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 N d 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 N d 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‐ N d 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.