Sooty Air Cuts China's Crop Yields.

China leads the world in grain production, harvesting more than 360 million tons each year. The growing affluence of its 1.2 billion inhabitants, however, has fostered a craving for more than China now grows. As a result, this nation imports grain. As China's population continues to swell, so will its demand for imports--leaving less for poorer grain-starved nations.

A new study finds that by cleaning its air, China might eliminate---at least in the near term--its need for imported grain.

Dust-size particles of soot and other pollutants have created a haze over much of China's grain belt. This pollution can significantly depress photosynthesis, reducing crop yields, the new study finds. In fact, its calculations suggest that the haze could be robbing farmers of more grain than China now imports.

Pollution analysts have viewed haze-causing particulates as a visibility-limiting nuisance and potential health threat. "This new paper looks at an impact that's never been considered," says study leader William L. Chameides of the Georgia Institute of Technology in Atlanta.

His international team put climate and pollution data into a computer model that simulates reactions between pollutants under local meteorological conditions. "From that," Chameides explains, "we get estimates of how much [haze] is in the atmosphere." Haze measurements collected throughout China invariably proved even larger than the estimated levels.

The model only accounts for pollution created by human activities, such as wood and coal burning, Chameides explains. It also ignores the role of particulates in gathering water into clouds.

The researchers put their computed values for haze into a crop-projection model that many governments use to predict harvests. To customize it for a given crop and region, scientists need some 10 years of meteorological and farm data. Such detailed information was only available for rural areas near Nanjing, about 200 miles west of Shanghai. Chameides' team then extrapolated its Nanjing findings to China's other farm regions.

Those harvest estimates, reported in the Nov. 23 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, show that haze may be depressing China's farm yields by 5 to 30 percent. The researchers project the biggest losses for the Sichuan Basin and Yangtze Delta, areas that have especially sooty air.

However, Chameides notes, because haze is a problem worldwide, it probably is diminishing crop yields in developing and industrial nations alike.

Though the new projections represent "a good effort," they remain by necessity "a bit rudimentary," says Cynthia E. Rosenzweig of NASA's Goddard Institute for Space Studies in New York. The harvest models, for instance, employ overly simplified relationships between crop growth and sunlight. The new study also estimated yield losses from haze in the absence of any other problems. In fact, Rosenzweig notes, yields can be limited at least as much by pests, water shortages, or insufficient nutrients as by haze.

The good news is that China may have begun a transition away from the coal burning that contributes to its serious haze problems, notes agricultural economist Lester Brown of the Worldwatch Institute in Washington, D.C. Cutting coal subsidies has raised the price of this fuel and limited its use, he says.

Moreover, the nation recently activated its first wind farm. "China has enough usable wind to easily double its current national electricity generation," Brown notes.