we're talking about a 1C variance, I'd like to know what calibrated instrument and organization has the data for review going back a bit further than 1979.
NOAA's GHCN: Global Historical Climatology Network, Its coverage includes data going back several centuries, with the earliest records starting around 1750.
HadCRUIT (Hadley Centre/Climatic Research Unit Temperature). Has marine (ship-borne and buoy) and terrestrial (land-based stations) sources.
NASA's GISS Surface Temperature Analysis has records from the 1880s.
As for calibration, I believe some places routinely calibrated their mercury tstats. But since that was a potential issue, we started using indirect measurements (eg: what I mentioned) to confirm certain climate trends.