Clocking the Heat: Projected Diurnal Patterns of Thermal Discomfort Across Saudi Arabia Under Future Climate Scenarios

Rising extreme temperatures driven by climate change are expected to significantly degrade outdoor thermal conditions, stretching the day of extreme heat and leaving fewer hours for comfortable and safe outdoor activity, while increasing the health risks associated with outdoor exposure. This study investigates the impact of climate change on thermal discomfort across the Kingdom of Saudi Arabia. Projections from two CMIP6 models, at 6-hour temporal resolution, were used to compute the Discomfort Index (DI) based on dry-bulb temperature and relative humidity, and to assess diurnal variations in thermal stress at 03 UTC (06:00 AST), 09 UTC (12:00 AST), 15 UTC (18:00 AST), and 21 UTC (00:00 AST) under SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. Changes were evaluated for the near (2021–2040), mid (2041–2060), and far future (2081–2100). Thermal discomfort across Saudi Arabia intensifies progressively from the historical period to the far future, exhibiting pronounced spatial and diurnal variability. Historically, daytime discomfort (09–15 UTC) had a mean DI value of ~25.4 °C, corresponding to uncomfortable conditions across most regions, with some areas, particularly in the southeast and coastal regions, reaching very uncomfortable conditions. Early morning and evening hours (03–21 UTC) were slightly lower, with mean DI values around 22.8–23.4 °C, corresponding to slightly uncomfortable conditions. Future projections indicate a substantial increase in discomfort magnitude, particularly in coastal and southeastern areas. In the near-future (2021–2040), mean DI values increase to ~25–26 °C during daytime and ~23 °C during early morning and evening hours. By the mid-future (2041–2060), at 09 UTC (12:00 AST), the southeast and coastal regions are very uncomfortable and can reach extremely uncomfortable conditions under SSP5-8.5, reflecting peak thermal stress during the day. In the far-future period (2081–2100), at 09 UTC (12:00 AST), mean DI values reach ~27–28.6 °C under SSP2-4.5 and SSP5-8.5 scenarios, with maximum values exceeding 32 °C in the southeast region under SSP5-8.5, corresponding to dangerous conditions, highlighting the severity of midday thermal stress and its potential impacts on outdoor activities and urban livability. Evening and early morning mean DI values also rise substantially compared to historical conditions, reaching ~25–27 °C (uncomfortable), with some regions, particularly in the southeast, reaching up to ~30.5 °C (extremely uncomfortable), indicating that nighttime relief is markedly reduced and thermal discomfort persists even outside peak daytime hours. These findings emphasize the necessity of adaptive strategies to ensure the resilience, safety, and comfort of outdoor environments under increasing heat stress.