Dim harvest: Asian air pollution has limited rice yields.

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.

The reduction in rainfall both decreases rice yield per acre and cuts the number of acres that can be farmed. "This shows that air pollution isn't just an urban problem," says Ramanathan.

He and his colleagues have analyzed India's rice harvests since 1966. They report in an upcoming Proceedings of the National Academy of Sciences that after improvements in agricultural techniques sparked dramatic yield increases in the mid-1960s, the annual growth of yields dropped to around 3 percent in the mid-1980s and has been stagnant since 2000. Although factors such as soil degradation and falling rice prices may have played a role in this decline, air pollution and greenhouse gases have contributed substantially, the researchers contend.

If the Asian brown cloud hadn't been present over India, increased precipitation would have boosted rice harvests by 10.6 percent each year between 1985 and 1998, the scientists say.

Rice yields would have been another 3.8 percent higher if atmospheric concentrations of greenhouse gases had remained stable during those years, says Ramanathan.

The new findings "combine several aspects of climate change and give a better idea of how crop yields might change in the future," says Lew Ziska, a plant physiologist with the Agricultural Research Service in Beltsville, Md. "When you look at climate change, it's not just about warming."

Says Peter Timmer, an agricultural economist at the Washington, D.C.-based Center for Global Development, "Brown-cloud pollution has already cost India millions of tons of food production."