Temporal Discomfort Variation in the Eastern Mediterranean City of Athens, Greece. Part 2: Climate Model Predictions for Near and Distant Future.

Human health is directly influenced by environmental conditions associated with temperature, relative humidity, wind and incoming solar radiation. Among these, temperature and humidity exert the greatest influence on thermal comfort and human well-being. To assess thermal stress, various bioclimatic indices have been developed to simplify the complex interactions between atmospheric variables and human perception, enabling meaningful comparisons across different climatic regions. Thom’s Discomfort Index (TDI), introduced in 1957, is the most widely accepted and the first physiological index of its kind, primarily designed to describe discomfort at the population level, has been analyzed across the globe in areas with different climatology.

The city of Athens has undergone a continuous urbanization process that started in the 1950s, with the construction of new high-rise buildings, leading to the subsequent appearance of the urban heat island phenomenon, which contributed to Athens basin's local heating and modification of its climatic characteristics, especially during summers. Concurrently, Athens has experienced intense regional climate change in recent decades (since the mid-1980s).

The aim of the present study is to explore the temporal evolution of thermal sensation and discomfort at the historic center of Athens, incorporating air temperature (T) and relative humidity (RH) climate model projections, on 3-hour time step, for two future periods: the near (2031–2060) and the distant (2071–2100) one. Thermal discomfort is assessed using TDI index, which combines temperature and relative humidity into a single metric. TDI values were calculated from 3-hourly outputs of four EURO-CORDEX regional climate models under the RCP4.5 (intermediate) and RCP8.5 (high-emission) scenarios (IPCC AR5). Model data were bias-corrected against observations form the National Observatory of Athens (Thissio station), one of the longest and most homogeneous urban meteorological records in the Mediterranean region. The correction was performed over a reference period (1976-2005) and subsequently applied to future projections.

The results reveal a pronounced increase in the frequency and duration of high thermal discomfort conditions in the city center. Under the RCP4.5 scenario, the number of intense discomfort days is projected to increase by 21-39 days by mid-century and by approximately 1-2 months by the end of the century. Under the RCP8.5 scenario, the increase is substantially larger and becomes dramatic, with intense discomfort conditions potentially extending by up to 3 additional months annually. The contrast between the two scenarios highlights the critical role of emission mitigation in limiting future heat stress.

The study of TDI shows that climate change does not merely raise temperatures, but drastically increases perceived discomfort and heat related risk, transforming long parts of the year into thermally uncomfortable periods, highlighting the need for urgent adaptation measures in urban planning and public health to reduce vulnerability to extreme heat.