In their January 2018 news release the NSIDC announced that using a compilation of maps, ship reports, and other records, NOAA has published monthly estimates of Arctic sea ice extent spanning 1850 to 2013.
The historical observations come in many forms: ship observations, compilations by naval oceanographers, analyses by national ice services, and others.
Details can be had here:
https://nsidc.org/the-drift/data-update/gridded-monthly-arctic-sea-ice-back-to-1850-for-analysis-or-browsing/The monthly charts are centred on the 15th of the month. In the user guide NSIDC recommend which colour palettes to use and map projection to use.
Using panoply and following the NSIDC guide, I produced this animated GIF on Sea Ice Concs for 1933.
