Worst-Case European Heat and Drought Storylines generated using Ensemble Boosting

Heat and drought extremes pose escalating socio-economic and ecological risks, yet the most severe combinations of these high-impact extremes possible today remain poorly understood. Using thousands of plausible ensemble-boosting current climate storylines, we reveal the risk for more intense drought compounding with far more extreme heat and fire weather than ever experienced over Europe in the recent past. The most extreme boosted heatwaves surpass historical extremes in both intensity and particularly in persistence, and also exceed levels considered extreme in a 3°C warmer world by large margins. Some of the most extreme heatwaves arise under severe soil moisture depletion, while others develop under strong surface temperature gradients in the North Atlantic and extreme heat in the nearby Mediterranean and Atlantic basins, underscoring the diversity of pathways to worst-case conditions. 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 storylines, we underscore the urgent need for well-informed adaptation strategies that sufficiently reflect these risks.