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In most global climate models, the multiple scattering of longwave radiation by particles like clouds has been neglected. As the single scattering albedoes of both ice and water clouds are between 0.4 to 0.7, the mutipple scattering of longwave radiation by cloud droplets increases the effective absorption path length. It enhances the absorption within the cloud layer as well as the lower atmosphere and thus have a warming effect to the climate system. The warming effect caused by multipple scattering exists for thick clouds as well as thin clouds. We have developed a radiative-convective model for the purpose of estimating the model sensitivity to the longwave scattering effect. Using this model, we find that, for cirrus clouds with optical depth larger than 0.5, the multipple scattering warming effect is bigger than 2 degrees.
