Worst-case European heat storylines generated using ensemble boosting

Abstract Extreme heat poses escalating socio-economic and ecological risks, yet the most severe high-impact heat extremes that would be possible today remain poorly understood. Using thousands of ensemble-boosting storylines, all plausible under current-climate conditions at least within the model world, we reveal the risk of far more intense and unprecedented heatwaves, which surpass historical extremes in both intensity and particularly in persistence by large margins, and greatly exceed levels considered extreme in a 3 °C warmer world. The most extreme heatwaves are preceded by severe soil moisture depletion, both locally and upstream of the region of extreme heat, as well as by strong ocean temperature gradients, with extremely warm anomalies in the nearby basins and cold anomalies in the subpolar North Atlantic region. Furthermore, our storyline simulations reveal an additional risk: worst-case heatwaves occur predominantly after another extreme heatwave. This highlights the potential for aggravated impacts due to decreased recovery times and intensified heat stress on humans, ecosystems and infrastructure made more vulnerable by the first event. Given the scale, intensity, and unprecedented successive and compounding nature of these worst-case heat storylines, we underscore the urgent need for well-informed adaptation strategies that sufficiently reflect these risks.