Vertical land motion component is underestimated in sea-level projections from northern Spain

Abstract

Abstract Coastal populations are susceptible to relative sea-level (RSL) rise and accurate projections are necessary for coastal adaptation. Local RSL rise may deviate from the global mean sea-level rise because of vertical land motion (VLM) but this is often inadequately estimated. Here, we estimated the VLM for the Oka estuary, northern Spain and compared it to the VLM component of sea-level projections in the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) and the Spanish National Climate Change Adaptation Plan (NCCAP). To estimate VLM, we updated the late Holocene RSL data from the Atlantic coast of Europe and compared it with two 3D glacial isostatic adjustment (GIA) models. Both models fit well with RSL data except in the Oka estuary. We derived a VLM rate of -0.96 mm/yr for the Oka estuary using the residuals of GIA misfits and further validated this rate with nearby Global Navigation Satellite Systems, satellite altimetry and tide gauge data. Incorporating the updated estimate of VLM in IPCC AR6 RSL projections under a moderate emissions scenario increased the rate of RSL rise by 14.4% by 2030, 10.6% by 2050, and 9.9% by 2150 compared to the original IPCC AR6 projections, and also increased the magnitude of RSL rise by ~ 60% by 2035 and 2090 compared with projections from the Spanish NCCAP. Our study demonstrates the potential and capability of late Holocene RSL data to constrain the VLM component of sea-level projections.