The jump in global temperatures in September 2023 is extremely unlikely due to internal climate variability alone

September 2023 was the warmest September on record globally by a record margin of 0.5 °C. Here we show that such a record-breaking margin is an extremely rare event in the latest generation of climate models, making it highly unlikely ( p ~ 1%) that internal climate variability combined with the steady increase in greenhouse gas forcing could explain it. Our results call for further analysis of the impact of other external forcings on the global climate in 2023.