Sea-level rise and time horizons for coastal infrastructure

Sea-level rise (SLR) will redefine the coastlines of the 21st century. For most coastal regions, global sea-level projections by the year 2100 are comparable in size to today’s extreme, but short-lived water-level events due to storms. Thus, the 21st century will see dramatic changes to coastal flooding regimes. So far, estimates of increased coastal flood frequency focus on endpoint scenarios, such as factors of increased flooding by 2050 or 2100. Here, we investigate the continuous shift in coastal flooding regimes by quantifying growth rates in the exceedance probability of extreme water levels. We find that the flood hazard potential increases exponentially with SLR, meaning that fixed amounts of SLR of approximately 1-10 cm double the chance of exceeding extreme water-level thresholds. Integrating these growth rates with established SLR projections, we find that the chance of extreme flooding doubles approximately every 5 years. Extrapolating this rate of growth into the future, we find that the present day 50-year extreme water level will be exceeded annually before 2050 for most US coastal regions. Looking farther into the future, the present day 50-year extreme water level will be exceeded during every high tide before 2100. These findings underscore the need for immediate adaptation planning.