Climate sensitivity and climate state

Abstract

The effective climate feedback/sensitivity, including its components, is a robust first order feature of the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled global climate model (GCM) and presumably of the climate system. Feedback/sensitivity characterizes the surface air temperature response to changes in radiative forcing and is constant, to first order, independent of the nature, history, and magnitude of the forcing and of the changing climate state. This "constancy" can only be approximate, however, and modest second order changes of 10–20% are found in stabilization simulations in which the forcing, based on the IS92a scenario, is fixed (stabilized) at year 2050 and 2100 values and the system is integrated for an additional 1000 years toward a new equilibrium. Both positive and negative feedback mechanisms tend to strengthen, with the balance tilted toward stronger negative feedback and hence weaker climate sensitivity, as the system evolves and warms. Some feedback mechanisms weaken locally, however, and an example of such is the ice/snow albedo feedback which is less effective in areas of the Northern Hemisphere where ice/snow has retreated. Changes in the geographical distribution of the feedbacks are modest and weakening feedback in one region is often counteracted by strengthening feedback in other regions so that global and zonal values do not reflect the dominance of a particular mechanism or region but rather the residual of changes in different components and regions. The overall 10–20% strengthening of the negative feedback (decrease in climate sensitivity) in the CCCma model contrasts with a weakening of negative feedback (increase in climate sensitivity) of over 20% in the Hadley Centre model under similar conditions. The different behaviour in the two models is due primarily to solar cloud feedback with a strengthening of the negative solar cloud feedback in the CCCma model contrasting with a weakening of it in the Hadley Centre model. The importance of processes which determine cloud properties and distribution is again manifest both in determining first order climate feedback/sensitivity and also in determining its second order variation with climate state.