Coastal sea level higher than assumed in most sea-level rise impact assessments: Revealing a methodological blind spot

The world’s coasts face the increasing risk of relative sea-level rise due to climate-induced sea-level rise and negative vertical land motion (i.e. land subsidence).  The impacts of relative sea-level rise and coastal (and compound) flooding are closely related to the land’s elevation relative to sea level. Consequently, the reliability of relative sea-level rise impact and flood exposure assessment heavily relies on the correct alignment of land elevation data and sea-level information. However, this is often not the case.

Based on a systematic evaluation of the scientific literature (385 studies), we found that over 90% of contemporary sea-level rise and coastal hazard impact assessments do not apply sea-level information in addition to land elevation data and therefore fail to properly align land elevation to observed coastal sea level. From the 10% of the assessments that combined sea-level and land elevation data, 9% contain incomplete methodological documentation (rendering the study irreproducible) and/or contain flaws in vertical datum conversion and dataset combination. Less than 1% properly align sea level and land elevation and provide full methodological documentation. Our meta-analyses revealed sea-level height to be globally on average 0.3 m higher than commonly assumed, with a disproportionate impact on the Global South and differences of more than 1 m in most affected regions in the Indo-Pacific. This translates into worldwide 37% more land and up to 68% more people exposed to a 1 m relative sea-level rise. As many of the reviewed studies inform policy reports (e.g. IPCC reports), the widespread underestimation of coastal exposure may have far-reaching implications for policymaking and coastal adaptation.

Our findings reveal a community-wide methodological blind spot which calls for systemic, cross-disciplinary changes. To overcome the methodological challenges to properly align coastal land and sea-level information and prevent future errors, we provide properly combined coastal elevation information referenced to local sea level. To ensure proper data integration and reproducibility of coastal impact assessments, we also recommend to introduce author declarations and review checklists into the scientific peer-review process. These actions will raise community-wide awareness on the current blind spot, prevent future error propagation and improve transparency and reproducibility of impact studies. This will lead to improved future sea-level rise and other coastal hazard assessments and strengthen the scientific information available for policy-informing reports, like the upcoming IPCC AR7 reports.