The Moderate-Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Earth Observing System (EOS) satellites provides an unprecedented opportunity to study aerosols from space with high accuracy and on a nearly global scale. However, difficulty with highly reflective arid and snow-covered lands introduces significant gaps in global or regional coverage that must be filled by some other means. This study provides a complete global coverage of an annual cycle of aerosol optical depth by combining the MODIS retrievals and Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) simulations weighted with the uncertainties in each product. The assimilated aerosol optical depths over land are better correlated with the ground-based Aerosol Robotic Network (AERONET) measurements than are either the MODIS retrievals or the GOCART simulations alone. The gaps in the MODIS retrievals are filled with values that are generally consistent with the AERONET aerosol climatology. The assimilated aerosol optical depths are in good agreement with the Advanced Very High Resolution Radiometer (AVHRR) aerosol climatology over the Atlantic and North Indian Oceans. In spring, large discrepancies between the MODIS retrievals in 2001 and the AVHRR climatology over the North Pacific are likely a result of extremely active transcontinental transport of Asian dust/pollutants to North America in the year 2001. Large model--satellite differences in the South Pacific and South Indian Oceans may be attributable to missing or underestimated sources in the model and/or cloud, whitecap, and glint contamination in satellite retrievals.