finding 12.4 : key-finding-12-4

As sea levels have risen, the number of tidal floods each year that cause minor impacts (also called “nuisance floods”) have increased 5- to 10-fold since the 1960s in several U.S. coastal cities (very high confidence). Rates of increase are accelerating in over 25 Atlantic and Gulf Coast cities (very high confidence). Tidal flooding will continue increasing in depth, frequency, and extent this century (very high confidence).

This finding is from chapter 12 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: This key finding is based upon several studies finding historic and projecting future changes in high-water probabilities for local-specific elevation thresholds for flooding.

Description of evidence base: Sweet et al.bbf3043e-9999-4f0e-8d0c-6012450d9d84 examined 45 NOAA tide gauge locations with hourly data since 1980 and Sweet and Park91aeffdb-e82f-4645-abe9-f6ea6909e979 examined a subset of these (27 locations) with hourly data prior to 1950, all with a National Weather Service elevation threshold established for minor “nuisance” flood impacts. Using linear or quadratic fits of annual number of days exceeding the minor thresholds, Sweet and Park91aeffdb-e82f-4645-abe9-f6ea6909e979 find increases in trend-derived values between 1960 and 2010 greater than 10-fold at 8 locations, greater than 5-fold at 6 locations, and greater than 3-fold at 7 locations. Sweet et al.,bbf3043e-9999-4f0e-8d0c-6012450d9d84 Sweet and Park,91aeffdb-e82f-4645-abe9-f6ea6909e979 and Ezer and Atkinsonb072d10e-db78-421e-a708-e2bdcb25de6e find that annual minor tidal flood frequencies since 1980 are accelerating along locations on the East and Gulf Coasts (>25 locationsbbf3043e-9999-4f0e-8d0c-6012450d9d84) due to continued exceedance of a typical high-water distribution above elevation thresholds for minor impacts.

Historical changes over the last 60 years in flood probabilities have occurred most rapidly where RSL rates were highest and where tide ranges and extreme variability is less (Sweet and Park 2014). In terms of future rates of changes in extreme event probabilities relative to fixed elevations, Hunter,158c0943-c768-4c4f-a4ca-e98108f93109 Tebaldi et al.,be9c98a9-915f-4d78-80b9-239b70f93ae2 Kopp et al.,38924fa0-a0dd-44c9-a2a0-366ca610b280 Sweet and Park91aeffdb-e82f-4645-abe9-f6ea6909e979 and Sweet et al.c66bf5a9-a6d7-4043-ad99-db0ae6ae562c all find that locations with less extreme variability and higher RSL rise rates are most prone.

New information and remaining uncertainties: Minor flooding probabilities have been only assessed where a tide gauge is present with >30 years of data and where a NOAA National Weather Service elevation threshold for impacts has been established. There are likely many other locations experiencing similar flooding patterns, but an expanded assessment is not possible at this time.

Assessment of confidence based on evidence: There is very high confidence that exceedance probabilities of high tide flooding at dozens of local-specific elevation thresholds have significantly increased over the last half century, often in an accelerated fashion, and that exceedance probabilities will continue to increase this century.

This finding was derived from figure -.2: Confidence / Likelihood

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