“It no longer takes severe weather to cause disruptive flooding along the coast,” NOAA’s National Ocean Service director Nicole LeBoeuf stated in 2023. What had previously been unimaginable now resounds as a everyday reality for millions who live along the U.S. coast. The intersection of two ruthless powers rising water and sinking land has created a crisis that is quietly but irresistibly redefining the nation’s coastal cities.
Sea levels rose during the past century by as much as 25 inches in Galveston, Texas, 18 inches in Norfolk, Virginia, and 16 inches in Atlantic City. They are not isolated instances. The NOAA Sea Level Rise Technical Report projects that, since 2050 to date, the coast of the U.S. will see another 10 to 12 inches of sea level rise essentially compressed into a few decades the first hundred years or so. Regional variations are forecasted, with the northern Gulf Coast and mid-Atlantic seeing the greatest increase. This rise is due to thermal expansion of the warming oceans and intensified melting of ice sheets and glaciers, confirmed by a May 2024 study that reports ice loss in Antarctica and Greenland is ahead of earlier projections.
Yet the crisis doesn’t break at the water’s edge. In a comprehensive report in Nature Cities, researchers used high-resolution satellite-based InSAR (Interferometric Synthetic Aperture Radar) to measure land subsidence in the 28 most populous U.S. cities. The findings are disheartening: in all the cities studied, at least 20% of the urban surface is subsiding, and in 25 of the 28 cities, at least 65% is subsiding. Houston is at the lead, with more than 40% of its land subsiding greater than 5 millimeters annually and individual districts compacting up to 5 centimeters per year. The perpetrators? Mainly, extensive continuous groundwater pumping, which accounts for 80% of measured subsidence, and oil and gas pumping in places like Texas.
The mechanisms are straightforward. Since groundwater is drawn out from aquifers consisting of fine-grained particles, the emptied pore spaces collapse, resulting in the ground surface above sinking. This doesn’t happen only in coastal cities; it also happens in interior cities such as Denver, Chicago, and Indianapolis, since they also subside due to the gradual settling of land that has previously been uplifted by old ice sheets. Even the sheer bulk of city infrastructure is in on the act New York’s million-plus buildings are weighing down on the earth, creating localized subsidence. The combination of rising water and sinking ground increases flood risk.
The cycle of “sunny day” or nuisance flooding once a rare nuisance is now a daily inconvenience. Philadelphia saw 17 days of tidal flooding in 2023, a wholesale increase attributed to a one-foot local sea level rise since the last century. NOAA’s recent outlook indicates that the country now sees a median of 5 more high tide flood days annually than in 2000, an increase of almost 200%. The Mid-Atlantic, for example, saw an average of 17 flood days between 2023-24, as Bar Harbor, Maine, and Charleston, South Carolina, both recorded new highs. The costs are real and hefty. Transit disruptions, business losses, and compromised stormwater drainage are now the norm in most cities. Nonprofit Climate Central has estimated that, over the lifespan of a typical 30-year mortgage, as many as 64,000 buildings and 637,000 properties will be at least partially below the tidal boundary level. More than 48,000 properties, largely in Louisiana, Florida, and Texas, could be below the high tide line by 2050.
For planners and engineers, the challenge for engineers is daunting. Satellite InSAR has revolutionized monitoring of land motion, enabling detection of vertical shifts to as much as a millimeter in 28-meter grids. That level of detail reveals not just uniform subsidence, but differential motion: one corner of a neighborhood falling at a different rate from the next-door neighbor, or even in some areas, actually rising because of rapid aquifer recharge. These variations exert stresses that can undermine the structural stability of bridges, highways, and buildings. Austin, San Antonio, and Fort Worth are some of the cities most vulnerable to such differential movement with almost 29,000 buildings under threat from such movement, according to the Nature Cities report. Engineering solutions for such combined hazards are shifting.
Traditional “hard” defenses levees, seawalls, and storm surge barriers remain significant, but their limitations are increasingly apparent as sea level and land subsidence both outpace historic trends. Managed retreat the intentional relocation of settlements and infrastructure away from hazard islands is becoming a plausible, albeit controversial, adaptation strategy. Nature-based solutions, such as restoring wetlands and creating artificial floodplains, are also being used to absorb excess water and protect cities.
As NASA Administrator Bill Nelson put it, “Science is indisputable and urgent action is required to mitigate a climate crisis that is well underway.” Combined, high-resolution earth observing, advanced hydrological modeling, and adaptive engineering lead the way, but the proactive window of adaptation opportunity dwindles. The record is now clear: the nation’s coastal fate will be as much decided by what is below our feet as by rising waters at our shores.
