Climate Change Impacts on Diurnal Temperature Range and Thermal Discomfort and Their Association in Selected Eastern Mediterranean Cities Using CMIP6 Projections

Climate projections indicate significant changes in temperature patterns and other meteorological parameters under different climate change scenarios, with temperature receiving special attention due to its influence on thermal conditions and human discomfort. This study examines the relationship between diurnal temperature range (DTR) and thermal discomfort in the five largest cities of Greece during summer. Thermal discomfort is assessed using Thom’s discomfort index (DI), where values ≥ 21 indicate the onset of thermal discomfort, focusing on thermal conditions at the upper (DIh) and lower (DIc) boundaries of daily variability. The analysis uses multiple CMIP6 projections for the reference period (1981–2010) and the near future (2031–2060) under the SSP2-4.5 and SSP5-8.5, representing intermediate and high greenhouse gas forcing pathways, respectively. The study aims to investigate associations between DTR and DI-based thermal discomfort. DTR is projected to increase in most cities in the near future relative to the reference period. This reflects a regional specific response that differs from the global tendency reported in the literature for minimum air temperatures (Tmin) to increase faster than maximum air temperatures (Tmax). Effect size analysis of DTR indicates generally small effects in Thessaloniki, medium to large effects in Larissa depending on the scenario, and large effects in Heraklion, Athens and Patra. Projected differences in DTR are consistent with the asymmetrical response of air temperature, specifically to the higher increase rate in Tmax than in Tmin in most cities. DI-based thermal discomfort shows a clear contrast between upper (DIh) and lower (DIc) boundaries of daily variability, reflected in higher discomfort classes for DIh and lower classes for DIc. Higher DTR values are associated with higher DIh-based thermal discomfort, while the corresponding association between DTR and DIc is weak or absent. The positive association observed for the DIh-based conditions is largely governed by the shared contribution of Tmax to both DTR and the discomfort index, whereas the absent or weak association for DIc-based conditions may reflect the weaker association between DTR and Tmin as well as the relatively smaller variability of Tmin.