The 2023 North Atlantic marine heatwave could have evolved into a far more extreme event

In 2023, the North Atlantic Ocean experienced a marine heatwave of unprecedented intensity and duration, raising the question of whether substantially more severe outcomes were physically plausible under comparable present-day preconditioning, and which processes could produce them. Here, we apply ensemble boosting to a coupled Earth system model to generate physically consistent alternative evolutions of a 2023-like event. The boosted ensemble produced marine heatwaves lasting up to 579 days, reaching maximum intensities of 4.5 °C above climatology and cumulative intensities of 717 °C·days, compared with 240 days, 2.9 °C and 224 °C·days in observations. Detailed analyses of the longest and most intense events highlight contrasting roles for persistent Euro-Atlantic high pressure, reduced turbulent heat loss, enhanced shortwave radiation and upper-ocean heat redistribution. Finally, we place these present-day storylines in the context of future warming, showing that events comparable to the observed 2023 event become increasingly common as global warming progresses.