Chapter 07, Fast and Slow Feedbacks in Future Climates

Fast and Slow Feedbacks in Future Climates ePub (Adobe DRM)

by: Ann Henderson-Sellers

The term "climate sensitivity" refers to the ratio of the steady-state increase in the global and annual mean surface air temperature to the global and annual mean radiative forcing. It is standard practice to include only the fast feedback processes, including changes in water vapor, in the calculation of climate sensitivity, but to exclude possible induced changes in the concentrations of other GHGs. Changes in climate in response to changes in GHG concentrations and other driving factors can be computed using relatively simple climate models in which the climate sensitivity is prescribed and the radiative forcing is computed from the concentrations of individual GHGs using simple formulae based on the results of detailed calculations. They can also be computed using 3D atmospheric general circulation models (AGCMs) coupled to a slab that represents the surface layer (mixed layer) of the ocean only. Alternatively, they can be computed using coupled 3D atmospheric and oceanic general circulation models (AOGCMs). In simple models, one can simply add up the individual radiative forcings to get the total global mean radiative forcing, and then apply the climate sensitivity obtained for a CO2 doubling in simulating the global mean temperature response. AML models and AOGCMs, on the other hand, have the flexibility to respond in separate ways to different forcing mechanisms.