In October 2018, the Intergovernmental Panel on Climate Change (IPCC) released a special report on “the impacts of global warming of 1.5 degrees C above pre-industrial levels.”
“Pre-industrial levels” is defined in the report as the the period from 1850 to 1900.
Not explained in the report, unless I missed it, is why I should feel confident in the scientific precision of air and sea surface temperatures taken in the 19th century.