We solve the radiative transfer equation for an atmosphere in radiative-convective equilibrium to derive an analytic solution of the equilibrium surface temperature. In our formulation, the equilibrium surface temperature is described in terms of the longwave optical thickness and the ratio of the optical-thickness scale height to the temperature scale height as well as the planetary albedo and solar constant.

Using this solution, climate sensitivity and its dependence on feedback processes are formulated. In the absence of feedback processes, the climate sensitivity parameter, typically=0.27Km2/W for the present Earth's climate, is found quite insensitive to the longwave optical thickness. Feedbacks with respect to the optical thickness, optical-thickness scale height, temperature lapse rate, and planetary albedo are compared with each other and their contributions to climate sensitivity are discussed.

For the water-vapor feedback, we found our estimates of climate sensitivity consistent both with past GCM studies and with observational studies. We also found a reasonable consistency among our result and past GCM studies for the ice/snow-albedo feedback. Although our formulation is simplistic, it will be useful to interpret observations and GCM results to clarify underlying physical mechanisms governing climate sensitivity.