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Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle
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Contributed by V. Ramanathan, January 25, 2005

Abstract
South Asian emissions of fossil fuel SO2 and black carbon increased≈6-fold since 1930, resulting in large atmospheric concentrations of black carbon and other aerosols. This period also witnessed strong negative trends of surface solar radiation, surface evaporation, and summer monsoon rainfall. These changes over India were accompanied by an increase in atmospheric stability and a decrease in sea surface temperature gradients in the Northern Indian Ocean. We conducted an ensemble of coupled ocean-atmosphere simulations from 1930 to 2000 to understand the role of atmospheric brown clouds in the observed trends. The simulations adopt the aerosol radiative forcing from the Indian Ocean experiment observations and also account for global increases in greenhouse gases and sulfate aerosols. The simulated decreases in surface solar radiation, changes in surface and atmospheric temperatures over land and sea, and decreases in monsoon rainfall are similar to the observed trends. We also show that greenhouse gases and sulfates, by themselves, do not account for the magnitude or even the sign in many instances, of the observed trends. Thus, our simulations suggest that absorbing aerosols in atmospheric brown clouds may have played a major role in the observed regional climate and hydrological cycle changes and have masked as much as 50% of the surface warming due to the global increase in greenhouse gases. The simulations also raise the possibility that, if current trends in emissions continue, the subcontinent may experience a doubling of the drought frequency in the coming decades.
Footnotes
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↵ † To whom correspondence should be addressed. E-mail: vram{at}fiji.ucsd.edu.
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Author contributions: V.R., J.T.K., and W.M.W. designed research; V.R., C.C., D.K., W.M.W., and Q.F. performed research; V.R., C.C., D.K., Q.F., D.R.S., and M.W. analyzed data; V.R. wrote the paper; T.B. and L.B. conducted model simulation; and L.B. made code changes.
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This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 30, 2002.
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Abbreviations: ABC, atmospheric brown cloud; BC, black carbon; GCM, general circulation model; GHG, greenhouse gas; MSU, microwave sounding unit; NIO, Northern Indian Ocean; PCM, parallel climate model.
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See accompanying Biography on page 5323.
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Freely available online through the PNAS open access option.
- Copyright © 2005, The National Academy of Sciences
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