What does it mean when the 2022 Interagency Technical Report says there will be a “flood regime shift” by 2050?

The 2022 technical report considers extreme water levels that span from rare events (1% annual chance of occurring) to more common events (10 times/year). Specifically, the extreme water levels are used to assess current and future flood exposure within the coastal floodplain while considering future sea level rise using NOAA’s height-severity categories of minor, moderate, and major high tide flooding. NOAA high tide flooding thresholds broadly define water levels where U.S. infrastructure becomes impacted. High tide flooding heights are calibrated to impact levels used in weather forecasting to trigger emergency responses, and are considered the best tangible way to communicate the impacts of extreme water levels to the public today and into the future. Minor high tide flooding, flooding about 2 feet (0.6 meters) above average high tide, is disruptive to communities where it occurs (e.g., stormwater backups and road closures). Moderate flooding, about 3 feet (0.9 meters) above average high tide, tends to cause more damage (e.g., to homes or businesses). Major flooding, which is about 4 feet (1.2 meters) above average high tide, is often quite destructive, requiring post-event repairs/rebuilding and sometimes evacuations.

The 2022 report explores how the annual frequencies of high tide flooding events are expected to change by 2050 if considering the local sea level rise scenario most aligned with the rise extrapolated from regional observations. . A flood regime shift is a concept used to describe how the annual flood frequency associated with a particular coastal flood type (i.e., NOAA minor, moderate, and major high tide flooding) changes to that of another because of sea level rise. For example, by 2050 the average annual frequency of NOAA moderate high tide flooding events along the U.S. coastline is expected to be greater than the average annual frequency of NOAA minor high tide flooding events occurring today. Put another way, with the average 1 foot (0.3 meters) of sea level rise that is expected to occur along the U.S coastline, tides and storm surge that cause minor and moderate high tide flooding today will cause moderate and major high tide flooding.

See the National Sea Level Explorer: Flooding Impacts for any location or Section 3.1 of the technical report for more information.