Reliable sea-level projections are crucial for assessing future coastal hazards, particularly in Southeast Asia, where densely populated low-lying regions are highly vulnerable. However, a lack of robust vertical land motion (VLM) estimates across the region hinders the accuracy of these projections.

Here, we estimate rates of vertical land motion (VLM) for 15 coastal cities across Southeast Asia using three complementary datasets: (1) Global Navigation Satellite System observations, where available; (2) newly available tide-gauge records; and (3) satellite altimetry data. We compare these estimates with the VLM component incorporated in the sea-level projections of the Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC AR6). The IPCC AR6 VLM estimates are derived from a limited number of tide-gauge records, which may introduce biases in regional assessments of relative sea-level change.

Our results reveal substantial temporal and spatial variability in VLM across Southeast Asia. Sites along the Malay Peninsula, in particular, experienced an abrupt transition from uplift to subsidence following the 2004/2005 Sumatra earthquakes. Estimated VLM rates range from uplift of 2.17 ± 0.33 mm/yr in Haiphong, Vietnam, to subsidence of 8.79 ± 0.48 mm/yr in Phuket, Thailand. These estimates differ markedly from the VLM signals used in IPCC AR6, indicating potential biases in existing sea-level projections that range from an overestimation of approximately 9 mm/yr in Bangkok, Thailand, to an underestimation of approximately 11 mm/yr in Cebu, Philippines. We incorporate our updated VLM estimates into probabilistic sea-level projections and assess the implications of relative sea-level rise for these 15 coastal cities. Our study underscores the need for accurate estimation of VLM for regional and local sea-level projections, to support regional and local adaptation planning.
 

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