Linear additivity of climate response for combined albedo and greenhouse perturbations

Using an atmospheric general circulation model with fixed cloud amounts and microphysical properties, and coupled to a mixed-layer static ocean, we perform idealized experiments to inquire into the linear characteristics of the modeled climate system's mean response to simultaneous greenhouse and Northern Hemisphere midlatitude albedo perturbations, two forcings deemed to be important during the present times. The two forcings are chosen to be equal and opposite in the global, annual-mean such that a linear behavior would be expected to lead to a complete offset of the global, annual-mean surface temperature change, which is indeed obtained. The monthly and annual zonal-mean surface temperature, and the annual zonal-mean precipitation responses to the combined forcings, also are reasonably similar to the sum of the responses to the individual forcings. The albedo forcing case casts a distinct signature on the circulation and precipitation changes in the northern and southern equatorial regions, which is absent for the greenhouse forcing case. The combined simulation yields a result similar to that for the albedo forcing case, one that is consistent with linear additive expectations.