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Rapid increases and extreme months in projections of United States high-tide flooding


Coastal locations around the United States, particularly along the Atlantic coast, are experiencing recurrent flooding at high tide. Continued sea-level rise (SLR) will exacerbate the issue where present, and many more locations will begin to experience recurrent high-tide flooding (HTF) in the coming decades. Here we use established SLR scenarios and flooding thresholds to demonstrate how the combined effects of SLR and nodal cycle modulations of tidal amplitude lead to acute inflections in projections of future HTF. The mid-2030s, in particular, may see the onset of rapid increases in the frequency of HTF in multiple US coastal regions. We also show how annual cycles and sea-level anomalies lead to extreme seasons or months during which many days of HTF cluster together. Clustering can lead to critical frequencies of HTF occurring during monthly or seasonal periods one to two decades prior to being expected on an annual basis.

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Fig. 1: Projections of annual counts of HTF days for the NOAA Intermediate SLR scenario.
Fig. 2: Impact of the nodal cycle.
Fig. 3: YOIs for the NOAA Intermediate SLR scenario.
Fig. 4: Extreme months and seasons.
Fig. 5: Years for which US coastal locations will experience HTF on a majority of days during annual and monthly windows.

Data availability

The tide-gauge sea-level data used in this analysis are publicly accessible and were obtained from the NOAA CO-OPS Data Retrieval API ( The NOAA SLR scenarios are publicly available and were obtained from the NOAA CO-COPS website (

Code availability

All code generated for the data analysis and figure creation is archived in a public repository47 under the GNU Affero General Public License v.3.0. The repository includes the Python environment, which provides the versions of all third-party libraries and packages used in this work.


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P.R.T. acknowledges support from NASA grant no. 80NSSC17K0564 and NOAA grant no. NA16NMF4320058. M.J.W. acknowledges support from NOAA grant nos NA17OAR4310110 and NA19OAR4310292. B.D.H. acknowledges support from the NASA Sea-Level Change Team (N-SLCT, WBS no. 105393).

Author information




P.R.T. designed the approach, performed the analyses and drafted the paper. M.J.W., B.D.H. and M.A.M. made substantive revisions. All authors made substantive contributions to the interpretation and communication of the results.

Corresponding author

Correspondence to Philip R. Thompson.

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The authors declare no competing interests.

Additional information

Peer review information Nature Climate Change thanks Kristina Dahl, Ben Hague and Hamed Moftakhari for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Projections of annual counts of high-tide-flooding (HTF) days compared to expectations from SLR and tides alone.

The four ensemble projections (blue) are identical to Fig. 1. The simple projection of HTF frequency based only on the SLR scenario and predictions of astronomical tides (gray) underestimates the frequency of HTF due to the exclusion of local mean sea level variability across a variety of time scales from high-frequency surge to decadal climate variability.

Extended Data Fig. 2 Years of inflection (YOIs) for the NOAA Intermediate Low SLR scenario.

The upper and lower panels correspond to the NOAA Minor and Moderate flooding thresholds, respectively. Position along the horizontal axis corresponds to the timing of the YOI. The vertical axis is projected ten-year increases in annual counts of HTF days following YOIs. Marker size corresponds to ten-year multipliers following the YOIs. Color denotes geographic region.

Extended Data Fig. 3 Projected changes in the seasonal cycle of HTF frequency.

Projections correspond to the YOI (blue) and 10 years later (orange) for the four US locations in Fig. 1 assuming the NOAA Intermediate SLR scenario. Shading shows the 10th–90th percentile intervals for each year and month.

Extended Data Fig. 4 Years for which U.S. coastal locations will experience HTF on a majority of days during annual and monthly windows.

Calculations assume the Intermediate Low SLR scenario. Years for which HTF is expected to occur on a majority of days on average during annual and monthly periods (top two rows) are compared to years for which flooding will first occur on a majority of days during a single month (bottom two rows). Marker colors denote station region. The vertical position of each marker within the rows is an arbitrary vertical offset to allow visual distinction between regions and individual locations.

Extended Data Fig. 5 Relationships between Δ99 and monthly counts of HTF days.

Examples correspond to the observed (gray and black) and fitted (orange and red) relationships for the month of January in (a) Honolulu and (b) Boston.

Supplementary information

Supplementary Data

Station metadata, NOAA derived flooding thresholds and YOIs for 89 US stations. The columns labelled 10A and 10M list the absolute changes and relative changes (that is, ten-year multipliers) in annual counts of HTF days during decades following the YOIs.

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Thompson, P.R., Widlansky, M.J., Hamlington, B.D. et al. Rapid increases and extreme months in projections of United States high-tide flooding. Nat. Clim. Chang. 11, 584–590 (2021).

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