Why climate models underestimate the exacerbated summer warming in Western Europe

Since 1980, mean summer temperatures in Western Europe have warmed three times faster than global mean temperatures. This strong warming of about 2.3 °C tends to be underestimated in model simulations, affecting both global and in particular regional climate models (RCMs). We demonstrate that the majority of global and regional climate model simulations exhibit weaker circulation-related warming contributions than observed, partly accounting for the discrepancy between observations and models. Crucially, most RCMs from the Coordinated Regional Downscaling Experiment (CORDEX) additionally underestimate the thermodynamic contribution to warming that occurs primarily in response to anthropogenic forcings. Because the driving global climate models of the CORDEX RCM simulations all provide at least sufficient, and typically even excessive global background warming, this partly compensates for the frequent lack of regional thermodynamic warming. We find that the main cause of the latter is the widespread use of constant aerosol concentrations in RCM simulations, such that the regional brightening and associated warming in Europe due to aerosol reductions in the past decades is not captured.  We infer a summer warming underestimation of about 0.5 °C since 1980 when relying on RCMs with constant rather than evolving aerosols over Western Europe, although this depends on the GCM–RCM ensemble subset. Locally, in parts of Eastern Europe with stronger aerosol reductions than further west, the discrepancies can exceed 1 °C. The use of constant aerosol representations not only contributes to the summer warming discrepancy in Europe but also impacts other seasons except winter. At the timescales of heat extremes, the aerosol representation-inflicted mismatch manifests even more clearly: heatwave intensity changes since 1980 are already underestimated by RCMs with constant aerosols by about 1°C in western Europe, and the warming discrepancies grow even larger in projections, exceeding 2 °C in large parts of Europe and at the end of the ongoing century. Our work highlights the importance of representing all relevant external forcings and associated responses in RCM simulations, as the added value of high-resolution climate projections is questionable when the strong regional brightening and warming in Europe and other regions is by design omitted.