How waves are accelerating global coastal overtopping 1

17 The world’s coastal areas are home to about 10% of the human population and support unique and dynamic ecosystems, offering € 18 trillions worth of environmental and societal benefits. Climate change and anthropogenic pressures are however exacerbating 19 devastating hazards such as episodic coastal flooding, the magnitudes of which remain highly uncertain to date. This study, for the first 20 time, presents global scale coastal overtopping estimates, which account for not only the effects of sea level rise, storm surge and wave 21 setup as traditionally done, but also that of wave runup and existing coastal protection measures. While the latter are widely recognized 22 as important determinants of episodic coastal flooding, they have hitherto been ignored in assessments thereof. Our results show that 23 the combination of tides and large wave runup events is the main contributor to episodic coastal overtopping. The Gulf of Mexico, 24 northern Europe, Mediterranean region, east coast of Africa, south east Asia, and north western Australia emerge as hotspots of episodic 25 coastal overtopping under the current climate. Future projections of overtopping with the the global mean sea level rise under “business26 as-usual” scenario RCP 8.5 indicate that the globally integrated number of annual overtopping hours will increase at a rate faster than 27 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 28 acceleration in coastal overtopping should be starting about now and will be clearly discernible by about 2050. Global overtopping has 29 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 30 projections presented here are anticipated to lay a solid foundation for the development of effective climate adaptation measures at 31 the identified hotspots, ideally through detailed local scale studies. 32 Non-peer reviewed EarthArXiv preprint 2/16 Context 33 Coastal flooding is threatening human societies (Hinkel et al., 2014; Vousdoukas et al., 2018) and infrastructures (Koks et al., 34 2019) and sea level rise is expected to exacerbate the situation in the decades to come. Over the next few decades, sea level 35 rise is projected to double the frequency of coastal flooding (Vitousek et al., 2017) possibly affecting an estimated global 36 population of nearly 1 billion people (Nicholls and Small, 2002; Neumann et al., 2015; Kulp et al., 2019). Regions with limited 37 water-level variability, i.e., short tailed flood-level distributions, that are located mainly in the Tropics, are likely to be the 38 most affected (Vitousek et al., 2017). In particular, the low-lying coasts of Africa and Asia are thought to be the most 39 vulnerable areas worldwide, at which an increase in flood occurrence could force population migration (Nicholls and 40 Cazenave, 2010). Without appropriate flood mitigation strategies, sea level rise will increase the frequency and magnitude 41 of flooding events (Tebaldi et al., 2012; Vitousek et al., 2017; IPCC report, 2018). 42 Total water level (EWL) at the coast results from several contributions (Figure 1). The main contributors differ according 43 to time scales and region; they include contributions by regional sea level ocean governed by the steric effect and circulation 44 (here referred as SLA), storm surge due to atmospheric pressure and winds (DAC), astronomical tide (AT) and wave effects 45 here named runup (R) including a time-averaged component (setup) and an oscillatory component (swash) (see Melet et al., 46 2018). 47 48 TWL = SLA + DAC + AT + R (1) 49 Despite the important role that ocean waves play in determining total water level at the coast (Beetham and Kench, 5