Development of the GEOS data assimilation system and the MERRA-2 project were funded by NASA’s Modeling, Analysis and Prediction program. Computational resources and support for the execution of MERRA-2 were provided by the NASA High-End Computing Capability Project and NASA Center for Climate Simulation. GEOS and MERRA-2 are the result of years of dedicated research, development, and analysis by many individuals at GMAO, whose efforts are greatly appreciated. We gratefully acknowledge the GMAO operations group for monitoring the production of MERRA-2 and the GMAO software integration group, who helped improve the performance and flexibility of GEOS. We thank Julio Bacmeister for his contribution to the development of the model physics, Qing Liu for processing the precipitation input data, and Allison Collow and Edmond Brent Smith for providing some of the figures for this paper. We also thank the GES DISC for providing online access to MERRA-2 products. Finally, we thank the three reviewers, whose comments and suggestions helped improve the paper substantially.