We have measurement stations that go back that far, and where we use the same instruments and methods today as back in the 19th century for the express purpose

We have measurement stations that go back that far, and where we use the same instruments and methods today as back in the 19th century for the express purpose to have a data series that can be interpreted that far back. A concrete data set: https://crudata.uea.ac.uk/cru/data/temperature/ https://crudata.uea.ac.uk/cru/data/temperature/ This includes the following list of original source data: https://crudata.uea.ac.uk/cru/data/temperature/crutem4/station-data.htm https://crudata.uea.ac.uk/cru/data/temperature/crutem4/stati... So you see, a lot of things go into making that one number for 1880, and there are many stations going back much further than that. Updating these data sets is a massive, careful, effort.