Sea-level rise exponentially increases coastal flood frequency

TLDR

Sea‑level rise, projected to reach 0.5–2 m by 2100, will transform coastal flooding regimes, turning rare extreme events into common occurrences and posing major risks to coastal communities worldwide. The study aims to quantify the continuous rate at which extreme water‑level events increase as sea level rises. The authors calculate continuous growth rates of extreme flooding by analyzing how sea‑level rise elevates the frequency of threshold‑exceeding water levels. The analysis shows that sea‑level rise causes the odds of exceeding critical water‑level thresholds to increase exponentially, with 1–10 cm of rise doubling flood odds, leading to a doubling every five years, and projecting that the current 50‑year flood level will be exceeded annually in most U.S.

Abstract

Abstract Sea-level rise will radically redefine the coastline of the 21 st century. For many coastal regions, projections of global sea-level rise by the year 2100 (e.g., 0.5–2 meters) are comparable in magnitude to today’s extreme but short-lived increases in water level due to storms. Thus, the 21 st century will see significant changes to coastal flooding regimes (where present-day, extreme-but-rare events become common), which poses a major risk to the safety and sustainability of coastal communities worldwide. So far, estimates of future coastal flooding frequency focus on endpoint scenarios, such as the increase in flooding by 2050 or 2100. Here, we investigate the continuous shift in coastal flooding regimes by quantifying continuous rates of increase in the occurrence of extreme water-level events due to sea-level rise. We find that the odds of exceeding critical water-level thresholds increases exponentially with sea-level rise, meaning that fixed amounts of sea-level rise of only ~1–10 cm in areas with a narrow range of present-day extreme water levels can double the odds of flooding. Combining these growth rates with established sea-level rise projections, we find that the odds of extreme flooding double approximately every 5 years into the future. Further, we find that the present-day 50-year extreme water level (i.e., 2% annual chance of exceedance, based on historical records) will be exceeded annually before 2050 for most (i.e., 70%) of the coastal regions in the United States. Looking even farther into the future, the present-day 50-year extreme water level will be exceeded almost every day during peak tide (i.e., daily mean higher high water) before the end of the 21 st century for 90% of the U.S. coast. Our findings underscore the need for immediate planning and adaptation to mitigate the societal impacts of future flooding.