Climate Matters•September 27, 2023
Rising Seas, Flooding Coasts
KEY CONCEPTS
As sea levels rise, flooding is becoming more common along U.S. coasts, where about 30% of the population lives.
High tide flooding can occur in the absence of heavy rainfall or storms, and the effects can be disruptive, dangerous, and costly for coastal communities.
The annual frequency of high tide flooding in the U.S. has more than doubled since 2000 — and is projected to more than triple again by 2050 as sea levels continue to rise.
According to NOAA outlooks for 2023, the annual risk of high tide flooding is expected to peak during late September and October in many locations along the East and Gulf Coasts.
Coastal communities face more floods
In our warming climate, it no longer takes a strong storm to flood streets, homes, businesses, and ecosystems along the coast.
High tide flooding — also called nuisance flooding or sunny day flooding — is becoming more common in the U.S., where 29% of the population lives in coastal counties.
As global sea levels rise, water levels along the coast rise too. As daily tides cycle over higher baseline water levels, high tide lines migrate farther inland or upslope — regularly flooding coastal areas that previously would have flooded only during storms or extreme rainfall events.
High tide flooding happens when water levels exceed 1-2 feet above the daily average high tide (the threshold for minor high tide flooding).
High tide flooding: disruptive, dangerous, and costly
Such flooding can clog storm water drains, flood streets, inundate coastal infrastructure and ecosystems, and affect freshwater supplies.
These impacts worsen during so-called king tides, the highest astronomical tides of the year. Flood risks compound if extreme high tides also overlap with heavy rains or storms.
Earlier this year, NOAA released a new interactive Monthly High Tide Flooding Outlook that shows how likely high tide flooding is on each day of the year at 98 coastal U.S. locations.
According to NOAA outlooks for 2023, the annual risk of high tide flooding is expected to peak during late September and October in many locations along the East and Gulf Coasts.
Warming planet, rising seas, flooding coasts
The oceans absorb 90% of the extra heat caused by carbon pollution. As oceans heat up, the volume of seawater expands — causing sea levels to rise. Melting glaciers and ice sheets also contribute to rising sea levels.
Global mean sea level rose 7.8 inches from 1901 to 2018, according to the latest IPCC reports. Sea levels have been rising faster since the late 1960s, and human activities are the primary cause.
Sea level rise is accelerating along U.S. coasts. According to NOAA, sea levels along U.S. coasts are expected to rise as much over the next 30 years (10-12 inches, on average) as they did over the last 100 years.
Rising sea levels have contributed to the increased frequency of coastal flooding in the U.S. — especially in the last two decades.
More high tide flood days in the U.S.
The annual frequency of U.S. high tide flooding has more than doubled since 2000. Flood frequency is projected to more than triple again by 2050 (relative to 2020) to reach a national average of 45 to 85 flood days per year.
The Northeast, Southeast, and Gulf Coasts have experienced the largest historical increase in high tide flooding.
Last year, eight U.S. locations saw record or record-breaking numbers of high tide flood days, including in Florida, Puerto Rico, Virginia, Georgia, and Washington state.
A strengthening El Niño could further boost coastal flooding during 2023-2024 along both the East and West Coasts due to regional effects on storms and sea level.
Coastal flooding: not only a coastal issue
The latest science is clear: we can expect more coastal flooding far into the future, due to rising emissions causing rising sea levels.
In the absence of measures to both rapidly curb emissions and reduce risk, coastal flood impacts are likely to become increasingly damaging and destructive.
People, ecosystems, and infrastructure along the coast will be directly affected by flooded homes, streets, and businesses, erosion, stormwater overflows, and reduced water quality.
But such impacts along U.S. coasts can have far-reaching effects.
U.S. coasts are essential for trade and supply chains we all depend on. They’re home to large and growing populations. Recreation, tourism, and fisheries are all thriving coastal industries. And ecosystems such as wetlands provide coastal protection, wildlife habitat, and carbon storage.
Thus, the entire country is directly or indirectly affected by the inundation of our coasts as sea levels continue to rise.
Reversing recent trends ultimately requires rapid and sustained emissions cuts. But even under low-emissions scenarios, sea levels will continue to rise.
Ways that coastal communities can reduce risk and enhance resilience to future flooding include raising roads, stabilizing shorelines, relocating low-lying buildings, and restoring coastal ecosystems.
LOCAL STORY ANGLES
What’s the flooding outlook in your local area?
NOAA’s Annual and Monthly High Tide Flooding Outlooks show when and where high tide flooding is most likely in 98 coastal U.S. stations, to guide seasonal planning and preparedness. The Annual Outlook also provides local high tide flooding projections for 2050. Both outlooks are integrated into NOAA’s Coastal Inundation Dashboard which provides 48-hour forecasts and local images of recent floods. The National Weather Service provides real time coastal flood watches, warnings, and advisories.
What’s at risk in your nearest coastal area?
Use NOAA’s Coastal County Snapshots to explore, visualize, and download data about flood hazards, sea level rise, and the marine and coastal economy in 850+ coastal counties across 37 coastal states and territories. Go deeper with NOAA’s Coastal Flood Exposure Mapper to locate people, infrastructure, and ecosystems exposed to flood hazards along U.S. coasts. Use Climate Central’s Risk Finder and Picturing Our Future tools to explore local risk and visualize how 190 landmark sites would be affected under different warming scenarios.
How are sea levels and coastal floods changing in your area?
The Interagency Sea Level Rise Scenario Tool from NASA and NOAA provides sea level rise projections for each decade through 2150 for: the entire U.S., major coastal regions, and individual tide gauge stations along the coast. Use The Climate Explorer, part of the U.S. Climate Resilience Toolkit, to chart past and future high tide flooding in your city. Climate Central’s Coastal Risk Screening Tool can be used to map areas at risk from sea level rise and coastal flooding under a range of warming scenarios. Use
CONTACT EXPERTS
Kate Silverstein (she/her/hers)
NOAA National Ocean Service
Can connect to experts on: Sea level rise and coastal flooding
Contact: katherine.silverstein@noaa.gov, oceanservicepress@noaa.gov
For questions about Climate Central’s suite of sea level rise tools, please contact: Kelly Van Baalen, project manager, at kvanbaalen@climatecentral.org.
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
NOAA defines three high tide flooding thresholds: Minor (disruptive) is about 1.75-2 feet above average high tide; Moderate (damaging) is about 2.75-3 feet above average high tide; Major (destructive) is about 4 feet above average high tide. Unless otherwise stated, text and graphics in this Climate Matters refer to the NOAA-defined minor flooding threshold.
Annual high tide flooding (at NOAA-defined minor threshold) and relative sea level rise data for 35 tide gauges over the 1950-2022 period were obtained from NOAA (click for detailed tide gauge information). Gauges were selected based on period of record and data completeness. National summary sea level rise and coastal flooding trends (1920-2020) were adapted from the interagency 2022 Sea Level Rise Technical Report (Figure 3.1).