Seasonal variability of trends in regional hot and warm temperature extremes in Europe

Extremely hot or warm weather over the course of the year may have significant impacts on many aspects of human life, economy, and the natural environment. Until now a comprehensive assessment of changes of extreme heat or warm events in Europe was hindered by the number of metrics employed, time periods examined, and most studies being conducted in the summer season only. This study was designed to systematically investigate long-term trends in the number of extremely hot or warm days in Europe over the course of the year, with a special focus on their spatial extent and intensity. Here, an extreme temperature event (ETE) is defined as a day with an unusually high temperature for a given location and season, even if such a temperature would not be considered extremely high in an absolute sense.

The research was conducted in five spatial domains representing different regions of Europe that together cover a large portion of the continent. The period from spring 1950 to winter 2019/2020 was considered using E-OBS gridded dataset v.21.0e. ETEs were analysed from a spatial perspective and were defined as days when the maximum air temperature exceeds the local percentile-based threshold across at least 10% of the area of the given domain. The severity of each ETE was assessed using the Extremity Index (EI) that combines information on the intensity and spatial range of an event. Given that EI is based on relative measures, it enables the comparison of results obtained in different geographic areas and seasons of the year. The main focus of the analysis was on the evaluation of seasonal EI trends and variability, as well as trends of frequency, spatial range, and intensity of ETEs in five spatial domains.

Climate warming in Europe during the studied 70-year period was accompanied by an increase in the frequency and severity of ETEs expressed in terms of their increasing intensity and spatial range. Yet, as our study demonstrates, every spatial domain is characterized by its own event pattern, and trends across Europe strongly vary geographically and seasonally. Our study highlights that examined trends of temperature extremes are accelerating and in the last 40 years the rate of change has been even more than three times greater than in the entire study period. The greatest changes were noted for summer season in Central Europe and Eastern Europe for the most recent 40-year period.