Large discrepancies in summer climate change over Europe as projected by global and regional climate models: causes and consequences

We assess the differences of future climate changes over Europe in summer as projected by state-of-the-art regional climate models (RCM, from the EURO-Coordinated Regional Downscaling Experiment) and by their forcing global climate models (GCM, from the Coupled Model Intercomparison Project Phase 5) and study the associated physical mechanisms. We show that important discrepancies at large-scales exist between global and regional projections. The RCMs project at the end of the 21st century over a large area of Europe a summer warming 1.5–2 K colder, and a much smaller decrease of precipitation of 5%, versus 20% in their driving GCMs. The RCMs generally simulate a much smaller increase in shortwave radiation at surface, which directly impacts surface temperature. In addition to differences in cloud cover changes, the absence of time-varying anthropogenic aerosols in most regional simulations plays a major role in the differences of solar radiation changes. We confirm this result with twin regional simulations with and without time-varying anthropogenic aerosols. Additionally, the RCMs simulate larger increases in evapotranspiration over the Mediterranean sea and larger increases/smaller decreases over land, which contribute to smaller changes in relative humidity, with likely impacts on clouds and precipitation changes. Several potential causes of these differences in evapotranspiration changes are discussed. Overall, this work suggests that the current EURO-CORDEX RCM ensemble does not capture the upper part of the climate change uncertainty range, with important implications for impact studies and the adaptation policies that they inform.