A physically based estimate of radiative forcing by anthropogenic sulfate aerosol

Estimates of direct and indirect radiative forcing by anthropogenic sulfate aerosols from an integrated global aerosol and climate modeling system are presented. A detailed global tropospheric chemistry and aerosol model that predicts concentrations of oxidants as well as aerosols and aerosol precur sors, is coupled to a general circulation model that predicts both cloud wa ter mass and cloud droplet number. Both number and mass of several external ly mixed aerosol size modes are predicted, with internal mixing assumed for the different aerosol components within each mode. Predicted aerosol speci es include sulfate, organic and black carbon, soil dust, and sea salt. The models use physically based treatments of aerosol radiative properties (inc luding dependence on relative humidity) and aerosol activation as cloud con densation nuclei. Parallel simulations with and without anthropogenic sulfa te aerosol are performed for a global domain. The global and annual mean di rect and indirect radiative forcing due to anthropogenic sulfate are estima ted to be -0.3 to -0.5 and -1.5 to -3.0 W m(2), respectively. The radiative forcing is sensitive to the model's horizontal resolution, the use of pred icted versus analyzed relative humidity, the prediction versus diagnosis of aerosol number and droplet number, and the parameterization of droplet col lision/coalescence. About half of the indirect radiative forcing is due to changes in droplet radius and half to increased cloud liquid water.