PNNL

Abstract

We evaluate the forcing and feedbacks to net shortwave radiation at the surface in the G4 experiment of the Geoengi- neering Model Intercomparison Project by analyzing outputs from six participating models. The G4 is a numerical experiment of solar radiation management (SRM) by injection of SO2, a sulfate aerosol precursor, into the lower stratosphere by 5 Tg/yr from 2020 to 2070 with adopting the representative concentration pathway of 4.5 W/m2 by 2100 (RCP4.5) as the baseline scenario. A single layer atmospheric model for shortwave radiative transfer is used to estimate the direct forcing of SRM and feedback effects from changes in water-vapor amount, cloud amount, and surface albedo (compared with RCP4.5) from the following model outputs: upwelling and downwelling shortwave radiation at the surface and at the top of the atmosphere for all-sky and clear-sky. Our analysis shows that the globally and temporally averaged SRM forcing ranges from -3.6 to -1.6 W/m2 depending on the models. The feedbacks from both water-vapor change and cloud amount change have heating effects ranging from about 0.4 to 1.5 W/m2 and weaken the effect of SRM by about half. The surface albedo changes have a cooling effect, which is locally strong (? 4 W/m2) in snow and sea ice melting region, but, in global average, it is minor.