Past and future coastal flooding in Pacific Small-Island Nations: insights from the Pacific Sea Level and Geodetic Monitoring (PSLGM) Project tide gauges

Abstract

Sea level rise is increasing the frequency of coastal flooding globally, and low-lying communities are particularly vulnerable. We present an assessment of historical and projected changes in coastal flooding in 11 Pacific small-island nations, using tide gauge data from the Australian Bureau of Meteorology. We derive impact-, event- and percentile-based thresholds to calculate historical exceedance frequencies. Projections of future exceedance frequencies are then made using the recent suite of Shared Socioeconomic Pathways (SSPs) emission scenarios (Sixth Assessment Report). We find that exceedances of the percentile thresholds have increased in the last decade at all locations, with sites seeing exceedances in months where exceedances were previously rare or unseen in the sea level record. In the future, daily threshold exceedances occur after 50–115 cm of sea level rise, depending on location. Such levels are currently projected to be reached between 2080 and 2130 according to high emissions scenario SSP5–8.5. Low emissions scenario, SSP1–1.9, shows sea level rise resulting in 25–75 days of exceedances by 2050 for the 11 locations. This increased frequency of coastal flooding highlights the changing nature of coastal flood risk in the Pacific, with extreme weather and wave events being increasingly unnecessary for inundation to occur. Further, this work highlights how underlying increases in coastal flooding frequency pose a growing risk of exacerbating inundation associated with extreme weather or waves. Better flood monitoring and reporting will improve the accuracy of impact thresholds, strengthening the relevance of the results presented here for coastal emergency and planning managers.