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EXECUTIVE SUMMARY
One of the most tangible signs of sea level rise is the increased frequency of high tide flooding
(HTF) occurring along the U.S. coastlines. HTF impacts coastal infrastructure and natural
systems (e.g., flooding of storm and wastewater systems, roadways, commercial/private
property, sandy beaches, and marshes) and typically begins when water levels exceed about
0.5 meter (1.75 feet) above high tide as measured by the National Oceanic and Atmospheric
Administration (NOAA) tide gauges. HTF levels are nationally calibrated against NOAA’s
National Weather Service and local emergency managers’ depth-severity thresholds used in
weather forecasting and impact communications to provide a consistent coastal-climate
resiliency standard. HTF damages infrastructure and creates other economic and ecosystem
impacts within coastal communities, which are largely responsible for finding and funding
mitigative solutions.
NOAA’s National Ocean Service provides annual reports (this is the seventh of an annual
series
1
) tracking historical HTF frequency changes, mapping current exposure footprints, and
providing next-year outlooks and multi-decadal projections of flood frequency changes. Decadal
projections are based upon a range of relative sea level (RSL) rise considered more likely to
occur based upon the Fourth U.S National Climate Assessment.
2
These reports are designed to
inform the public of the accelerating upward trajectory of climate-driven coastal flooding, and
support partners and coastal communities as they plan and make decisions regarding increasingly
disruptive and expensive impacts.
In 2020 (May 2020–April 2021), U.S. coastlines continued to experience HTF at a rate
(4 days/year—national median) that is twice what it was just 20 years ago. In total, 14 locations
tied or broke HTF records along the U.S. Southeast Atlantic and Gulf coastlines due to a
combination of effects from a record-breaking hurricane season and/or continued RSL rise (12
locations tied/set records for RSL levels). For perspective, as compared to HTF frequencies
typical in 2000, these regions experienced a (median values) 400–1100% increase in 2020 (e.g.,
14 and 22 days for Charleston, S.C. and Bay Waveland, Miss. in 2020 versus 2 and 3 days in
2000, respectively). Over 80% of East and Gulf Coast tide gauge locations are now experiencing
annual acceleration (nonlinear rise) in HTF frequencies with most others linearly increasing.
For 2021 (May 2021–April 2022), continued acceleration in HTF and its impacts are expected
with 3–7 days (national median) of HTF likely under near-neutral conditions of the El Niño
Southern Oscillation. The 2021 HTF Outlook is 7–15 days along the Western Gulf, 6–11 days
along the Northeast Atlantic, 3–7 days along the Southeast Atlantic and Eastern Gulf, and
upwards of 3 and 7 days along the Southwestern and Northwestern Pacific, respectively. No HTF
is likely along Hawaii or the U.S. Pacific or Caribbean coastlines. That said, this outlook does
not discount that local flooding might occur at water levels less than the HTF height threshold
used here, and it does not explicitly account for wave, rainfall, and elevated groundwater effects.
By 2030, the national median HTF frequency rate is likely to increase about 2–3 times (7–15
1
An interactive map of this year’s results is available at
https://tidesandcurrents.noaa.gov/HighTideFlooding_AnnualOutlook.html
2
https://nca2018.globalchange.gov/chapter/appendix-3/