Effect of the temperature dependence of gas absorption in climate feedback

[1] In the context of climate feedback associated with temperature change, there exist two potential mechanisms that affect the outgoing longwave radiation (OLR) and the downward longwave radiation (DLR). One is the ‘‘Planck’’ effect that determines the blackbody thermal emission at a considered temperature. The other is the ‘‘absorptivity’’ effect, in which a temperature change causes a change in gas absorptivities and thus influences the longwave radiative transfer. By using the line-by-line computed radiative Jacobians, which quantify the sensitivity of the radiative fluxes to a perturbation in the atmospheric temperature, the absorptivity effect is separated from the Planck effect. The absorptivity effect is further partitioned into components, with each one having a distinct physical meaning. It is demonstrated that the absorptivity-induced changes in the longwave radiation are individually significant even though the net effect is largely one of cancellation. As a consequence, the Planck effect dominates the overall OLR and DLR sensitivities to temperature change. The absorptivity effect tends to counteract the Planck effect. This tendency is particularly significant for the DLR and is more prominent for a warmer climate, with the result being a reduction in the surface warming.