Satellite estimate of global aerosol indirect forcing by marine warm clouds

The effect of anthropogenic atmospheric aerosols on global cloud radiative forcing is termed the aerosol indirect effect (AIE). The Intergovernmental Panel on Climate Change (IPCC) estimated the AIE to be the single largest uncertainty in global radiative forcing. The AIE comprises two components: the intrinsic effect of aerosol variations on cloud albedo and the extrinsic effect of aerosol variations on fractional cloud cover. Of the world's clouds, low-level marine clouds exert the largest impact on the planet's albedo. We perform a comprehensive analysis of multiple A-Train satellite measurements of co-located aerosols and clouds over the world's oceans for the period August 2006 to December 2009, comprising over 3.7 million single-layer marine warm cloud pixels. AIE is dictated by changes in cloud liquid water path as aerosol loading increases, strongly tied to precipitation and meteorological constraints. Using the local anthropogenic aerosol fraction estimated from GEMS (Global and regional Earth-system Monitoring using Satellite and in-situ data), we estimate the intrinsic component of global oceanic warm cloud AIE as -0.56 ± 0.17 W m^(-2). The corresponding estimate for the extrinsic AIE is -1.46 ± 0.37 W m^(-2), although this estimate is subject to some uncertainty and may be overestimated. These results suggest that present-day aerosol indirect forcing is substantial, implying that, with aerosol levels expected to remain roughly at present levels, future warming will accelerate as greenhouse gas levels continue to rise.