How can those «140 years of records with actual thermometer measurements» support a claim of accuracy of 0.1 degree? (realclimate.org)
Comparing that data allows scientists to derive past temperature data for several centuries before accurate thermometer measurements were available. (ucsusa.org)
The Climatic Research Unit had legal agreements with certain countries that allowed them to use thermometer measurements, but they were legally bound not to distribute the data. (discovermagazine.com)