2008: Equilibrium climate sensitivity: Is it accurate to use a slab ocean model

The equilibrium climate sensitivity of a climate model is usually defined as the globally averaged equi-librium surface temperature response to a doubling of carbon dioxide. This is virtually always estimated in a version with a slab model for the upper ocean. The question is whether this estimate is accurate for the full climate model version, which includes a full-depth ocean component. This question has been answered for the low-resolution version of the Community Climate SystemModel, version 3 (CCSM3). The answer is that the equilibrium climate sensitivity using the full-depth ocean model is 0.148C higher than that using the slab ocean model, which is a small increase. In addition, these sensitivity estimates have a standard deviation of nearly 0.18C because of interannual variability. These results indicate that the standard practice of using a slab oceanmodel does give a good estimate of the equilibrium climate sensitivity of the full CCSM3. Another question addressed is whether the effective climate sensitivity is an accurate estimate of the equilibrium climate sensitivity. Again the answer is yes, provided that at least 150 yr of data from the doubled carbon dioxide run are used. 1