A sinking feeling came over me after reading a study of satellite data published in Nature Communications, which revealed that most of the U.S. East Coast, home to an estimated 118 million people (about a third of all Americans), is suffering from subsidence of 3 mm annually—with some areas subsiding by as much as 10 mm a year.
The major cities most affected in the subsiding area are New York; Boston; Atlantic City, NJ; Norfolk, VA; and Charleston, SC, each with a subsidence rate of over 5 mm a year.
A rise of 3 mm, the thickness of two pennies, may not seem like much if you are filling a bathtub, but that is the same order of magnitude as the current global sea level rise, which has sped up to 3.7 mm a year and is ringing alarm bells in coastal areas all over the world. A negative land motion of 3 mm and a positive sea level rise of 3.7 mm per year is a 6.7 mm rise of the sea level relative to the subsiding land—more than double the sea level rise coastal dwellers were expecting.
Terra Firma Is Anything but Firm
Movement of the Earth under our feet is happening all the time but is normally too slow for us to detect—unless there is an earthquake. Historically, we have been slow to accept that the Earth moves at all. It wasn’t until the 17th century that Galileo theorized that the Earth was moving around the Sun. It took until 1968, when a landmark paper titled “Seismology and the New Global Tectonics” was published in the Journal of Geopolitical Research, to convince us that the Earth’s mantle is anything but stable and that continents are giant land plates floating over a sphere of molten lava. The study of what happens at the interfaces of the continental plates and interfaces of land and water is even more recent.
“When you can measure what you are speaking about, and express it in numbers, you know something about it; when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind,” said Lord Kelvin, who created the concept of absolute temperature that bears his name.
Only recently were there satellites that gave us the numbers we needed to learn of subsidence on a world scale. The researchers of the East Coast subsidence report pored over data collected by the Sentinel-1 satellites from 2015 to 2020 and ALOS (Advanced Land Observing Satellites). The interferometric synthetic aperture radar provided the researchers with “unprecedented” resolution and accurate elevations over broad areas, overcoming limitations of previous methods of measurement such as field observations from local ground-based operations and tidal data. The researchers were able to map the Eastern seaboard in 38 million pixels, with each representing a 50 m square with mm accuracy in elevation.
Cause of Subsidence
Although land subsidence occurring all along the coastline may look as if it could be caused by natural oceanic action, the consensus among geologists is that it is caused by man-made activity. Pumping out groundwater for farms or cities has been known to lower the land substantially. Venice, sinking since the day it was built—the result of building on top of wood piles sunk into the mud of the Venetian Lagoon—sank even faster when local industries pumped out the water below it, a practice that was banned after World War II. Parts of Tokyo sank 13 feet during the 20th century due to groundwater pumping.
Millimeter accuracy of satellite data is not needed to measure the subsidence of farmland in California’s Central Valley, some of which has dropped a staggering 20 feet in the last 65 years, according to a 2022 Stanford University study, 10 feet during the last 10 years coinciding with one of the worst droughts in the state’s history.
The Central Valley, though only 1 percent of U.S. agricultural area, produces 40 percent of the country’s fruits, vegetables and nuts annually. The need for water is desperate because rainfall is completely absent in the hottest months and only 5 to 10 inches of rain annually is forcing farmers to deplete the water underground. More than 80 percent of irrigation water comes from wells.
But the need for groundwater increases every year with the increasing population. The Tulare Basin, the lower third of the Central Valley, was determined to still be sinking at the rate of 1 foot per year, according to a joint UC Berkeley and JPL analysis of satellite data.
Subsidence was detected by satellite data under the Champlain Towers South north of Miami Beach, which partially collapsed in 2021, though in all fairness, the collapse resulted from multiple factors, including lax building codes and improper maintenance.
Damming rivers leads to subsidence of wetlands as silt is prevented from being carried into deltas and wetlands and raises its elevation and compensates for oceanic erosion. The massive levees built to save New Orleans from Mississippi River floods are actually helping to sink it because they prevent silt from being deposited and raising the ground level.
Other Effects of Subsidence
In addition to subsidence raising the relative sea level rise, it is detrimental in other ways. The infiltration of seawater from flooding seeps into depleted aquifers and reduces freshwater storage. Saltwater changes everything. Forests die, making “ghost forests,” agricultural land is ruined, and last but certainly not least, in vacation spots like Miami Beach and Venice, tourists are turned off.