Heat Extremes Within Hot Summer Seasons Intensify More Than in Average Summers Under Global Warming

Abstract

The intensity and persistence of heat extremes increase in a warming climate. Warmer temperatures imply more hot days and longer hot periods per summer. However, beyond summer mean temperature change, changes in the timing of hot days within the season increase the likelihood of long‐lasting uninterrupted hot periods in many mid‐latitude regions and particularly in Europe. Both changes in atmospheric circulation and soil moisture have been discussed as drivers of heat persistence, yet the precise role of both drivers remains unclear. Here we compare simulated extreme hot summers in current climate, with a corresponding statistical return time of 100 years, to similarly rare summers from pre‐industrial climate. The extreme summers were obtained by applying a rare event algorithm to the fully coupled CESM2 climate model; initialized in early June from a wide variety of ocean and land surface initial conditions. Our results demonstrate that the persistence and intensity of temperature anomalies beyond summer mean temperature change (i.e., relative to each climate's climatology) has increased considerably in 100‐year summers. These changes are driven by comparatively small changes in soil moisture and its seasonal cycle of summer drying, and corresponding changes in land‐atmosphere energy fluxes. These changes in sub‐seasonal evolution of heat within hot summers occur despite no change in summer mean temperature anomalies of 100‐year summers. The findings imply that the next mega‐summer in western Europe likely differs from past summer extremes with more persistent and intense heatwaves during peak summer and in the second half of the season.