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 (Nc) with concentrations of aerosol soluble ions is developed as a basis for the calculation. Explicit dependence on Nc 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 Nc 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 Nc 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 Nc 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.