Increased central and northern European summer heatwave intensity due to the forced changes in internal climate variability

In the past two decades, the intensity of European summer heatwaves has strongly increased due to anthropogenic emissions and associated rising global mean temperatures. On the one hand, the anthropogenic forcing is causing an increase in European summer temperatures, shifting European summer temperature distributions towards warmer values and intensifying European summer heatwaves. On the other hand, the anthropogenic forcing is expected to affect the internal climate variability under global warming, changing the variability of European summer temperatures. While the effects of the forced changes in internal variability have been long debated for mean or maximum summer temperatures, the effects of the forced changes in internal variability on European summer heatwave intensity under increasing global warming levels remain unknown. Using four state-of-the-art global climate model large ensembles, we find that the forced changes in internal variability will intensify central and northern European summer heatwaves. In central and northern Europe, soil moisture is projected to decrease, leading to frequent moisture limitations, enhancing land-atmospheric feedback, and increasing heatwave intensity and variability. On the contrary, the forced changes in internal variability will weaken southern European summer heatwaves. Southern Europe is projected to face significant soil moisture depletion, leading to more stable moisture-depleted conditions that reduce extreme temperature variability and heatwave intensity. Our findings imply that while adaptation to increasing mean temperatures in southern Europe should suffice to reduce the vulnerability to increasing European summer heatwave intensity, adaptation to increased temperature variability will also be needed in central and northern Europe.