Assessing Current Coastal Subsidence at Continental Scale: Insights From Europe Using the European Ground Motion Service

Beside climate-change-induced sea-level rise (SLR), land subsidence can strongly amplify coastal risk in flood-prone areas. Mapping and quantifying contemporary vertical land motion (VLM) at continental scales has long been a challenge due to the absence of gridded observational products covering these large domains. Here, we fill this gap by using the new European Ground Motion Service (EGMS) to assess the current state of coastal VLM in Europe. First, we compare the InSAR-based EGMS Ortho (Level 3) with nearby global navigation satellite systems (GNSS) vertical velocity estimates and show that the geodetic reference frame used to calibrate EGMS strongly influences coastal vertical land velocity estimates at the millimeter per year level and this needs to be considered with caution. After adjusting the EGMS vertical velocity estimates to a more updated and accurate International Terrestrial Reference Frame (ITRF2014), we performed an assessment of VLM in European low elevation coastal flood plains (CFPs). We find that nearly half of the European CFP area is, on average, subsiding at a rate faster than 1 mm/yr. More importantly, we find that urban areas and populations located in the CFP experience a near -1 mm/yr VLM on average (excluding the uplifting Fennoscandia region). For harbors, the average VLM is even larger and increases to -1.5 mm/yr on average. This demonstrates the widespread importance of continental-scale assessments based on InSAR and GNSS to better identify areas at higher risk from relative SLR due to coastal subsidence. Land subsidence increases the risk of flooding in low-lying coastal zones by amplifying relative sea-level rise (SLR). In this study, we assess for the first time current coastal land subsidence at the scale of Europe using the new Copernicus European Ground Motion Service (EGMS) that was released in 2022. Our results suggest that nearly half of the low-lying coastal areas in Europe are currently subsiding at a rate faster than 1 mm/yr on average. We find that coastal subsidence is higher on average in areas hosting more people, urban centers and critical infrastructure. This raises concerns that coastal subsidence, and therefore relative SLR, tends to be underestimated in Europe and presumably in many other regions around the world. Our study demonstrates that emerging continental-scale land motion services such as EGMS are useful to better characterize the issue and anticipate coastal risks and adaptation accordingly. Continental-scale coastal land motion assessment in Europe is performed based on the European Ground Motion Service released in 2022 European flood-prone coastal cities subside on average at 1 mm/yr (Fennoscandia excluded) The geocentric reference frame used to calibrate continental-scale land motion affects the results for geodetic (non-geophysical) reasons