Worst-Case European Heat Storylines generated using Ensemble Boosting

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 plausible ensemble-boosting current climate storylines, we reveal the risk for 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. Worst-case boosted sotrylines depict extreme heatwave intensities four to six times more intense than those observed in 2003 and 2018 over Europe, the most extreme years on record, occurring concurrently with soil moisture deficits or sea surface temperatures more than one standard deviation more intense. These findings reveal the potential for far more intense heat compounding with other system stressors in unprecedented levels.

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 work reveals 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.