Reducing the uncertainty in estimated global direct aerosol radiative forcing by observations

Aerosols directly affect the radiative balance of the Earth through absorption and scattering of solar radiation. On the whole, aerosols are estimated to cool the climate, although the contributions of absorption (heating) and scattering (cooling) of sunlight have proved difficult to quantify. Recent estimates of global direct aerosol radiative forcing range from -0.6 W m-2 to 0.0 W m-2. The range of these estimates arises in part from challenges in determining the anthropogenic fraction of aerosols and in accounting for the effect of underlying cloud coverage on aerosol radiative forcing. Here, we integrate a comprehensive suite of satellite and ground-based observations to constrain total aerosol amount, its anthropogenic fraction, the vertical distribution of aerosol and cloud, and the co-location of cloud and overlying aerosols. We find that biomass burning and industrial aerosols tend to be less abundant above cloud than at the same heights in clear skies. Direct aerosol radiative forcing since pre-industrial time is estimated to be in the range of -0.3 to -0.1 W m-2. Our study demonstrates that extensive use of observations can significantly reduce the uncertainty in estimated direct aerosol forcing.