Dim Harvest: Asian Air Pollution Has Limited Rice Yields

Article excerpt

Thick clouds of air pollution over southern Asia and increased concentrations of greenhouse gases in the atmosphere worldwide have restricted rice harvests in India for the past 2 decades, a new analysis suggests.

Aerosols, such as volcanic ash and industrial soot, typically cool Earth's surface by reflecting some solar radiation back into space. This phenomenon somewhat counteracts the planet-warming effect of increased concentrations of gases such as carbon dioxide, says V. Ram Ramanathan, a climate scientist at the Scripps Institution of Oceanography in La Jolla, Calif.

However, after reviewing crop records and past research, Ramanathan and his colleagues suggest that the cooling action of the so-called Asian brown cloud that hangs over much of India hasn't countered global warming's negative consequences on rice harvests. For one thing, the cooling effect occurs at the wrong time of day, they say.

Increased concentrations of greenhouse gases raise nighttime temperatures, says Ramanathan. But air pollution blocks radiation only in the daytime, he notes.

In previous studies, each 1[degrees]C increase in average nighttime temperature decreased rice yield in the Philippines about 10 percent (SN: 7/10/04, p. 29), and in India, the air pollution was shown to reduce rice yields between 6 and 17 percent.

Beyond their cooling action, thick clouds of high-altitude pollution tend to stifle precipitation. The abundance of small particles in the atmosphere results in water droplets that are too tiny to fall as rain (SN: 3/11/00, p. 164). Furthermore, says Ramanathan, the clouds of pollution decrease evaporation at ground level and thereby reduce the amount of water vapor available to form rain. …