Twin indicators of coastal vulnerability: land subsidence and mangrove extent change in Dili, Timor-Leste

Lik Ren Tai 

Mangrove forests and land subsidence are major drivers of coastal vulnerability rarely quantified together outside the well-studied deltas of Southeast Asia. This study provides the first satellite-based, twin-indicator assessment of coastal vulnerability for Dili, Timor-Leste, combining Sentinel-1 InSAR time series analysis of land subsidence with Global Mangrove Watch extent change over a multi-decadal baseline. Subsidence was detected across the urban coastal footprint (mean -0.53 cm/yr, locally exceeding -4 cm/yr) and was strongly spatially clustered, but its attribution to groundwater extraction did not survive proper spatial statistical control. Mangrove extent increased significantly over both a full 1996-2020 record and a 2007-2020 subset, contrary to national reports of long-term decline, although gross turnover exceeded net change at every transition and the most recent period showed a net loss. Mangrove-present zones fell within subsidence clusters at roughly twice the area-wide rate and none fell within stable or uplifting clusters, a categorical association not matched by a continuous relationship between the two indicators. No evidence emerged for a direct subsidence contribution to mangrove loss over the short InSAR-mangrove overlap window. Combined with regional sea-level-rise projections, the observed subsidence trend implies most of the study area could face over 0.5 m of compound relative sea-level rise by 2100 under either emissions scenario. These findings provide the first satellite geodetic measurement of subsidence in Timor-Leste and show that Dili's remaining mangroves and its subsiding zones share the same low-lying terrain, a convergence relevant to coastal planning regardless of the causal mechanisms linking them.