Thermal Comfort Across the Iberian Peninsula at High Resolution: Variability, Trends, and Hotspots

Understanding how thermal comfort and heat stress have evolved under a warming climate is essential for assessing risks to human health and ecosystems, as well as for informed regional planning. This study aims to provide a high-resolution assessment of thermal comfort conditions and their evolution across the Iberian Peninsula (IP), characterized by complex topography and coastal–inland contrasts, from 1985 to 2020, using the Copernicus European Regional ReAnalysis (CERRA) dataset. Thermal extremes were characterized using summer days (SU), scorching days (SU35), tropical nights (TR), torrid nights (TR25), and the Excess Heat Factor (EHF), to evaluate spatial and temporal variations in heat-related stress during the northern hemisphere summer season (from June to September). Results revealed a clear intensification of thermal extremes, particularly in inland and southern regions. The frequency of hot days and warm nights has increased significantly, indicating a growing exposure to both daytime and nocturnal heat stress. Compound indicators (SU + TR and SU35 + TR25) exhibit a moderate yet consistent increase in the combined occurrence of hot days and nights, underscoring the persistence of extreme heat events. The EHF analysis revealed strong regional contrasts in heatwave intensity and duration, with the highest mean and maximum EHF values occurring over northeastern and western IP, and notable upward trends in frequency and duration in the south and east regions. Overall, these findings highlight the increasing risk of prolonged and more intense heatwaves, reinforcing the need for adaptive planning in public health, infrastructure resilience, and climate adaptation policies at regional and local scales. In this study, summer thermal comfort and heat-stress conditions across the Iberian Peninsula (IP) were assessed using the Copernicus European Regional ReAnalysis (CERRA) dataset for the 1985 − 2020 period. Thermal extremes were characterized for the summer season (June−September) using indices for hot days and warm nights (SU, SU35, TR, TR25), compound day−night occurrences (SU + TR and SU35 + TR25), and the Excess Heat Factor (EHF) to quantify heatwave intensity, frequency, and duration. Spatial patterns and temporal trends were mapped to identify regional hotspots and evolving exposure. Results indicate a clear intensification of thermal extremes, most pronounced in inland and southern areas, alongside increasing concurrence of hot days and warm nights. EHF highlights strong regional contrasts in heatwave patterns, with increasing frequency and duration in the central and eastern regions. The graphical abstract summarizes the workflow and the main outcomes relevant to public health planning, infrastructure resilience, and climate adaptation strategies. Summer days and tropical nights show solid upward trends since 1985. Compound heat events are becoming more common, pointing to more persistent events. EHF trends show longer and more intense heatwaves, especially in central and eastern areas. EHF reveals sharp regional contrasts in heatwave severity, supporting targeted adaptation planning.