We developed a new ensemble-based data-assimilation system based on a global aerosol climate model and performed a 1-month assimilation experiment using satellite optical measurements from MODIS onboard TERRA and AQUA to estimate the direct radiative effect (DRE) of aerosols. Using the assimilated data field, monthly averaged optical thickness (AOT) was estimated to be 0.15 ± 0.030 (a 52.0% increase over a priori), and the root mean-square difference (RMSD) between modeled values and MODIS measurements was reduced by 28.4%. Independent validation using globally distributed AERONET measurements showed that the a posteriori data achieved better agreement with 82.5% of 80 AERONET sites. However, improvements in Ångström exponents were limited (50.0% of sites). Using the assimilated aerosol field, we modeled the aerosol DRE. A posteriori whole- and clear-sky DREs at the top of the atmosphere were estimated to be −1.1 ± 0.35 and −2.5 ± 0.49 W/m2, respectively, in May 2007 and were close to previously reported measurement-based estimates.