Direct Radiative Forcing of Anthropogenic Aerosols over Oceans from Satellite Observations

Anthropogenic aerosols play an important role in the atmospheric energybalance. Anthropogenic aerosol optical depth (AOD) and its accompanyingshortwave radiative forcing (RF) are usually simulated by numerical models.Recently, with the development of space-borne instruments and sophisticatedretrieval algorithms, it has become possible to estimate aerosol radiativeforcing based on satellite observations. In this study, we have estimatedshortwave direct radiative forcing due to anthropogenic aerosols over oceansin all-sky conditions by combining clouds and the Single Scanner Footprintdata of the Clouds and Earth's Radiant Energy System (CERES/SSF) experiment,which provide measurements of upward shortwave fluxes at the top ofatmosphere, with Moderate Resolution Imaging Spectroradiometer (MODIS)aerosol and cloud products.We found that globally averaged aerosol radiative forcing over oceans in theclear-sky conditions and all-sky conditions were -1.03±0.48 W m^-2 and -0.34±0.16 W m^-2, respectively. Direct radiative forcing byanthropogenic aerosols shows large regional and seasonal variations. In someregions and in particular seasons, the magnitude of direct forcing byanthropogenic aerosols can be comparable to the forcing of greenhouse gases.However, it shows that aerosols caused the cooling effect, rather thanwarming effect from global scale, which is different from greenhouse gases.