In semi-arid cities such as Tehran, declining urban green space and expanding Built-Up areas are typically assumed to intensify surface heating. However, long-term observations suggest that several districts may have experienced cooling within the Built-Up fabric, while green spaces have lost part of their expected influence. This paradox indicates that conventional assumptions about vegetation's thermal advantage may not fully explain semi-arid urban dynamics. The novelty of this study lies at analyzing Built-Up cooling dynamics in Tehran by integrating two decades (2000 - 2020) of land use change and urban green space (UGS) dynamics parameters with cooling indices for land surface temperature (LST) of urban green spaces (TUGS) and LST of Built-Up areas (TBU), evaluated across several buffer distances in 22 districts. False discovery rate (FDR) controlled correlations of TUGS and TBU with, morphological, demographic, and atmospheric variables were analyzed to evaluate the cooling efficacy of the TBU and TUGS indices. Results showed a notable green space loss in several districts, with per - capita provision declining by up to 125%, while surface LST in parts of the built up fabric fell by roughly 0.7 - 2.8 degrees C. TUGS weakened across many districts, with reductions of 0.3 - 0.6 units at 100 m and up to 1 - 1.5 units at 300 - 500 m, whereas TBU shifted by about 1 - 2 units in mid-range buffers toward stronger relative cooling. Lower sky - view factor (SVF) generally aligned with enhanced cooling, while greater horizontal / vertical density and human pressure were associated with reduced cooling capacity. Bivariate color mapping revealed scale - dependent and increasingly heterogeneous cooling patterns, with Built-Up morphology playing a more prominent role at larger buffer distances over time. Collectively, these patterns suggest that thermal behavior in semi - arid cities may be non - linear, with Built-Up areas under certain configurations exhibiting cooling comparable to, or occasionally exceeding, that of green spaces.