New normal? Attributing the unprecedented global heat in September 2023

Between July 2023 and June 2024, each month set a new temperature record, all exceeding the 1.5 °C threshold established by the Paris Agreement. The global mean surface temperature anomaly peaked in September 2023 at 1.8 °C above pre-industrial levels, exceeding the previous record by an unparalleled 0.5 °C.

Using a probabilistic attribution framework, we assess the likelihood of this global heat. Our analysis shows that both the absolute temperature anomaly and the year-to-year temperature difference were extremely unlikely from an observational perspective, and are generally not reproduced by CMIP6 models. Yet, the occurrence probability of the absolute temperature anomaly rises sharply within just a few years into the future. In contrast, the temperature jump from September 2022 to 2023 remains highly unlikely throughout the next decades, even under higher warming levels. A process-based analysis highlights water vapour feedback and resulting longwave forcing as key drivers of the heat build-up in September 2023, with no indication of nonlinearities in the climate models. Our findings suggest that the September 2023 temperature jump was an extremely rare event which would strongly challenge our understanding of the climate system if it were to reoccur.