DOI: 10.1126/science.1160606 , 1309 (2008); 321Science et al.Daniel Rosenfeld,Precipitation?Flood or Drought: How Do Aerosols Affect www.sciencemag.org (this information is current as of September 5, 2008 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/321/5894/1309version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/321/5894/1309#otherarticles, 13 of which can be accessed for free: cites 51 articlesThis article http://www.sciencemag.org/cgi/collection/atmosAtmospheric Science : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. is aScience2008 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on September 5, 2008 www.sciencemag.orgDownloaded fromFlood or Drought: How Do AerosolsAffect Precipitation?Daniel Rosenfeld,1* Ulrike Lohmann,2Graciela B. Raga,3Colin D. O’Dowd,4Markku Kulmala,5Sandro Fuzzi,6Anni Reissell,5Meinrat O. Andreae7Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect oncloud properties and the initiation of precipitation. Large concentrations of human-madeaerosols have been reported to both decrease and increase rainfall as a result of their radiativeand CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rainout too quickly to mature into long-lived clouds. On the other hand, heavily polluted cloudsevaporate much of their water before precipitation can occur, if they can form at all given thereduced surface heating resulting from the aerosol haze layer. We propose a conceptual modelthat explains this apparent dichotomy.Cloud physicists commonly classify thecharacteristics of aerosols and cloudsinto “maritime” and “continental” regimes,where “continental” has become synonymouswith “aerosol-laden and polluted.” Indeed, aero-sol concentrations in polluted air masses aretypically one to two orders of magnitude greaterthan in pristine oceanic air (Fig. 1) (1). How-ever, before humankind started to change theenvironment, aerosol concentrations were notmuch greater (up to double) over land thanover the oceans (1, 2). Anthropogenic aerosolsalter Earth’s energy budget by scattering andabsorbing the solar radiation that energizes theformation of clouds (3–5). Because all clouddroplets must form on preexisting aerosol par-ticles that act as cloud condensation nuclei (CCN),increased aerosols also change the composi-tion of clouds (i.e., the size distribution of clouddroplets). This, in turn, determines to a large ex-tent the precipitation-forming processes.Precipitation plays a key role in the climatesystem. About 37% of the energy input to theatmosphere occurs by release of latent heatfrom vapor that condenses into cloud dropsand ice crystals (6). Reevaporation of cloudsconsumes back the released heat. When wateris precipitated to the surface, this heat is left inthe atmosphere and becomes available to ener-gize convection and larger-scale atmosphericcirculation systems.The dominance of anthropogenic aerosolsover much of the land area means that cloud com-position, precipitation, the hydrological cycle,and the atmospheric circulation systems are allaffected by both radiative and microphysical im-pactsofaerosols,andarelikelytobeinadiffer-ent state relative to the pre-industrial era.The Opposing Effects of Aerosolson Clouds and PrecipitationThe radiative effects of aerosols on clouds most-ly act to suppress precipitation, because they de-crease the amount of solar radiation that reachesthe land surface, and therefore cause less heat tobe available for evaporating water and energiz-ing convective rain clouds (7). The fraction ofradiation that is not reflected back to space bythe aerosols is absorbed into the atmosphere,mainly by carbonaceous aerosols, leading toheating of the air above the surface. This sta-bilizes the low atmosphere and suppresses thegeneration of convective clouds (5). The warmerand drier air thus produces circulation systemsthat redistribute the remaining precipitation (8, 9).For example, elevated dry convection was ob-served to develop from the top of heavy smokepalls from burning oil wells (10). Warming ofthe lower troposphere by absorbing aerosolscan also strengthen the Asian summer monsooncirculation and cause a local increase in precipi-tation, despite the global reduction of evaporationthat compensates for greater radiative heatingby aerosols (11). In the case of bright aerosolsthat mainly scatter the radiation back to space,the consequent surface cooling also can alteratmospheric circulation systems. It has beensuggested that this mechanism has cooled theNorth Atlantic and hence pushed the IntertropicalConvergence Zone southward, thereby contrib-uting to the drying in the Sahel (12, 13).Aerosols also have important microphysicaleffects (14). Added CCN slow the conversion ofcloud drops into raindrops by nucleating largernumber concentrations of smaller drops, whichare slower to coalesce into raindrops or rimeonto ice hydrometeors (15, 16). This effect wasshown to shut off precipitation from very shal-low and short-lived clouds, as in the case ofREVIEW1Institute of Earth Sciences, Hebrew University of Jerusa-lem, Jerusalem 91904, Israel.2Institute for Atmosphericand Climate Science, ETH Zürich, 8092 Zürich, Switzerland.3Universidad Nacional Autónoma de México, Mexico City04510, Mexico.4School of Physics and Centre for Climateand Air Pollution Studies, Environmental Change Institute,National University of Ireland, Galway, Ireland.5Depart-ment of Physics, University of Helsinki, Post Office Box 64,Helsinki 00014, Finland.6Istituto di Scienze dell’Atmosferae del Clima–CNR, Bologna 40129, Italy.7BiogeochemistryDepartment, Max Planck Institute for Chemistry, Post OfficeBox 3060, D-55020 Mainz, Germany.*To whom correspondence should be addressed. E-mail:[email protected] =