The regional equilibrium response to a doubling of the atmospheric concentration of CO2

In this chapter we examine the characteristics and geographical patterns of the climatic changes that are simulated by 3-D AGCMs to result from a doubling of atmospheric CO₂. These climatic changes are computed by first running a model with the present (or near-present) concentration of CO₂ until it has settled into a statistically constant climate, then suddenly increasing the concentration of CO₂ and allowing the simulated climate to reach a new, statistically constant state. In the vast majority of cases, a doubling of the atmospheric CO₂ concentration is imposed. The difference between the two climates is then determined. In some cases a second simulation, without increasing the CO₂ concentration, is continued from the exact point where the CO₂ increase was applied. This simulation is referred to as the "control" simulation, and this is what is compared with the perturbed simulation. The resultant climatic change is often referred to as the "equilibrium" climatic change, although the term "steady state" climate change is preferable and will be used here interchangeably with the term "equilibrium" climatic change. In order to achieve a new equilibrium climate after only a few decades of simulated time, only the mixed layer of the ocean is included. If the AGCM were coupled to an OGCM, representing the full depth of the ocean, then about 1000 years of simulated time would be required before the new equilibrium would be achieved. This in turn would require a prohibitively large amount of computer time. Owing to the absence of the deep ocean in a mixed layer-only simulation, the horizontal heat transport by the ocean for the present climate has to be specified and then held constant as the climate changes.