The Future of Sinking Coastal Cities in Java

Rapid urbanization and groundwater extraction in Java’s coastal cities are driving land subsidence at rates that far exceed global sea-level rise, amplifying flood risk and coastal vulnerability. Using Sentinel‑1 data from 2016 to 2023, we generated an InSAR-derived vertical land motion map for the island and integrated it with geology, land cover, elevation, and socioeconomic datasets. The InSAR time series was processed using a Small Baseline Subset approach and calibrated against 59 GNSS stations. Subsidence patterns were analysed through spatial correlation and Random Forest regression to identify relationships between ground motions and lowland alluvial plains in the northern part of the island. Results show that 10% of urban areas are subsiding faster than 14 mm/yr, and 5% exceed 28 mm/yr, with hotspots such as Pekalongan and Demak reaching city-wide median subsidence rates of 87 mm/yr and 43 mm/yr, respectively. Subsidence is most pronounced in the low‑lying northern plains, where the shallow geology is dominated by unconsolidated alluvium and clay-rich deposits. Currently, at least 7 million people and 920,000 buildings are exposed to subsidence ≥14 mm/yr in coastal Java. Projections under high-emission scenarios (RCP 8.5) indicate that by 2100, up to 1.4 million residents could live at or below sea level when land subsidence is combined with climate change-driven sea-level rise. Our findings underscore the need for locally tailored and integrated adaptation strategies that include urban development planning, groundwater regulation, and coastal defences, to mitigate the compounding hazards from subsidence and climate-driven sea-level change.