Prolonged summer seasons over Europe: a sensitivity study

Heat waves and flash droughts are becoming more severe due to ongoing climate change. These events are typically associated with the summer season, which in Europe is traditionally defined as a three-month period from June to August. Rising temperatures, however, challenge this conventional definition, as summertime-like temperatures are recorded more often outside this interval. For example, April 2024 as well as the turn of April and May 2025 were marked by unseasonably high temperatures exceeding 30°C in Central and Western Europe. In this study, we employ alternative definitions of the summer season based on the persistence of temperatures above specific thresholds (both absolute and relative) to study shifts in its onset and termination across Europe between the 1961–1990 and 1995–2024 periods. We also investigate differences between two gridded datasets (E-OBS and ERA5) to address uncertainties arising from the data source. Preliminary results indicate prolonged summer seasons across most of Europe, with an opposite tendency found in the British Isles, the Pannonian lowland, and the Black Sea coast. This suggests spatially diverging regional pattern in the summer season prolongation, representing important climate hazard associated with climate change.