Coevolution of Extreme Sea Levels and Sea-Level Rise Under Global Warming

Design of coastal defense structures like seawalls and breakwaters can no longer be based on stationarity assumption. In many parts of the world, an anticipated sea-level rise (SLR) due to climate change will constitute present-day extreme sea levels inappropriate for future coastal flood risk assessments since it will significantly increase their probability of occurrence. Here, we first show that global annual maxima sea levels (AMSLs) have been increasing in magnitude over the last decades, primarily due to a positive shift in mean sea level (MSL). Then, we apply non-stationary extreme value theory to model the extremal behavior of sea levels with MSL as a covariate and quantify the evolution of AMSLs in the following decades using revised probabilistic sea-level rise projections. Our analysis reveals that non-stationary distributions exhibit distinct differences compared to simply considering stationary conditions with a change in location parameter equal to the amount of MSL rise. With the use of non-stationary distributions, we show that by the year 2050 many locations will experience their present-day 100-yr return level as an event with return period less than 15 and 9 years under the moderate (RCP4.5) and high (RCP8.5) representative concentration pathways. Also, we find that by the end of this century almost all locations examined will encounter their current 100-yr return level on an annual basis, even if CO2 concentration is kept at moderate levels (RCP4.5). Our assessment accounts for large uncertainty by incorporating ambiguities in both SLR projections and non-stationary extreme value distribution parameters via a Monte Carlo simulation.Plain Language Summary The gradual increase of global mean sea level (MSL), known as sea-level rise (SLR), is one of the biggest climate change-related concerns of the 21st century. SLR poses a threat to seaside communities and can lead to an increase in the frequency and severity of coastal flooding. Thus, evaluating the future expected number of extreme sea-level events and their magnitudes is critical for sustainable coastal flood risk management. In this work, we show significant increasing trends in extreme sea levels at a "pseudo-global" scale, cross-examined by different trend detection methods. Hence, we model coming coastal flood hazard with the assumption that MSL will continue to rise in the future. This is the first global-scale study that utilizes a physical covariate, that is, MSL for explicit statistical modeling of the evolving coastal flood hazard. By using revised SLR projections that account for uncertainty, we show a substantial increase in coastal flood hazard by end of the century, either under a moderate or a high CO2 concentration pathway.