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The Flip Side of Global Warming : Agriculture: An EPA-commissioned study says the greenhouse effect would actually boost overall farm production in the U.S.

TIMES SCIENCE WRITER

Although global warming caused by the greenhouse effect would make it more difficult to grow crops in many regions of the United States, overall farm productivity would actually increase because of the increased amount of carbon dioxide available in the air to plants, a multiuniversity study will report today.

In what is billed as the “first quantitative estimate” of changes in crop yields and water use arising from global warming, the study’s authors conclude in the British journal Nature that the United States will not be converted into a food-importing nation by climatic change, as some researchers have feared.

They also report that U.S. farmers would benefit by $1.5 billion to $3.5 billion per year largely because the adverse effects of a warmer climate would be more than offset by higher productivity.

The study’s projections are generally more benign than some earlier, but much less detailed, predictions. Although some biologists have argued that increased carbon dioxide levels would accelerate insect activity and weed growth with potentially ruinous effects on crops, the study’s authors conclude that is not likely.

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But the two different computer models used by the researchers give grossly different projections of global warming’s effect on consumers. One model says that consumers would save $9.3 billion per year because of increased agricultural productivity. The other says that consumers would pay an extra $13.3 billion per year because of higher prices.

The study’s authors cautioned that their results are, in effect, a guess based on the best information available and “do not predict the future.”

“This still can be considered a global experiment that nobody has ever conducted before, so the outcome is not known yet,” said agronomist L. Hartwell Allen Jr. of the University of Florida in Gainesville, one of the study’s authors.

The greenhouse effect arises because carbon dioxide in the atmosphere allows sunlight to reach the Earth’s surface and warm it, while preventing the radiation of heat into space--a phenomenon similar to that by which the glass panes of a greenhouse keep it warmer than outside temperatures.

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The carbon dioxide comes primarily from the burning of fossil fuels. Its concentration in the atmosphere has grown by more than 25% since the beginning of the industrial age in the 1850s, and scientists expect its concentration to double within the next 50 to 75 years if emissions continue to grow at their current rates.

As a result of that increased carbon dioxide concentration, some researchers believe that the Earth’s average surface temperature has already increased by as much as 1 1/2 degrees Fahrenheit and that it will increase by as much as nine degrees by the year 2030.

That projection is controversial for a number of reasons, including the belief of some climatologists that increased warming will lead to greater cloud formation that would ameliorate some of the effects of carbon dioxide.

Climatic change caused by global warming could have major effects on agriculture. Researchers have noted that the primary growing regions for many crops would move northward, such as from the United States into Canada, and that many crops would require much more irrigation. But no one has previously studied the economic effects of such changes, according to the authors.

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The study, sponsored by the U.S. Environmental Protection Agency and headed by Richard M. Adams of Oregon State University in Corvallis, used two different computer models of climatic change caused by increased carbon dioxide concentrations--one developed by NASA’s Goddard Institute for Space Studies in New York City and one developed at Princeton University in New Jersey.

The primary difference between these two models lies in their rainfall predictions. The Princeton model projects substantial decreases in rainfall throughout the United States, while the NASA model projects such decreases primarily in the Southeast. Currently, experts say, there is no conclusive evidence on which to base a choice between the two models.

Those models were linked to other, well-documented computer models that show changes in productivity of wheat, corn and soybeans caused by changes in atmospheric carbon dioxide, temperature and rainfall.

The main projection of the computer simulations is that increased productivity caused by higher carbon dioxide levels can offset many of the other effects produced by global warming. If carbon dioxide concentrations were to double and other environmental conditions remained unchanged, for example, soybean production would be increased by 35%, wheat by 25% and corn by 10%, according to the models.

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Furthermore, Allen noted, increased growth would occur without the need for additional water because plants use water more efficiently when exposed to higher carbon dioxide concentrations.

Critics such as biologist Fakhri Bazzaz of Harvard University have argued, however, that as plants grow faster, their leaves and fruit may become less nutritious because they do not get enough nutrients from the soil. That may affect their palatability, and may cause insects to eat larger quantities to meet their food requirements.

Bazzaz also noted that increased carbon dioxide would accelerate the growth of weeds, which would in turn compete with crops.

Allen, however, said, “That is a non-issue” because farmers would undoubtedly use more pesticides and herbicides and would apply more fertilizer as productivity increases. Although overall U.S. agricultural productivity probably would not change much, “There will be significant regional shifts in U.S. agriculture,” said Cynthia Rosenzweig of the Goddard Institute. The most severely affected area would be the Southeast, where, according to the Princeton model, the demand for irrigation would increase by 147% by 2030. Some of the biggest gains would occur in the northern regions, which would have a longer frost-free growing season.

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California agriculture probably would not be affected much. Most studies indicate that it would be one of the last regions affected by greenhouse warming.


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