Multidecadal analysis of the dynamics of bioclimatic stress caused by thermal extremes in Romania

The current trend of rising temperatures may lead to significant changes in bioclimatic conditions, with direct implications for thermal comfort and population health. However, regional-scale analyses in climatically transitional areas remain limited, despite their high variability and relevance for population exposure. This study analyses the evolution of bioclimatic stress in Bihor County (western Romania) over a 60-year period (1961–2020), using a set of bioclimatic indices that quantify heating and cooling stress. The analysis is based on multiannual meteorological data from nine stations representative of the main relief units of the reference area. The results indicate a significant intensification of bioclimatic stress due to heating during 1991–2020 compared to the reference period (1961–1990), with the HUMIDEX index increasing by approximately 13%–15% (≈+2.2 °C) and the wet-bulb globe temperature index by about 5%–6% (≈+0.6 °C). At some stations, the duration of heat-related stress extended by one to two months. In contrast, bioclimatic stress due to cooling decreased, as reflected by a reduction in the wind chill index (≈−6%) and an increase in wind chill temperature values by approximately 15.9% (≈+1.3 °C), indicating milder cold-season conditions and an expansion of thermal comfort intervals. This dual pattern was observed at all analysed stations, with differences in intensity determined by altitude. The results align with recent warming trends observed at regional and European scales and emphasise the relevance of bioclimatic indices in assessing the impact of bioclimatic stress on thermal comfort and potential risks to population health.