Spatial divergence in urbanization–ecological environment coordination in the coastal city belt of the Yangtze River Delta: Evidence from pixel-level time-series remote sensing

Abstract

Achieving sustainable coastal development requires a quantitative understanding of the coordination between urban growth and ecological conditions. In this study, we developed a pixel-level remote sensing framework to assess long-term urbanization–environment coupling in the coastal city belt of the Yangtze River Delta (YRD), China, across three decades (1992-2021). Urbanization intensity was characterized using a Comprehensive Nighttime Light Index (CNLI) derived from calibrated annual nighttime light data, while ecological conditions were represented by a time-series Remote Sensing Ecological Index (ts-RSEI) constructed from annual Landsat composites. These indicators were integrated using four-quadrant analysis and a Coupling Coordination Degree (CCD) model to evaluate spatiotemporal dynamics and sustainability trajectories. The main results were as follows: (1) The urbanization levels increased rapidly, with the mean CNLI rising by 0.39. Shanghai consistently represented the peak of urbanization development (ΔCNLI = 0.502), exerting a notable radiative driving effect on surrounding cities. (2) The environment experienced degradation amid fluctuations, with the mean ts-RSEI decreasing by 0.033. This degradation exhibited a bimodal spatial distribution centered around Shanghai (Δts-RSEI = −0.153) and Zhoushan (Δts-RSEI = −0.051). (3) The development in the coupling coordination relationship between urbanization and the environment shifted from Quadrant II to Quadrant IV, with the CCD increasing from 0.356 to 0.684 before declining to 0.432, showing an inverted U-shaped trend. Spatially, metropolitan core areas shifted from relatively coordinated to comparatively unbalanced states, whereas northern, southern, and suburban coastal zones showed gradual improvement. This spatial divergence reflects differentiated coastal development pathways shaped by land–sea resource endowment, ecological constraints, and policy-driven infrastructure optimization. By providing spatially explicit evidence of sustainability transitions in a rapidly urbanizing coastal megaregion, this study offers practical insights for integrated coastal zone management, ecological redline delineation, and adaptive coastal governance.