The ECMWF's first atmospheric reanalysis of the 20th Century is known as ERA-20C and it provides a long, gridded dataset of many climate variables, spanning 1900-2010. Observations assimilated include surface pressures from the International Surface Pressure Databank v3.2.6 and ICOADS v 2.5.1, and surface winds over the oceans from ICOADSv2.5.1. Upper-air and satellite data are omitted.
For details of ERA-20C are available at http://www.ecmwf.int/en/research/climate-reanalysis/era-20c
ECMWF also released an "AMIP style" (Atmospheric Model Intercomparison Project) version of the 20th Century reanalysis, known as ERA-20CM, which includes no data assimilation. It's an ensemble of ten atmospheric model integrations for the years 1899–2010.
For more information on ERA-20CM please see Hersbach et al. (2015)
ERA-20C and ERA-20CM are forced with the same time-varying SSTs, sea ice concentrations from HadISST22.214.171.124, and radiative forcings from CMIP5.
For a comparison of ECMWF's ERA-20C with NOAA's 20CR see https://climatedataguide.ucar.edu/climate-data/era-20c-ecmwfs-atmospheric-reanalysis-20th-century-and-comparisons-noaas-20cr
Note that ERA-20CMv0 is an experimental version and public access to this data will be disabled in the future. Existing users of ERA-20CMv0 should migrate to the final version as soon as possible.
ERA-20C, ERA-20CM and ERA-20CMv0 may be downloaded from the ECMWF Public Data Server .