Discomfort Indices’ Variation in Athens, Greece: Present Status and Future Estimations.

Human health is directly affected by various environmental conditions associated with temperature, relative humidity, prevailing wind and incoming solar radiation. Among these climatic parameters, temperature and humidity have the greatest influence on human health and on thermal comfort sensation. In general, thermal comfort appears at air temperatures between 18^oC and 23^oC, relative humidity between 35% and 70%, with less or no air motion. Researchers have proposed various rational or empirical bioclimatic indices and models to assess human’s thermal comfort. These indices simplify the interpretation of the complex effects of prevailing atmospheric conditions on human comfort, allowing comparisons between different climatic regions. Two of the most widely accepted indices, are the ones of Humidex (HMDX) and Thom's Discomfort Index (TDI). HMDX represents the perception of heat in humid areas, while TDI indicates the contribution of temperature and humidity to thermal comfort.

For the city of Athens, a continuous urbanization process that started in the 1950s, with new high-rise buildings, and the subsequent appearance of the urban heat island phenomenon contributed to Athens basin’s local heating and modification of its climatic characteristics, especially during summers, characterized by long periods of sunshine, high temperatures and varying levels of humidity. At the same time, in recent decades (since the mid-1980s), Athens has experienced intense regional climate change.

The aim of the present study is to examine the temporal evolution of thermal sensation and discomfort, incorporating indices based on Temperature (T) and Relative Humidity (RH) records from NOA’s meteorological station on Thissio hill at the historic center of Athens (lat.: 37.972 N, log: 23.717 E, alt.: 107m), on an hourly basis , for the period between 1971 and 2005. In addition to the historical data, T and RH climate model projections up to 2100 are also used, after performing a bias correction procedure based on station data. HMDX and TDI were also estimated from 3-hourly T and RH projections (from a sub set of 3 regional climate models), for two different greenhouse gas emission scenarios the intermediate scenario - RCP 4.5 and the extreme with high emissions - RCP 8.5 (IPCC AR5). The analysis showed that, under the intermediate scenario an increase in the number of days of intense discomfort is expected in the city center of Athens by 15-20 days in the near future (up to 2060), and 25-30 days for the distant future (up to 2100). As for the extreme scenario, the corresponding increase in the number of days of intense discomfort reaches 25-30 days in the near and 60-65 days in the distant future, highlighting the expected heat exhaustion conditions and the need for urgent adaptation measures.