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Science 5 September 2008:
Vol. 321. no. 5894, pp. 1309 - 1313
DOI: 10.1126/science.1160606
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Review

Flood or Drought: How Do Aerosols Affect Precipitation?

Daniel Rosenfeld,1* Ulrike Lohmann,2 Graciela B. Raga,3 Colin D. O'Dowd,4 Markku Kulmala,5 Sandro Fuzzi,6 Anni Reissell,5 Meinrat O. Andreae7

Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.

1 Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel.
2 Institute for Atmospheric and Climate Science, ETH Zürich, 8092 Zürich, Switzerland.
3 Universidad Nacional Autónoma de México, Mexico City 04510, Mexico.
4 School of Physics and Centre for Climate and Air Pollution Studies, Environmental Change Institute, National University of Ireland, Galway, Ireland.
5 Department of Physics, University of Helsinki, Post Office Box 64, Helsinki 00014, Finland.
6 Istituto di Scienze dell'Atmosfera e del Clima–CNR, Bologna 40129, Italy.
7 Biogeochemistry Department, Max Planck Institute for Chemistry, Post Office Box 3060, D-55020 Mainz, Germany.

* To whom correspondence should be addressed. E-mail: daniel.rosenfeld{at}huji.ac.il

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