Urbanization has significantly transformed land-use patterns and continues to influence thermal comfort in cities. This study explores how built-up areas contribute to reducing thermal discomfort in semi-arid regions, focusing on Tehran from 2000 to 2024. Landsat 7 and satellite data were used to classify land use, derive Land Surface Temperature (LST), and assess thermal stress through the Discomfort Index (DI). To evaluate cooling performance, several indicators were applied, including Temperature and Discomfort Index variations for both Urban Green Spaces (UGS) and built-up areas (Temperature Urban Green Space (TUGS), Temperature Built-up (TBU), Discomfort Index Urban Green Space (DIUGS), and Discomfort Index Built-up (DIBU)), as well as Apparent Temperature differences (Apparent Temperature Urban Green Space (ATUGS), and Apparent Temperature Built-up (ATBU)). During the study period, built-up areas expanded from 55% to 68%, while UGS coverage peaked at 18% in 2014 before declining to 14% in 2024. Despite their ecological importance, green spaces showed only a slight cooling influence, with mean LST decreasing by just-0.13 degrees C. However, DI dropped more noticeably (-0.51 degrees C), indicating the importance of humidity in shaping thermal comfort. Meanwhile, UGS temperatures increased from 36.0 degrees C to 38.7 degrees C, whereas built-up zones experienced a minor cooling trend. Cooling indices revealed a gradual loss of cooling benefits from UGS and an enhanced moderating effect from built-up structures. TUGS decreased from 2.92 to-0.40, while TBU increased from 1.09 to 2.56. Similarly, ATUGS weakened from-3.54 to 0.36, whereas ATBU improved from-1.09 to-2.77. Correlation analysis showed that larger UGS patches still offer better cooling, although patch complexity has limited impact. Overall, the findings underscore growing urban heat pressure in Tehran and the weakening role of green spaces in temperature regulation. At the same time, they highlight the emerging significance of built-up morphology and shading features in improving thermal comfort across a rapidly urbanizing landscape. (c) 2026 COSPAR. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.