Evaluation of Heatwave Severity in Southeastern Europe Through the Excess Heat Factor

The analysis of extreme weather events is a key aspect of present-day climate change research due to their significant socio-economic and ecological impacts. This study investigates the spatiotemporal variability of extreme heat events in Southeastern Europe using the Excess Heat Factor (EHF), which has proven to be an effective metric of heatwave severity. Building on the concept of Heatwave Event Moments (HEMs), we developed a framework to identify and analyze heatwaves using both reanalysis and observational data. We computed a suite of metrics to quantify heatwave severity at the local scale across Köppen–Geiger climate zones using an updated observational dataset developed for our previous study of extreme heat events in the region. We also introduced a new ranking of regional heatwaves for the period 1950–2024, based on the effective duration of events, using the ERA5-Land reanalysis by the European Centre for Medium-Range Weather Forecasts. Our findings indicate a marked increase in heatwave days and heatwave duration at both local and regional scales in recent decades. While heatwaves during the study period were predominantly of low to moderate severity, a comparison of the two 30-year periods, 1961–1990 and 1991–2020, revealed a clear trend toward longer-lasting heatwaves and increasing heat stress across all climate zones. “Hot spots” were detected mainly in the northwestern part of the study domain, the eastern Balkan Peninsula, and several coastal areas. The study also demonstrates the ability of EHF to represent the temporal evolution, spatial extent, and intensity of the historical heatwave events, such as the severe heatwave of 2007 and the prolonged heatwave of 2024.