Coastal populations experience sea level rise at least twice as large as the global average

Abstract

Coastal subsidence can significantly increase rates of relative sea level change over and above climate-related changes, and has been reported to be particularly large in densely populated regions. However, due to the small scale variability of vertical land motion (VLM) and the sparsity of VLM observations, there is currently still low confidence in VLM and the uncertainties this introduces in sea level change estimates. Therefore, we synergize currently available VLM observations (from GNSS, InSAR, tide gauges, and satellite altimetry) to investigate the impact of VLM on relative sea level changes and their uncertainties, as well as the implications for coastal populations. We find that the average contemporary (1995-2019) global relative sea level change experienced by coastal populations (5.14 mm/year) is more than twice as large as the average coastal relative sea level change (2.09 mm/year). This is because rates of subsidence tend to be higher in densely populated areas. A significant part of the coastline (representing 40% of the coastal population) lacks measurements or access to the direct measuring stations which are necessary to constrain VLM rates estimated with InSAR. Missing, or imprecise VLM constraints contribute to the increased population-weighted uncertainties in the relative sea level change trends of ~3 mm/year. Thus, future community efforts are needed to improve the observational database in the disproportionately exposed and densely populated coasts in order to reduce uncertainties in estimates of future relative sea-level rise and their societal implications.