First observational estimates of global clear sky shortwave aerosol direct radiative effect over land

Using one year (2000-2001) of merged Multiangle Imaging SpectroRadiometer (MISR), Moderate Resolution Imaging SpectroRadiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) data sets from NASA's Terra satellite, we estimate the top of atmosphere cloud-free direct radiative effect (DRE) of aerosols over global land areas. The global mean shortwave DRE is -5.1 +/- 1.1 Wm(-2) although substantial regional variability in DRE over land exists due to differences in aerosol properties and land cover types. This value is consistent with those reported in the literature although this is the first observational estimate of the global DRE over land using satellite data alone. Future studies need to separate the anthropogenic component of aerosols from satellite data to examine aerosol climate forcing over global scales.