The researchers initially correlated measured temperatures for the months of June, July, and August of 2023 with those recorded in the period 1850 to 1900. They discovered that the average temperature in summer 2023 was 2.07°C warmer than that of the summers of the phase from 1850 to 1900, which the Intergovernmental Panel on Climate Change (IPCC) uses as a reference period for pre-industrial temperatures. To generate a more extensive comparison, the team then made use of an existing international archive of meteorological data that had been reconstructed with the help of tree rings and reaches back as far as year 1 of the Common Era. "What we found as a result was that summer 2023 was the hottest even over this very long period of time and was 2.20°C warmer than the mean summer temperature since year 1 CE,"
so 2.0C already hit but part of that is because 1850-1900 baseline is lower than we previously thought.