TWH – In February of 2022, the National Oceanic and Atmospheric Administration (NOAA) issued a report, “Global and Regional Sea Level Rise Scenarios for the United States.” NOAA, a U.S. agency, only discussed sea level rise along U.S. coastlines.
This most recent report revised NOAA’s 2017 estimates for rising sea levels over the next century. NOAA used sea levels in the year 2000 as their baseline. In the next 30 years, sea levels along the lower 48 U.S. states would rise between 0.25 meters (0.8 feet) and 0.3 meters (1 foot). This rise will not be uniform across all U.S. coasts but would be most severe along the Gulf and East coasts. By 2100, NOAA forecast sea levels rising along the U.S. coastline between 0.6 meters (2 feet) and 2.2 meters (7.2 feet). By 2150, they forecast a rise in sea levels along U.S. coastlines between 0.8 meters (2.6 feet) and 3.9 meters (12.8 feet).
The report states “Global mean sea level (GMSL) rise is a direct effect of climate change.” NOAA attributes this linkage to two factors. First, heat expands water. Second, as water stored on land in ice sheets and glaciers melt, the volume of water in the ocean increases.
Storm surges and tidal flooding
Sea level rise takes two forms: storm surges and tidal flooding.
Any coastal storm will push seawater onto the shoreline. Intense storms, like hurricanes and nor’easters, push large amounts of water further inland. People call those pushes “storm surges.”
Tidal flooding may be a new concept for some people. These floods occur in the absence of rain. They result from the alignments of the Earth, the Moon, and the Sun at the new and full Moons. The Moon attracts and moves water. The high tide line moves further inland. Sometimes, science flirts with enchantment.
Normally, storm drains move water from streets and parking lots into waterways. Rising sea levels bring tidal action into these storm drains. In tidal flooding, storm drains go “in reverse.” They start to move water from those waterways onto streets and parking lots. Those streets and parking lots can no longer drain. Water rises and threatens buildings, cars. ecosystems, and septic tanks.
As sea levels rise, both low and high tides move beyond their “normal range.” When the low tide line extends further inland, the sea claims more of the land.
Tidal and storm surge flooding will increase in both frequency and intensity. These tidal floods are already occurring. People have also called them “sunny day floods” or “nuisance floods.”
Tidal flooding has increased in many coastal cities. In 2000, New York City and Norfolk, Virginia each had five days of a minor, but disruptive, tidal flooding. In a minor tidal flood, waters reach 0.55 meters (1.8 feet) above the average high water line. In 2020, tidal flooding had increased in both cities. It grew to between 10 to 15 days per year. Miami, Florida, and Charleston, South Carolina showed a similar pattern.
Across the US, minor tidal flooding has more than doubled between 2000 and 2020. Current projections have it doubling again between 2020 and 2030.
NOAA reported that by 2050 moderate tidal flooding will increase by a factor of ten. In a moderate tidal flood, waters reach 0.85 meters (2.8 feet) above the average high water line. In 2050, NOAA expects moderate tidal flooding to occur as often as minor tidal flooding had in 2020.
Major high-tide flooding will increase by a factor of five. In a major tidal flood, waters reach 1.2 meters (3.9 feet) above the average high water line. In 2050, major high-tide floods will occur as often as moderate ones did in 2020.
Relative sea-level rise
In 2050, NOAA forecast that the US national average sea-level rise would be between 0.25 meters (0.8 feet) to 0.3 meters (1 foot). Sea levels rise will not rise uniformly. Local factors will cause sea levels to differ. Grand Isle, Louisiana has already had sea levels rise 0.9 meters (3 feet). In contrast, Juneau, Alaska has already had sea levels fall by 1.2 meters (4 feet).
The boundary between sea and land is a liminal, unstable place. Every day, that boundary changes with the tides. Contrary to human perception, the land is also moving.
Many people know that plate tectonics causes some plates to move down, up, or sideways. If the land is rising, it works against rising sea levels.
Fewer people know that non-tectonic movement occurs as upper layers of land press down on soft sediments. That pressure causes the underlying sediment to compact and the land above to sink. If the land is sinking, it works with rising sea levels.
The measure, average sea level rise, describes sea level rise at the global level. Differences in how the land is moving give “relative sea-level rise” more utility at the local level.
Along the U.S. East and Gulf coasts, the land itself is slowly sinking downwards. Along the U.S. West Coast, the land is rising. Different regions of the U.S. will differ in their experience of climate change. The West will see increased drought and wildfires. The East and Gulf will see rising sea levels and flooding.
Threats linked to sea-level rise
On the U.S. East and Gulf Coasts, relative seawater rise will challenge current flood control measures.
Coastal flooding caused by Beta in Corpus Christi, Texas as viewed from the JFK Causeway (September 21, 2020) – Image credit: Robloxsupersuperhappyface – CC BY-SA 4.0
Current flood control strategies rely on assumptions about the frequency and intensity of floods. The NOAA report warns that many of those assumptions will not be valid in 2050. Planners and politicians will have to develop new policies and structures to contain rising relative sea levels. The current built environment will have to adapt to an ever-changing series of new normals. Current ecosystems will also face threats from rising sea levels and temperatures.
This report discusses the serious challenges of the next 30 years. The challenges of the next 100 years will be dire.
The full NOAA report, “Global and Regional Sea Level Rise Scenarios for the United States” is available online. A related but much shorter NASA report is also available.
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