Air pollution in northern hemisphere may affect world food supply.

Photochemical smog caused by emissions from the burning of fossil fuels now covers major portions of the northern hemisphere. As a result, many of the world's most productive agricultural regions are exposed to harmful amounts of air pollutants. These exposures, which will likely increase in the coming decades as a result of industrial growth, may raise the costs of food productio in the United States and Europe, and lead to food shortages in China and other developing countries.

The warning comes in a new report in the April issue of Science. Written by scientists at the Georgia Institute of Technology and the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, the finding are based on a computer model of the global distribution of air pollution and its geographical correlation with food crop production.

The researchers estimate that by the year 2025, 30-75% of the world's cereal crops will be grown in areas with potentially harmful levels of ozone, an air pollutant produced in photochemical smog.

Currently between 10-35% of these staple crops, including wheat, corn, and rice are grown in areas where ozone pollution levels exceed 50-70 parts per billion. This is the concentration threshold that has been observed to cause reductions in the yields of food crops.

"The widespread effect of air pollution on food crop production is a prime example of the linkage that exists between the health and vitality of our economy and our environment," argues Dr. William L. Chameides, director of the School of Earth and Atmospheric Sciences at Georgia Tech, and lead author of th report.

Research carried out under the auspices of the National Crop Loss Assessment Program in the 1980s indicated that crop losses from exposure to ozone pollutio cost the U.S. economy several billion dollars annually.

The Science report by Chameides and colleagues, P.S. Kasibhatla, J. Yienger, an H. Levy, suggests these effects are common to agriculturally productive regions throughout much of the northern hemisphere and will likely worsen in the coming decades if nations expand their industrial activities without instituting appropriate pollution control measures.

"The costs of food crop exposures to air pollution are largely hidden from the consumer in the wealthier nations like the U.S.," explained Chameides, "because these countries control food supplies and prices through policies that limit national surpluses."

These costs ultimately exact some economic and human toll, and Chameides believes that they need to be carefully assessed before the full benefits of pollution control can be understood and effective plans for economic developmen formulated.

While photochemical smog and high concentrations of ozone pollution are most often associated with urban centers, more diffuse, regional ozone pollution episodes have been found to extend over thousands of miles and encompass agricultural as well as urban areas. The repetition of these episodes over a growing season can produce a pattern of chronic exposure that ultimately reduce crop yields.

"The problem appears to be most severe in three broad regions of the northern hemisphere," says Chameides. These regions, which the authors call "Continental-Scale Metro-Agro-Plexes" (CSMAPS) are located in eastern North America, Europe, and eastern China and Japan, and are characterized by an intermingling of urban, suburban, and agricultural areas.

The authors note that collectively the CSMAPS comprise only 23% of the Earth's continental area, but account for over 60% of the world's food crops and over 75% of all commercial energy consumption and fertilizer usage.

The levels of crop-damaging air pollutants are projected to increase most markedly in developing nations. In China, rapid economic and industrial expansion is projected to more than double the emissions of air pollutants that produce photochemical smog by the year 2025, a time when rising populations wil likely be straining world food production capacities.