We all just lived through our first 2-degree Celsius day November 20, 2023, 6:25 PM ET On Friday, November 17, 2023, the Earth appeared to have crossed a threshold into new climatic territory. That day was the first that the average air temperature near the surface of the Earth was 2 degrees Celsius warmer than preindustrial levels. Saturday was the second. The planet has been this hot before, but never in the era relevant to modern humanity.  For those two days, we were the furthest we have ever been from the average climate of 1850–1900, the time just before humans began industrializing in earnest and adding large quantities of carbon dioxide to the atmosphere. We are now a large margin away from the climate in which nearly all of human history has played out. The news of the 2-degree Celsius days came first from Samantha Burgess, the deputy director of the Copernicus Climate Change Service, which published the results from a model that uses observations to estimate global climate conditions in real time. The numbers …