 Atmospheric reanalysis datasets are in heavy use in climate research, because they provide complete gridded data at regular time steps over long time periods, both at the surface and above, for a wide range of variables, including ones that are difficult to measure with instruments. Many reanalysis datasets cover a few decades of the twentieth century and are produced using as much of the available observational data as is feasible (see Dee et al. 2016 for an overview); when satellite data are available, this can amount to millions of observations for each analysis period in the sequence. An interesting exception is NOAA’s 20th Century Reanalysis (20CR) project: it covers the entire twentieth century and assimilates only surface pressure observations and observed monthly sea surface temperatures and sea ice distribution (Compo et al. 2011). 20CR is not informed by any temperature observations over land, yet its results indicate twentieth century global warming over land similar to that seen in station-based temperature datasets.