Many studies in the last few years have been dedicated to the increasing temperatures and extreme heat in Europe since the second half of the 20th century because of their adverse effects on ecosystems resilience, human health, and quality of life. The present research aims to analyze the spatio-temporal variations of extreme heat events in Southeastern Europe using daily temperature data from 70 selected meteorological stations and applying methodology developed initially for the quantitative assessment of hot weather in Bulgaria. We demonstrate the suitability of indicators based on maximum temperature thresholds to assess the intensity (i.e., magnitude and duration) and the tendency of extreme heat events in the period 1961–2020 both by individual stations and the Köppen’s climate zones. The capability of the used intensity-duration hot spell model to evaluate the severity of extreme heat events has also been studied and compared with the Excess Heat Factor severity index on a yearly basis. The study provides strong evidence of the suitability of the applied combined approach in the investigation of the spatio-temporal evolution of the hot weather phenomena over the considered domain.

Southeast Europe is considered one of the most vulnerable regions in the context of climate change, and projected future summer warming is expected to exceed global rates significantly. Despite the importance of this problem, there have been few studies that utilized Coupled Model Intercomparison Project Phase 5 (CMIP5) Global Circulation Models (GCMs) and the multi-model ensemble approach to examine extreme heat events on a regional scale. Additionally, the NEX-GDDP dataset, successfully applied in other parts of the world to assess extreme heat, has not yet been utilized for Southeast Europe. This study aims to fill that gap, presenting the time evolution and spatial distribution of extreme heat events in Southeast Europe for the historical period 1950–2005 and for the expected future climate up to the end of the 21st century based on the NEX-GDDP dataset. In order to leverage the strengths of the multi-model ensemble approach, a set of purpose-tailored indicators, such as the annual number of hot days, the maximum number of consecutive hot days, and hot spell duration at different thresholds, is computed by the daily maximum temperature data from all datasets, produced by NEX-GDDP (21 for the historical period and 42 for the future period). The E-OBS dataset is used as a reference for evaluating the NEX-GDDP’s capability to simulate the features of the observed historical extreme heat events. The results show that the multi-model ensemble can satisfactorily capture the occurrence of extreme heat events in the historical period, and therefore it is reasonable to assume that the NEX-GDDP dataset has the potential to reproduce such extremes in the projected future. The study provides clear evidence that the persistence and spatial extent of extreme heat will increase significantly. Some indicators that were not relevant for the historical period due to the high-temperature threshold will become helpful in assessing extreme heat in Southeast Europe in the latter part of the century. Thus, under the RCP8.5 scenario, the area-averaged duration of hot spells at 32 °C and 34 °C will increase from near zero in 1976–2005 to 60 and 45 days, respectively, by the end of the century. The indicators used in the study may be helpful for decision-makers to implement climate change mitigation strategies and actions adequately. The findings are consistent with general tendencies in maximum temperatures considered in our previous works but also with the outcomes of recent studies dedicated to the future climate of the region.