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 See under Ashes.
See also: Ash and industrial soot, typically cool Earth's surface Noun 1. Earth's surface - the outermost level of the land or sea; "earthquakes originate far below the surface"; "three quarters of the Earth's surface is covered by water"
surface by reflecting some solar radiation solar radiation,
n the emission and diffusion of actinic rays from the sun. Overexposure may result in sunburn, keratosis, skin cancer, or lesions associated with photosensitivity. back into space. This phenomenon somewhat counteracts the planet-warming effect of increased concentrations of gases such as carbon dioxide carbon dioxide, chemical compound, CO2, a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. , says V. Ram Ramanathan, a climate scientist at the Scripps Institution of Oceanography Scripps Institution of Oceanography: see California, Univ. of. in La Jolla La Jolla (lə hoi`yə), on the Pacific Ocean, S Calif., an uninc. district within the confines of San Diego; founded 1869. The beautiful ocean beaches, in particular La Jolla shores and Black's Beach, and sea-washed caves attract visitors and , 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 The Asian Brown Cloud is a layer of air pollution covering parts of the northern Indian Ocean, India, Pakistan, and parts of South Asia, Southeast Asia, and China. In proper humidity conditions it forms haze. 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 evaporation, change of a liquid into vapor at any temperature below its boiling point. For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the amount of surface exposed, the humidity 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 The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States 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."