The warmest summer in the tree ring reconstruction before industrial times was year 246, but the researchers say that even this does not come close to the recent warmth.
Even taking account of the large uncertainties, the authors say summer 2023 surpassed this range of natural climate variability by 0.5C at the very least.
It is also worth looking at the graph later in the article, which shows that over the past 2k years the temperature has maintained a baseline, and post-industrial era it turned into a hockey stick.