Assessing the plausibility of unprecedented events: a process-based approach applied to month-long heatwaves in Western Europe

Climate model based storylines of individual climate and weather events are increasingly used to quantify impacts, vulnerability, or stress-test infrastructure to inform adaptation decisions. Here, we present an approach to test the plausibility of unprecedented climate storylines based on physical conformity, internal consistency, and historical precedent. We apply this approach to assess the plausibility of month-long heatwaves in Western Europe that would exceed existing record temperatures by more than 5 K. These heatwaves are based on model simulations using ensemble boosting, a computationally efficient method to simulate unprecedented events. We compare these unprecedented heatwaves with historical heatwaves in a reanalysis data set, using standardised anomalies relative to a time-evolving climatology of relevant physical variables. We show that these unprecedented heatwaves are associated with well-known physical drivers that are similar to historical heatwaves, including strong anticyclones and low soil moisture. The anomalies of these drivers are more intense in magnitude, but similar in their spatial patterns and in their relationships with each other. However, we also reveal substantial discrepancies between historical and unprecedented heatwaves, which are related to atmospheric blocking, land-atmosphere interactions, and their temporal persistence. Despite these discrepancies, we conclude that these long-lasting, unprecedented heatwaves should not be ruled out as implausible and thus highlight the need to anticipate such events when planning adaptation measures. Similar approaches can be used to assess the plausibility of other types of extreme events.