Global anthropogenic aerosol direct forcing derived from satellite and ground‐based observations

Abstract

A global estimate of the direct effects of anthropogenic aerosols on solar radiation in cloudy skies is obtained by integrating satellite and ground‐based observations with models of aerosol chemistry, transport, and radiative transfer. The models adopt global distribution of aerosol optical depths (from MODIS), clouds, water vapor, ozone, and surface albedo from various satellite climatology. Gaps and errors in satellite derived aerosol optical depths are filled and corrected by surface network (AERONET), and an aerosol chemical‐transport model (GOCART) by using statistical techniques. Using these derived aerosol properties and other related variables, we generate climatological monthly mean anthropogenic aerosol forcing for both clear and average cloudy skies. Unless otherwise stated, our estimates are for average cloudy skies, also referred to as all sky conditions. The global annual mean direct forcing is −0.35 Wm−2 (range of −0.6 to −0.1 Wm−2) at the top‐of‐the atmosphere (TOA), +3.0 Wm−2 (range of +2.7 to +3.3Wm−2) in the atmosphere, and −3.4 Wm−2 (range of −3.5 to −3.3 Wm−2) at the surface. The uncertainty of about 10–20% in the surface and atmosphere forcing translates into a six fold uncertainty in the TOA forcing because the TOA forcing is a small sum of two large terms (surface and atmosphere) of opposing signs. Given the current state of observations and modeling, it is very difficult to further reduce the uncertainty in the estimated TOA forcing. The major contributors to the uncertainty in atmospheric absorption are from the uncertainty in the vertical distribution of aerosols and the single scattering albedo of aerosols. The TOA forcing in clear skies is a factor of two different, while the surface and atmosphere forcing terms differ by only about 10–25%. Another major finding of this study is that the reduction in the surface solar radiation is a factor of 10 larger than the reduction in net solar (down minus up) radiation at TOA. The TOA forcing changes sign regionally, whereas the surface forcing is always negative. Thus caution must be exercised against relying too strongly on assessing the aerosol impacts based solely on global mean forcing. Aerosols over the NH contribute about 64% to the global surface forcing. Regionally the populated tropical regions contribute the most to the global surface forcing, with Asia the largest contributor. Roughly 49% of the total surface forcing is over the oceanic regions. Most of the previous global aerosol forcing estimate studies were conducted with a chemical transport model coupled to a general circulation model with model generated aerosols and cloudiness. Thus the present study, which adopts observed aerosol properties and observed three dimensional cloudiness, provides an independent approach for estimating the aerosol forcing.