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 sealevel 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. Sea-level rise is slow, yet consequential 1 and accelerating 2. Upper-end sea-level rise scenarios could displace hundreds of millions of people by the end of the 21 st century 3. However, even small amounts of sea-level rise can disproportionately increase coastal flood frequency 4,5. A multitude of oceanic processes affect both mean and extreme water levels, such as the tide, tropical and extratropical storms, climatic cycles (e.g., El Nino/Southern Oscillation), oceanic eddies, and circulation patterns 6-11. Hence, the frequency and severity of coastal flooding varies on a multitude of time scales. Yet, the persistent trend and acceleration of sea-level rise have a profound interaction with transient extreme events 12. In theory, sea-level rise progressively increases the frequency and severity of flooding 5. In practice, the monotonic increase in flooding, driven by elevating long-term mean sea level, is often overshadowed by interannual variability in extreme events 13 , which will likely continue through the middle of the 21 st century 14. Many have quantified future increases in potential coastal flood frequency by deriving 'multiplying factors' 15 , 'amplification factors' 16 ...
