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

The 2022 technical report consider 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 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 today and in the future to the public. 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), whereas moderate flooding, about 3 feet (0.9 meters) above average high tide, tends to cause more damage (e.g., to homes or businesses) and 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 explore how the annual frequencies of high tide flooding are expected to change by 2050 considering the local sea level rise scenario that closely aligns with the rise associated with the regional observation-based extrapolations. The concept of a flood regime shift is 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 annual frequency of NOAA moderate high tide flooding is expected to occur on average along the U.S coastline at a frequency greater than the NOAA minor high tide flooding events occur today. Or put another way, after about 1 foot (0.3 meters) of sea level rise that is expected to occur on average along the U.S coastline, tides and storm surges that today cause minor and moderate high tide flooding 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.