Global indirect radiative forcing caused by aerosols: IPCC (2007) and beyond, in: Clouds in the perturbed climate system: Their relationship to energy balance, atmospheric dynamics, and precipitation, edited by
Anthropogenic aerosols are thought to exert a signifi cant 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 b...
| Main Authors: | , , , |
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| Format: | Text |
| Language: | English |
| Published: |
MIT Press
2009
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.540.6608 http://www.gfdl.noaa.gov/cms-filesystem-action/user_files/cjg/pubs/haywood_et_al_2009.pdf |
| Summary: | Anthropogenic aerosols are thought to exert a signifi cant 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 quantifi cation of the radiative forcing attributable to the fi rst 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 defi nitions 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 defi nition of radiative forcing, a straightforward scaling between an agent’s forcing and the temperature change it induces, is signifi cantly 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 effi cacy, will be required when comparing indirect aerosol effects with other radiative forcings. We suggest using the radiative fl ux perturbation associated with a change from preindustrial to present-day composition, 452 J. Haywood et al. calculated in a global climate model using fi xed sea surface temperature and sea ice, as a supplement to IPCC ... |
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