Storylines for month-long heatwaves and associated heat-related mortality impacts over Western Europe

Countless heat records were broken in recent years, leading to thousands of heat-related deaths. This raises the question of how much worse heat-related mortality could become in coming years if a potential worst-case heatwave lasts for several weeks or reaches unprecedented intensity. Here, we develop impact storylines for worst-case heatwaves and associated heat-related mortality in France, Germany, and Switzerland. We compare several physical climate storyline approaches to quantify plausible extreme heatwaves and combine these with empirical heat-mortality relationships. We find physically consistent week-long heatwaves possible in the climate of 2020 that exceed the observed 7-day record temperatures by more than 5°C and associated mortality impacts exceeding the observed maximum by 30-90%. Even more severe consequences would arise from possible five-week heat periods of unprecedented intensity, which would lead to more than a doubling of impacts. Developing these impact storylines can inform the stress-testing of socio-economic systems for preparing appropriate emergency response capacities.