A recent study has revealed that the top 28 most populous cities across the United States are experiencing subsidence, intensifying the threats of floods and infrastructural damage. The primary driver of this phenomenon is the extraction of groundwater, which is needed to support the expanding needs of residents and businesses.

Published in Nature Cities on Thursday, the study highlights that subsidence is a widespread issue. “This is something that’s happening everywhere and needs attention across the board,” noted Matt Pritchard, a geophysicist at Cornell University who did not participate in the research.

It’s been well-documented that various cities globally, especially those near coasts like New Orleans in the U.S., are sinking. This sinking, or subsidence, can be attributed to both natural and human-induced factors, including ongoing geological adjustments post-ice age, the construction of dams that block sediment from reaching river deltas, and the sheer weight of urban structures.

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While detailed analyses have been conducted for cities like Miami, most data on urban subsidence have been derived from limited on-site measurements, which lack the detail necessary to accurately assess infrastructure risks. To overcome this, the study’s researchers employed interferometric synthetic aperture radar (InSAR) technology, which uses satellite-based measurements. Leonard Ohenhen, a postdoctoral researcher at Columbia University’s Lamont-Doherty Earth Observatory and co-author of the study, likened InSAR to playing catch with a wall: “The rate at which the ball returns tells you if the wall is moving towards or away from you.” This technology allowed the team to detect minute vertical shifts in the earth’s surface, with a precision down to the millimeter in grid sections measuring 28 meters each.

The findings were stark: all 28 cities, each with over 600,000 residents, displayed some degree of sinking, with 25 cities showing that at least 65 percent of their area was affected. Cities like New York, Chicago, Houston, Dallas, Fort Worth, Columbus, Seattle, Denver, and Detroit saw average sinking rates exceeding two millimeters per year. Houston, Fort Worth, and Dallas recorded the most significant rates, each averaging over four millimeters annually. In Houston, more than 42 percent of the city is subsiding by over five millimeters per year, and 12 percent by more than ten millimeters annually.

Though these figures might seem minor, the cumulative effect over years is significant, Pritchard explains, emphasizing the value of detecting these shifts early on, even when they are subtle. The varying rates of subsidence within a city can lead to uneven settling, causing structural damages and tilting of buildings. He pointed to the 2021 Surfside condo collapse in Florida as a potential example of damage linked to such ground movement.

Of the 5.6 million buildings surveyed in these cities, about 29,000 are situated in areas particularly susceptible to subsidence. While not all these buildings are doomed to suffer damage—factors like soil type, construction practices, and age are also influential—it highlights regions where more detailed evaluations are crucial.

Subsidence also increases flood risks by creating depressions that trap water, forming what Ohenhen refers to as “subsidence bowls.” Instead of water flowing smoothly across what was once flat terrain, it now accumulates in these depressions.

The study also found that a staggering 80 percent of the observed subsidence was linked to groundwater extraction, underscoring the need to manage water resources carefully, especially in drought-prone areas like Texas. “When a drought hits, the likelihood of further subsidence skyrockets due to increased groundwater depletion,” Ohenhen warned.

Ultimately, Pritchard believes that the issue of subsidence should be a concern not just in well-known cases like New Orleans but in all urban areas, regardless of size. “This is a widespread issue that merits broader attention,” he concluded.