Rafael Almar, Harold Diaz, Erwin W. J. Bergsma, Roshanka Ranasinghe, Angélique Melet, Fabrice Papa, Michalis Vousdoukas, Panagiotis Athanasiou, Olusegun Dada, Luis Pedro Almeida

The world’s coastal areas are home to about 10% of the human population and support unique and dynamic ecosystems, offering € trillions worth of environmental and societal benefits. Climate change and anthropogenic pressures are however exacerbating devastating hazards such as episodic coastal flooding, the magnitudes of which remain highly uncertain to date. This study, for the first time, presents global scale coastal overtopping estimates, which account for not only the effects of sea level rise, storm surge and wave setup as traditionally done, but also that of wave runup and existing coastal protection measures. While the latter are widely recognized as important determinants of episodic coastal flooding, they have hitherto been ignored in assessments thereof. Our results show that the combination of tides and large wave runup events is the main contributor to episodic coastal overtopping. The Gulf of Mexico, northern Europe, Mediterranean region, east coast of Africa, south east Asia, and north western Australia emerge as hotspots of episodic coastal overtopping under the current climate. Future projections of overtopping with the the global mean sea level rise under “business-as-usual” scenario RCP 8.5 indicate that the globally integrated number of annual overtopping hours will increase at a rate faster than that of the global mean sea level rise itself. This study also shows that, under the RCP 8.5 sea level rise trajectory, the projected acceleration in coastal overtopping should be starting about now and will be clearly discernible by about 2050. Global overtopping has increased almost by 1.5 from 1993 by now and will reach values more than 50 times larger by the end of the 21st century. The global projections presented here are anticipated to lay a solid foundation for the development of effective climate adaptation measures at the identified hotspots, ideally through detailed local scale studies.