Long-term impact of 3-D urban expansion on the thermal environment in the Yangtze River Delta region, China

Rapid urbanization is reshaping landscapes worldwide, concentrating populations and economic activities while intensifying environmental challenges. Understanding the long-term impacts of three-dimensional (3-D) urban expansion on the thermal environment is essential for advancing the United Nations’ Sustainable Development Goals and mitigating climate change. While the urban heat island (UHI) effect has been extensively studied, most research has overlooked how regional-scale long-term thermal dynamics evolve under sustained 3-D urban expansion. Previous studies have largely focused on single or several cities in short time periods, leaving the long-term regional consequences of 3-D expansion poorly understood. Here, we examined the Yangtze River Delta (YRD) region from 2000 to 2020 to assess how long-term 3-D (horizontal and vertical) urban expansion shapes land surface temperature (LST) and surface urban heat island intensity (SUHII). During this period, impervious surface area (ISA) increased from 16493.90 km² to 51738.24 km², while mean building height (BH) showed a slight overall increase, indicating the coexistence of rapid horizontal expansion and moderate vertical urban growth. Both annual mean LST and SUHII show a fluctuating but overall upward trend during daytime and nighttime. In a 3-D context, LST and SUHII were positively correlated with impervious surface coverage (ISC) and BH. Notably, when the horizontal urban intensity gradient was in the range of 0–0.4, the value of LST was relatively low. The increase in SUHII along the horizontal urban intensity gradient at nighttime was smaller than at daytime, and there was an obvious inflection point (ISC = 0.8). In addition, when the vertical urban intensity gradient ranged from 20 to 30 m, it had the most unfavorable impact on the thermal environment of the YRD region. These findings provide novel insights into the long-term thermal impacts of 3-D urban expansion. They highlight critical thresholds of urban form that exacerbate heat stress, offering valuable implications for formulating effective urban heat adaptation policies in rapidly urbanizing regions.