Conclusions and Outlook

Clouds play a crucial role in determining Earth's energy balance. This must be accurately represented in climate models, if the models are to be used with confidence to project future climate change. Much of the variation in climate sensitivity observed in current climate models can be attributed to differences in the treatment of clouds, as evidenced by model-to-model variation in cloud feedback. Microphysical processes are now recognized to exert strong influences on cloud dynamics and that the influences of clouds on short- and longwave radiation must be understood and accurately represented in climate models. In addition, the strong coupling between aerosols and clouds has large influences on climate and climate change. An increased atmospheric aerosol burden alters the microphysical properties of clouds and influences short- and longwave radiation and the locus and intensity of precipitation.

One of the better understood influences of aerosols on clouds is a reduction of the amount of solar radiation absorbed by Earth-atmosphere system, as quantifi ed by net shortwave radiation at the TOA and a similar decrease in shortwave radiation reaching the surface. The negative radiative forcing of anthropogenic aerosols competes with greenhouse gas warming as a forcing of climate change and in altering evaporation and precipitation. Although much has been learned about these effects, they are not understood be well enough to be fully represented in climate models. None of the transient climate model simulations conducted thus far accounts for all of the known aerosol-cloud interactions; thus the net effects of aerosols on clouds and climate deduced from global climate models cannot be considered conclusive. Therefore, the cloud feedback and sensitivity of Earth's climate system remain highly uncertain. One reason is that aerosol-cloud interactions take place on microscale and thus are at best crudely represented in GCMs.

In terms of aerosols and clouds, the principal areas for future development in global climate models are twofold. The treatment of clouds themselves requires improvement in all aspects, if models are to represent accurately cloud feedbacks in a greenhouse-warmed world. A good representation of cloud dynamics, including entrainment, is especially important for the representation of convective clouds and boundary layer clouds. Cloud microphysical processes are important for the conversion of cloud particles into precipitation-size particles. The treatment of aerosol-cloud interactions needs to be improved as well, if aerosol radiative forcing is to be accurately quantified.

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