Thermal bioclimate in Greece based on the Universal Thermal Climate Index (UTCI) and insights into 2021 and 2023 heatwaves

The research on the thermal bioclimate is increasingly valuable in the context of climate change. Human well-being is threatened not only from direct exposure to extreme thermal conditions and complications from underlying health issues but also from disruptions of health and social services. This study aims at examining the thermal bioclimate in Greece. Data of ERA5-Land reanalysis dataset were used to estimate the Universal Thermal Climate Index (UTCI, in degrees C). The analysis covered a 30-year period (1991-2020) examining UTCI climatology and trends. Additionally, the intensity of two recent heatwaves which occurred in summers 2021 and 2023 was assessed. During the period 1991-2020, the UTCI equivalent temperature ranges between -49.8 degrees C and 51.9 degrees C [mean +/- standard deviation (sd) = 13.3 +/- 12.2 degrees C] classifying thermal stress from 'extreme cold stress' to 'extreme heat stress'. Across the country, the mean UTCI ranges from 7.5 degrees C to 17.8 degrees C (mean +/- sd = 13.3 +/- 2.1 degrees C), with a decrease of 0.5 degrees C per latitude degree. Mountainous regions exhibit the lowest UTCI equivalent temperatures, while inland plains display higher UTCI equivalent temperatures, especially in the eastern regions of the country. An increasing trend in mean annual UTCI (mean = 0.05 degrees C/year) is identified, more pronounced in higher latitude regions. Both the 2021 and 2023 heatwaves were exceptionally severe, with about 99% of the population to experience mean UTCI equivalent temperatures falling within stress categories of 'moderate heat stress' and above. The 2023 heatwave set a new record with the highest absolute maximum UTCI (52.1 degrees C) and number of heat stress hours compared to the period 1991-2020. The bioclimatic maps developed in this study analyze the spatial variability in the thermal conditions across Greece identifying high-risk areas. These findings can contribute to impact studies for tourism, energy efficiency, urban design and public health purposes.