Coastal change in Southeast Asia from geological to contemporary timescales: a review

Southeast Asia's coasts are among the world's most physically and ecologically sensitive environments, facing escalating threats from climate change, rising seas, extreme events and pervasive land subsidence. Understanding the magnitude, rate and trajectories of coastal transformation is critical for adaptation planning and hazard mitigation. This review synthesises current knowledge of coastal landform change across Southeast Asia (SEA), emphasising spatial and temporal variability driven by geological, climatic and anthropogenic boundary conditions. While globally sea levels are rising, tectonic deformation and vertical land motion dominate local trajectories, resulting in large spatial variability in relative sea-level rise (RSLR), offsetting SLR in uplifting areas and amplifying it in subsiding areas, where rates exceed 8 mm/yr, rivalling those of the Holocene marine transgression and surpassing global averages. The evolution of coastal landform types in response to boundary controls is examined. Paleo-reconstructions reveal that the Holocene transgression inundated ~2.3 million km² of the Sunda Shelf, fundamentally resetting the land-sea interface of SEA countries. Sea-level fall from the mid-Holocene highstand drove extensive coastal deposition, generating >100,000 km² of lowlands over the past 6,000 years, with progradation rates of 10¹-10² m/yr. These geologically young landscapes now face rapid, multidirectional change under contemporary forcing. Coastal change in SEA is complex, and rather than uniform erosion and inundation, the region exhibits a mosaic of responses, from stability to rapid progradation and island migration. Contemporary rates of change are comparable to those documented across the Holocene (10² m/yr), but are responding to a broader suite of drivers, including tectonic deformation, variable RSLR, extreme events and intense human activity. Attribution remains elusive, constrained by sparse spatial coverage, uneven representation of landform types and limited temporal resolution of data. Addressing these gaps requires comprehensive, site-specific studies of both geological and contemporary dynamics across all coastal typologies, supported by high-resolution monitoring and integrated modelling. Such efforts will provide the empirical foundation needed for informed coastal management and adaptation strategies in one of the world's most vulnerable regions.