Good, broadly available, consistently measured temperature data from daily mercury record keeping was mainly only really done in UK and colonial U.S. until the late 1800s. Even then most of the world did not maintain standards for temperature stations until the early to mid 20th century. Global temperature estimates that actually span the whole globe were not really possible until the weather satellite era, around the late 1970s. So there is a big problem when comparing the precision and breadth of modern temperature data with historical estimations for a number of reasons. Not least of which is that we do not really see the sample rates of historical data necessary to estimate the periodic, even as small as decadal, swings to any high degree of accuracy.