Indirect forcing by anthropogenic aerosols: A global climate model calculation of the effective-radius and cloud-lifetime effects

A global climate model (GCM) that includes a physically based cloud scheme is used to calculate the indirect radiative forcing due to the modification of liquid-water cloud properties by anthropogenic aerosols. The distribution of cloud-droplet number concentration Nd required by the cloud scheme is estimated empirically from monthly mean fields of sulfate mass generated by a chemical transport model. The effects of anthropogenic changes in Nd are considered in the calculation of precipitation (the "cloud-lifetime" effect) and of the droplet effective radius used in the shortwave and longwave radiation schemes (the "effective-radius" effect). The modeled cloud-droplet effective radii for present-day conditions agree quite well with satellite-retrieved values, although the land-ocean and hemispheric contrasts are weaker in the model than in the observations. The total indirect forcing is -2.1 W m(-2), including a small longwave forcing of +0.1 W m(-2). The forcing results from a 1% increase in cloudiness, a 6% increase in liquid water path, and a 7% decrease in droplet effective radius. The breakdown of the total indirect forcing into the effective-radius and cloud-lifetime effects is estimated by performing separate GCM experiments in which each effect is included individually. The estimated forcings due to the effective-radius and cloud-lifetime effects are -1.2 and -1.0 W m(-2), respectively. The calculated forcings show some sensitivity to the autoconversion threshold, the sulfate-Nd relation, and the vertical distribution of sulfate, but in each case the cloud-lifetime forcing is at least 25% of the total indirect forcing. These results suggest that the cloud-lifetime effect should not be ignored in future calculations of the indirect forcing due to anthropogenic aerosols.