The 20th Century Reanalysis (Version 2) Dataset was produced by the National Oceanic and Atmospheric Administration (NOAA) and University of Colorado's Cooperative Institute for Research in Environmental Sciences (CIRES), members of the international Twentieth Century Reanalysis (20CR) project.

This dataset collection contains twentieth century global climate reanalysis data, including output from each 56 members of the ensemble runs, their means, spreads and input observational data. The reanalysis was performed in various runs, each producing a 5 year period of output data.

The data were produced through international cooperation under the auspices of the international Atmospheric Circulation Reconstructions over the Earth (ACRE) initiative, and working groups of the Global Climate Observing System (GCOS) and the World Climate Research Program (WCRP). The 20CR project produced comprehensive global atmospheric circulation data spanning from late 19th century to the early 21st century, having only assimilated surface synoptic pressure observations and using observed monthly sea-surface temperature and sea-ice distributions from the HadISST dataset, courtesy of the Met Office Hadley Centre, as boundary conditions. Analyses every 6 hours on a 2 degree grid were produced to give the most likely state of the atmosphere based on a 56 ensemble member runs. Means, spreads and all ensemble members for each time step are available in the dataset archived at the BADC. The current 20CR output produced spans from 1871 to present with the BADC copy being updated when possible.