Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 6 2004 10.5194/acpd-4-7561-2004 http://www.atmos-chem-phys-discuss.net/4/7561/2004/ http://www.atmos-chem-phys-discuss.net/4/7561/2004/acpd-4-7561-2004.html http://www.atmos-chem-phys-discuss.net/4/7561/2004/acpd-4-7561-2004.pdf 7561 7614 2004-11-17 Global indirect aerosol effects: a review U. Lohmann J. Feichter ETH Institute of Atmospheric and Climate Science, Schafmattstr. 30, CH-8093 Zurich, Switzerland Max Planck Institute for Meteorology, Bundesstr. 53, D-20146 Hamburg, Germany Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei, they may inhibit freezing and they could have an influence on the hydrological cycle. While the cloud albedo enhancement (Twomey effect) of warm clouds received most attention so far and traditionally is the only indirect aerosol forcing considered in transient climate simulations, here we discuss the multitude of effects. Different approaches how the climatic implications of these aerosol effects can be estimated globally as well as improvements that are needed in global climate models in order to better represent indirect aerosol effects are discussed in this paper.