Global climate response to anthropogenic aerosol indirect effects: Present day and year 2100

Aerosol indirect effects (AIE) are a principal source of uncertainty in future climate predictions. The present study investigates the equilibrium response of the climate system to present-day and future AIE using the general circulation model (GCM), Goddard Institute for Space Studies (GISS) III. A diagnostic formulation correlating cloud droplet number concentration (N_c) with concentrations of aerosol soluble ions is developed as a basis for the calculation. Explicit dependence on N_c is introduced in the treatments of liquid-phase stratiform clouds in GISS III. The model is able to reproduce the general patterns of present-day cloud frequency, droplet size, and radiative balance observed by CloudSat, Moderate Resolution Imaging Spectroradiometer, and Earth Radiation Budget Experiment. For perturbations of N_c from preindustrial to present day, a net AIE forcing of -1.67 W m^-2 is estimated, with a global mean surface cooling of 1.12 K, precipitation reduction of 3.36%, a southward shift of the Intertropical Convergence Zone, and a hydrological sensitivity of +3.00% K^-1. For estimated perturbations of N_c from present day to year 2100, a net AIE forcing of -0.58 W m^-2, a surface cooling of 0.47 K, and a decrease in precipitation of 1.7% are predicted. Sensitivity calculations show that the assumption of a background minimum N_c value has more significant effects on AIE forcing in the future than on that in present day. When AIE-related processes are included in the GCM, a decrease in stratiform precipitation is predicted over future greenhouse gas (GHG)-induced warming scenario, as opposed to the predicted increase when only GHG and aerosol direct effects are considered.