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.
Hitting 2 degrees Celsius for two days does not mean that we have passed 2 degrees Celsius in the way that experts have been warning of for years; meeting the Paris Agreement goals-to keep the planet "well below" exactly that threshold-is a matter of long-term averages. To pass 2 degrees Celsius more permanently would mean months or years of 2-degree-smashing days. These temperatures are both an anomaly and a preview-the product of the particular conditions of 2023, and the product of choices that will turn such anomaly into routine.
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