19 Global Indirect Radiative Forcing Caused by Aerosols IPCC (2007) and Beyond Jim Haywood 1 , Leo Donner 2 , Andy Jones 1 , and Jean-Christophe Golaz 2 1 Met Office, Exeter, U.K. 2 Geophysical Fluid Dynamics Laboratory/NOAA, Princeton University, Princeton, NJ, U.S.A. Abstract Anthropogenic aerosols are thought to exert a significant indirect radiative forcing be- cause they act as cloud condensation nuclei in warm cloud-forming processes and ice nuclei in cold cloud-forming processes. Although many of the processes associated with the perturbation of cloud microphysics by anthropogenic aerosols were discussed, IPCC (2007) provided only an estimate of full quantification of the radiative forcing attributable to the first indirect effect (which they referred to as the cloud albedo ef- fect). Here we explain that this approach is necessary if one is to compare the radia- tive forcing from the indirect effect of aerosols with those from other radiative forc- ing components such as that from changes in well-mixed greenhouse gases. We also highlight the problems in assessing the effect of anthropogenic aerosols upon clouds under the strict definitions of radiative forcing provided by the IPCC (2007). Although results from global climate models, at their current state of development, suggest that an analysis of indirect aerosol effects in terms of forcing and feedback is possible, a key rationale for the IPCC’s definition of radiative forcing, a straightforward scaling between an agent’s forcing and the temperature change it induces, is significantly com- promised. Feedbacks from other radiative forcings are responses to radiative perturba- tions, whereas feedbacks from indirect aerosol effects are responses to both radiative and cloud microphysical perturbations. This inherent difference in forcing mechanism breaks down the consistency between forcing and temperature response. It is likely that additional characterization, such as climate efficacy, will be required when comparing indirect aerosol effects with other radiative forcings. We suggest using the radiative flux perturbation associated with a change from preindustrial to present-day composition, From the Strüngmann Forum Report, Clouds in the Perturbed Climate System: Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation Edited by Jost Heintzenberg and Robert J. Charlson. 2009. © MIT Press ISBN 978-0-262-01287-4