There is no average temperature but one can look at a range of temperatures across a timeline. Jan Esper shows recent temperatures easily falls within the range of normal temperatures when viewed across a 2,000 year timeline. Roman and Medieval periods were as warm or warmer than recent temperatures without catastrophic climate changes. Warmer temperatures have been associated with longer growing seasons and increased agricultural production.
Even that graph should be considered highly suspect, since there is no way to accurately measure temperatures historically before modern instrumentation existed.
What was the temperature in Philadelphia when the Declaration of Independence was signed on July 4th of 1776?
You couldn’t get an accurate answer to this question even if you could resurrect all of the signatories of the Declaration today and ask them this question. Instead, you would learn that the temperature was one of the following: (1) hot, (2) very hot, (3) kind of mild, or (4) unseasonably cool. That’s it. Good luck with that “science.”