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Science 12 May 2000:
Vol. 288. no. 5468, pp. 1042 - 1047
DOI: 10.1126/science.288.5468.1042
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Reports

Reduction of Tropical Cloudiness by Soot

A. S. Ackerman, 1* O. B. Toon, 2 D. E. Stevens, 3 A. J. Heymsfield, 4 V. Ramanathan, 5 E. J. Welton 6

Measurements and models show that enhanced aerosol concentrations can augment cloud albedo not only by increasing total droplet cross-sectional area, but also by reducing precipitation and thereby increasing cloud water content and cloud coverage. Aerosol pollution is expected to exert a net cooling influence on the global climate through these conventional mechanisms. Here, we demonstrate an opposite mechanism through which aerosols can reduce cloud cover and thus significantly offset aerosol-induced radiative cooling at the top of the atmosphere on a regional scale. In model simulations, the daytime clearing of trade cumulus is hastened and intensified by solar heating in dark haze (as found over much of the northern Indian Ocean during the northeast monsoon).

1 NASA Ames Research Center, Moffett Field, CA 94035, USA.
2 University of Colorado, Boulder, CO 80309, USA.
3 Lawrence Livermore National Laboratory, Livermore, CA 94551, USA.
4 National Center for Atmospheric Research, Boulder, CO 80301, USA.
5 Scripps Institution of Oceanography, La Jolla, CA 92093, USA.
6 Science Systems and Applications, Greenbelt, MD 20771, USA.
*   To whom correspondence should be addressed. E-mail: ack{at}sky.arc.nasa.gov

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