Revealing trends in extreme heatwave intensity: applying the UNSEEN approach to Nordic countries

Abstract

Abstract The increase in heatwave intensity, causing heat stress and crop failures in many regions is a concerning impact of global climate change. In northern Europe, significant interannual variability previously prevented robust assessments of trends in heat extremes. However, with a large-ensemble seasonal hindcasts and archived forecasts dataset covering 1981–2022 multiple realisations of weather patterns can be pooled and assessed. What are recent trends of extreme temperatures? Has the risk for a 100-year heatwave event increased in Northern Europe? We apply the UNSEEN (UNprecedented Simulated Extremes using ENsembles) approach to assess the credibility of the model ensemble and use non-stationary extreme value analysis to quantify recent trends in extreme 3-day heatwaves in late spring and early summer (May to July). We find significant non-stationarity and positive trends in annual maximum heatwave intensity. We also show that heatwave volatility, i.e. the risk of clearly outstanding heatwaves, is highest in central Scandinavia.