You’ll also see a lot of “studies” use either 1860 or 1910 (or there about) as a reference. No coincidence that - those were cool years. In the normal fluctuations and cycles of temperature, there are going to be highs, and there are going to be lows. So if you pick a time when those fluctuations and cycles result in several years of relatively low temperature... Well, then everything else starts looking pretty warm in comparison. Data manipulation 101.