the response of communities and ecosystems to global change," said Curtis.

In most of these studies, the plants were exposed to roughly twice the amount of carbon dioxide currently in the atmosphere (the gas makes up about 0.036 percent of atmospheric gases today). This is the amount of the gas that many scientists predict will be present in the atmosphere in the year 2100, said Curtis.

"Unlike controversy surrounding global warming, the increase in atmospheric carbon dioxide itself is beyond argument," said Irakli Loladze, an ecologist at Princeton University in New Jersey.

The researchers found that elevated carbon dioxide levels had an overwhelmingly positive effect on the reproductive traits of crop plants. The analysis revealed a 25 percent increase in total seed weight, a 19 percent increase in the number of flowers, a 16 percent increase in number of seeds, and a 4 percent increase in individual seed weight.

The scientists detail their findings in the October issue of the botanical journal New Phytologist.

Paying the Price

The study confirms what many people expected about fruit and seed yields, said Curtis.

"The surprise is that nitrogen levels actually go down" with elevated carbon dioxide levels, reducing the nutritional value of these foods, he said. Nitrogen is most commonly present in the form of protein.

Nitrogen levels were 15 to 20 percent lower in the seeds of many crop species tested. There are several theories why the nitrogen content decreased. One is that the nitrogen is simply more diluted amongst the larger mass of fruits and seeds, said Curtis.

Alternatively, it could be that the plants become more efficient at photosynthesis—converting sunlight and carbon dioxide into glucose, providing the energy needed for growing—under high carbon dioxide levels, reducing their requirement for nitrogen.

Some crop species known as legumes—including peas and soybeans—didn't follow this trend. These plants showed an increase in yield and no change in nutritional value.

The increase in productivity does not make up for the fall in nutritional value of the crops, said Loladze. "Plants provide 84 percent of worldwide calorie intake, but they also are the major source of essential nutrients," he said.

Loladze's own research has shown that levels of other micronutrients such as iron, zinc, and iodine may also fall in carbon dioxide enriched species. High carbon dioxide levels may result in less "essential [dietary] elements per calorie," he said.

While both crops and wild species showed an average of 30 percent total increase in growth, wild plants tested didn't allocate this growth in the same way as crop plants. They were more likely to use the additional resources for protection in the form of chemical deterrents or toughened leaves, than in increased reproductive output, the researchers say.

"Six thousand to 10,000 years of breeding have caused domestic plants to put all their energy into seeds and fruits; this leaves them very poorly equipped to deal with the vagaries of nature," said Curtis.

"But that's okay because we protect them from insects, browsing animals, and disease. Wild plants do not have this luxury," he said.

In addition, the composition of wild plant communities may be adversely affected by increasing carbon dioxide levels as some plants will be better able to take advantage of the resources than others, said the researchers.

"There will be winners and losers," said Curtis. "We may end up with a bunch of fast growing weedy species."

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