The importance of underestimated local vertical land motion component in sea-level projections: A case study from the Oka estuary, northern Spain

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Vertical land motion (VLM) is an important component in relative sea-level (RSL) projections, especially at regional to local scales and over the short to medium term. However, VLM is difficult to derive because of a lack of long-term instrumental records (e.g., GPS, tide gauge). Geological data offer an alternative, revealing RSL histories over thousands of years that can be compared with glacial isostatic adjustment (GIA) models to isolate VLM.Here, we present a case study from the Oka estuary, northern Spain. We apply two GIA models for the Atlantic coast of Europe with different ice model inputs (ICE-6G_C and ANU-ICE) but the same 3D Earth model. Both models fit well with the late Holocene RSL data along the Atlantic coast of Europe, with misfit statistics < 1.5, except the Oka estuary region, where both models show notable misfits with misfit statistics > 4.5. The significant misfits of both models in the Oka estuary region are indicative of local subsidence. The nearby GPS (station SOPU) with 15 years records shows a VLM rate of -0.96 ± 0.57 mm/yr (subsiding) compared to -0.15 ± 0.40 mm/yr to -2.48 ± 0.37 mm/yr elsewhere along the Atlantic coast of Europe. The VLM rate of SOPU accounts for the misfit between the GIA models and late Holocene RSL data, which decreases by ~90% from > 4.5 to ~0.5 after the subsidence correction of the late Holocene RSL data. The VLM rate incorporated in IPCC AR6 projections in Oka estuary is ~0.18 mm/yr (uplifting), which is contradictory in direction. Therefore, the projected sea-level rise rate is underestimated by 19 - 25% by 2030, 14 - 20% by 2050 and 9 - 26% by 2100 under the five Shared Socioeconomic Pathway (SSP) scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5). Our study indicates the importance of considering local/regional VLM component in sea-level projections.