Cloud optical properties, in particular the optical thickness [tau] affect the earth-atmosphere radiation budget, and their potential changes associated with climate changes may induce feedback effect. A one-dimensional radiative-convective model was used to illustrate that the difference in the vertical distribution of the radiative forcing between CO[sub 2] increase and changes of solar constant can result in a different [tau] feedback. A general circulation model, done by Wang et al., indicates that these trace gases provide an important radiative energy source for the present climate. Because the radiative-forcing behavior of CO[sub 2] is different from that of these other gases, the simulations also show that different radiative forcing can lead to quite different climatic effects. A one-dimensional model is used to investigate the [tau] feedback associated with trace gases. Because of the different changes in the [tau] vertical distributions, the [tau] feedback is calculated to be a small negative value for a CO[sub 2] increase, larger negative values for increases of trace gases, and the strongest negative feedback for CFCS. Similar experiments were also conducted using a revised version of the Somerville and Remer [tau] scheme. The results indicate that the negative feedback for CO[sub 2] increases for a singlemore » cloud layer becomes much smaller when multiple-layer clouds are used. Because this scheme assumes a strong functional dependence of the local temperature, the [tau] feedback is also found to be sensitive to model dimensionality. In addition, the strength of the [tau] feedback calculated from both schemes depend on the vertical distribution of cloud cover for the control climate, indicating the complexity of cloud-radiation interactions. Clearly, more observational and theoretical studies are needed to understand the cloud microphysics and their relation to large-scale climate variables.« less