Global direct aerosol radiative forcing, as constrained by comprehensive observations

Abstract

Aerosols directly affect the radiative balance of the Earth through absorption and scattering of solar radiation. Although the contributions of absorption (heating) and scattering (cooling) of sunlight have proved difficult to quantify, the consensus is that anthropogenic aerosols cool the climate, partially offsetting the warming by rising greenhouse gas concentrations. Recent estimates of global direct aerosol radiative forcing are −0.35 ± 0.5 Wm−2, and these estimates depend either entirely or heavily on aerosol simulation. Here, we integrate a comprehensive suite of satellite and ground-based observations to constrain total AOD, its fine-mode fraction, the vertical distribution of aerosols and clouds, and the co-location of clouds and overlying aerosols. We find that fine-mode forcing is −0.46 Wm−2 (−0.54 ~ −0.39 Wm−2). Fine-mode aerosols include sea salt and dust aerosols, and we find that these natural aerosols pose a very large cooling (−0.44 ~ −0.26 Wm−2) when constrained by observations. When the contribution of these natural aerosols is subtracted from the fine-mode forcing, the net becomes −0.10 (−0.28 ~ +0.05) Wm−2. The net forcing arises from carbonaceous, sulfate and nitrate aerosols. Despite uncertainties in the anthropogenic fraction of these aerosols, this −0.28 ~ +0.05 Wm−2 range compels the direct aerosol forcing to be near zero.