Coastal sea level higher than assumed in most hazard assessments: Implications for coastal resilience and policy

The world’s low-lying and densely populated coasts are at risk of climate-induced sea-level rise from climate change and accelerating coastal subsidence. Sustainable coastal adaption strategies need adequate coastal land and population exposure assessments for coastal hazards like storm surges, coastal (compound) flooding and future relative sea-level rise. The accuracy of coastal impact exposure assessments strongly depends on the proper alignment of both land elevation and sea level data, referenced to a common vertical datum. Shortcomings in this alignment result in incorrect assessment of contemporary coastal sea-level height, which consequently introduces errors into coastal exposure and risk assessments.

Here we unravel shortcomings and errors in the fundamental aspect of vertical datum alignment in most contemporary sea-level rise and coastal hazard impact assessments based on a systematic scientific literature evaluation. More than 99% of the evaluated assessments handled sea-level and land elevation data inadequately and/or contained shortcomings in the methodological documentation, leading to systematic underrepresentation of contemporary coastal sea level. Our meta-analyses on global and regional scales revealed that globally coastal sea-level height is on average 0.3 m higher than often (>90%) assumed, with a disproportionate impact on the Global South and differences of more than 1 m in most affected regions in the Indo-Pacific. We find that worldwide 37% more land and up to 68% more people will fall below sea level following a 1 m relative sea-level rise than frequently assumed, implying the necessity for a potentially much sooner implementation of coastal adaptation strategies.  Many reviewed studies informed policy reports (e.g., IPCC AR6), which may have led to misjudged coastal exposure and risk. To improve future coastal hazard and impact assessments, we provide ready-to-use combined products of land elevation and coastal sea level. We also recommend adding dedicated author declarations and review checklists into the scientific peer-review process to catch errors before publication and uphold scientific standards. Applying these measures in policy reports (including IPCC assessments) will enable verification of methodological robustness of cited coastal-hazard studies, strengthening the reliability of scientific evidence (e.g. global climate risk rankings) that informs policy and underly UN-level discussions (e.g. funding priorities, or Loss and Damage negotiations).