Estimating the roles of radiation and dynamics in the climate system's equilibrium response to enhanced greenhouse forcing

A simple two-zone model of the climate system, which has previously been used to study climate stability (Bates, Tellus, 51A, 349-372, 1999) is used to study the system's equilibrium response to enhanced greenhouse forcing. The positive lower tropospheric water vapour/infrared radiative feedback and the negative evaporative feedback associated with the atmospheric angular momentum cycle and the surface winds which it induces are included in the model. Also included is the poleward transport of heat by ocean currents and its estimated sensitivity to variations in the curl of the surface wind stress.

The model's global mean equilibrium surface temperature increase resulting from an enhanced greenhouse forcing of 4W/m2 (the estimated increase resulting from a doubling of CO2 in the absence of other change) is found to be 1.3 K. Unlike complex coupled models, which show a comparable change, the simple model allows the relative roles of radiation and dynamics in determining the temperature change to be clearly distinguished. The model results suggest that the dynamical factors are of great importance.