Coastal urban land expansion and adaptation gaps in a rising sea era: evidence from 24 global cities

The accelerating rise in global sea levels, presents a growing conflict with seaward expansion, while adaptation efforts remain fragmented as land-use planning often fails to adequately account for sea-level threats. This study uses multi-source geospatial data and spatial statistical methods to analyze urban expansion patterns, driving mechanisms, and adaptation responses across 24 representative coastal cities from 1985 to 2022. Our results demonstrate that coastal cities have experienced phased built-up area expansion, and that 71 percent have expanded toward the sea. The extent of seaward expansion differs widely depending on economic development levels, topographic constraints, and policy interventions. Driver analysis reveals economic factors and social factors explain 71% and 75% of seaward expansion variance respectively, while relative sea-level rise and climate policies show limited explanatory power. Through our development-adaptation framework, we identified four distinct urban development typologies, including New Orleans with its engineered resilience supported by community partnerships, Jakarta with its contested land reclamation projects, Mumbai with its rapid urban expansion shaped by social equity challenges, and Xiamen with its ecological transformation guided by sponge city practices. These findings provide a multi-scale analytical framework that can support climate adaptation strategies aimed at balancing urban development needs with long-term coastal sustainability.