US coasts face a crisis as land sinks and seas rise

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A slow-moving crisis of sinking land and rising water is playing out along America's coastline.

In the past 100 years, sea levels have climbed about a foot or more in some U.S. cities – 11 inches in New York and Boston, 12 in Charleston, S.C., 16 in Atlantic City, 18 in Norfolk, Virginia, and 25 in Galveston, Texas, according to the National Oceanic and Atmospheric Administration..

Seas are forecast to rise from 8 inches to 23 inches along the nation’s coasts by 2050, with the higher increases along the northern Gulf Coast and mid-Atlantic. Every inch of additional water is expected to move farther inland making flood events worse and putting more properties at risk.

Meanwhile, in many coastal areas, the land is sinking, making flooding an even greater issue.

What is causing sea level rise along U.S. coastlines?

◾As atmospheric temperatures increase, the ocean absorbs more heat, which makes the water expand in volume. It's the same principle of physics that causes the liquid in a thermometer to expand as it gets hotter.

◾Glaciers and ice sheets are melting at an accelerated rate, especially in Greenland and Antarctica, adding more water to the oceans. A study published on May 20 found that the ice is melting even faster than expected.

Even a little rise in water means lots more flooding

Higher high tides, supercharged by rising sea levels, could flood all or parts of an estimated $34 billion worth of real estate along the nation’s coasts within just 30 years, a 2022 report found.  

It's not just at the shore. Inland areas are also finding themselves hit by "sunny day" flooding. Philadelphia had 17 days of flooding in 2023, with a substantial increase in high tide flooding near the Delaware and Schuylkill rivers. Sea levels in the area have risen about one foot over the last century, according to the city.

Within the span of a 30-year mortgage, as many as 64,000 buildings and roughly 637,000 properties along the ocean and its connecting waterways could be at least partially below the tidal boundary level, the nonprofit Climate Central stated in a report.

More than 48,000 properties could be entirely below the high tide lines by 2050, mostly in Louisiana, Florida and Texas.

Subsidence is another growing problem

Another problem is that in many coastal areas, the land is sinking, making flooding an even greater issue.

Called "subsidence," the most common cause of the sinking is "massive ongoing groundwater extraction," a study published on May 8 found, though other forces are at work in some places. The cities include not just those on the coasts, where sea level rise is a concern, but many in the interior.

Using high-resolution satellite-based measurements of land subsidence, researchers found sinking in the 28 most populous U.S. cities. In every one studied, at least 20% of the urban area is sinking – and in 25 of 28 cities, at least 65% is sinking.

The nation's fastest-sinking city was Houston, with more than 40% of its area dropping more than 5 millimeters (about 1/5 inch) per year, and 12% sinking at twice that rate.

“We’re taking excess groundwater out of the ground and lowering the groundwater table,” Jennifer Walker, an assistant professor at Rowan University, previously told Paste BN. Add to that soil compaction and the risks of flooding are increasingly significant.

'Sunny day' and 'nuisance' flooding

Coastal residents have become used to storm surges that can send potentially ruinous floodwaters into their communities. But the double whammy of sea level rise and subsidence brings another risk – "nuisance or "sunny day" flooding during high tides.

Damaging floods that decades ago happened only during a storm now happen more regularly, such as during a full-moon tide or with a change in prevailing winds or currents, according to an annual report produced by NOAA.

This type of flooding leads to disruptions such as road and business closures and longer commute times, occurring when tides reach anywhere from 1 to 2 feet above the daily average high tide, depending on location.

In just 25 years, the nation is expected to experience an average of 45 to 85 high-tide flooding days a year. Long-term projections are based on the ranges of expected relative sea level rise of about a foot, on average, across the U.S. by 2050, NOAA said.

"It no longer takes severe weather to cause disruptive flooding along the coast," Nicole LeBoeuf, director of NOAA’s National Ocean Service, said in 2033.

NOAA's annual average high tide flooding days

NOAA's projections for high tide flooding days in 2030 and 2040, an annual average based on an intermediate sea level rise scenario:

Contributing: Dinah Voyles Pulver