Indirect aerosol forcing, quasi forcing, and climate response

The component of the indirect aerosol effect related to changes in precipit ation efficiency (the second indirect or Albrecht effect) is presently eval uated in climate models by taking the difference in net irradiance between a present-day and a preindustrial simulation using fixed sea surface temper atures (SSTs). This approach gives a "quasi forcing,'' which differs from a pure forcing in that fields other than the initially perturbed quantity ha ve been allowed to vary. It is routinely used because, in contrast to the f irst indirect (Twomey) effect, there is no straightforward method of calcul ating a pure forcing for the second indirect effect. This raises the questi on of whether evaluation of the second indirect effect in this manner is ad equate as an indication of the likely effect of this perturbation on the gl obal-mean surface temperature. An atmospheric global climate model (AGCM) is used to compare the evaluatio n of different radiative perturbations as both pure forcings (when availabl e) and quasi forcings. Direct and indirect sulfate aerosol effects and a do ubling of carbon dioxide (CO2) are considered. For evaluation of the forcin gs and quasi forcings, the AGCM is run with prescribed SSTs. For evaluation of the equilibrium response to each perturbation, the AGCM is coupled to a mixed layer ocean model. For the global-mean direct and first indirect effects, quasi forcings diffe r by less than 10% from the corresponding pure forcing. This suggests that any feedbacks contaminating these quasi forcings are small in the global me an. Further, the quasi forcings for the first and second indirect effects a re almost identical when based on net irradiance or on cloud-radiative forc ing, showing that clear-sky feedbacks are negligible in the global mean. Th e climate sensitivity parameters obtained for the first and second indirect effects (evaluated as quasi forcings) are almost identical, at 0.78 and 0. 79 K m(2) W-1, respectively. Climate sensitivity parameters based on pure f orcings are 0.69, 0.84, and 1.01 K m(2) W-1 for direct sulfate, first indir ect, and 2 x CO2 f forcings, respectively. The differences are related to t he efficiency with which each forcing excites the strong surface-albedo fee dback at high latitudes. Closer examination of the calculations of the firs t indirect effect as a forcing and quasi forcing shows that, although they are in reasonable agreement in the global mean, there are some significant differences in a few regions. Overall, these results suggest that evaluatio n of the globally averaged second indirect effect as a quasi forcing is sat isfactory.