Major US Cities Face Sinking Crisis: New Study Reveals Widespread Land Subsidence
A groundbreaking study has revealed a concerning trend across the United States: the nation’s largest cities are sinking. This phenomenon, known as land subsidence, is primarily driven by the unsustainable extraction of groundwater, posing a significant threat to infrastructure and urban stability. The study, utilizing high-resolution satellite data, provides the most detailed analysis to date of subsidence patterns in 28 of the most populous US cities.
The research, led by Leonard Ohenhen of the Lamont-Doherty Earth Observatory, paints a stark picture of widespread sinking. Alarmingly, the study found that at least 20% of the urban area in every city examined is experiencing subsidence. Even more concerning, in 25 of the 28 cities, at least 65% of the area is sinking. This widespread sinking is not limited to coastal cities already vulnerable to sea-level rise; many interior cities are also facing this silent crisis.
Ohenhen emphasized the novelty and importance of the study, stating, "This is the first high-resolution, satellite-based measurement of land subsidence across the 28 most populous U.S. cities, and for several of these cities, it is the first time we have such detailed insights." The data provides critical information for urban planners, infrastructure managers, and emergency responders, enabling them to better prepare for and mitigate the impacts of subsidence.
The study identifies Houston, Texas, as the fastest-sinking city in the nation. More than 40% of Houston’s area is dropping by more than 5 millimeters (approximately 1/5 inch) per year, with 12% of the city sinking at twice that rate. In some localized areas, the land is sinking as much as 5 centimeters (2 inches) per year. Other Texas cities, including Fort Worth and Dallas, are also experiencing significant subsidence.
Subsidence is often described as an "invisible threat" because its effects can be subtle and gradual, often going unnoticed for extended periods. However, even small amounts of downward land movement can have significant consequences for the structural integrity of buildings, roads, bridges, and dams. "When land shifts downward, even just a little bit, the structural integrity of buildings, roads, bridges, and dams can be profoundly impacted," Ohenhen explained.
Ohenhen highlighted several early warning signs of subsidence that are frequently overlooked. These include cracks in walls, foundations, or around windows and doors; uneven or sloping floors; doors and windows that no longer close properly; warped roads or buckling pavement; tilting fences or utility poles; and fire hydrants sticking out more than they once did. Recognizing these signs is crucial for early detection and timely intervention.
The study attributes the primary cause of subsidence to massive, ongoing groundwater extraction. According to a statement from Lamont-Doherty, when water is withdrawn from aquifers composed of fine-grained sediments and the aquifer is not replenished, the pore spaces formerly occupied by water can collapse. This collapse leads to compaction below the surface, resulting in sinkage at the surface. In Texas, the problem is further aggravated by the extraction of oil and gas.
Interestingly, the study identified three cities – San Jose, Memphis, and Jacksonville – where data indicates slight "uplift." However, Ohenhen clarified that the uplift detected is an average across the city and is minimal, ranging from 0.1 to 0.3 millimeters per year, which he considers "almost negligible." He emphasized that this does not mean the entire city is uniformly rising and that these areas can be considered relatively stable.
The long-term consequences of subsidence are far-reaching. Even seemingly small changes can accumulate over time, exacerbating weaknesses within urban infrastructure and increasing flood risks. "A lot of small changes will build up over time, magnifying weak spots within urban systems and heighten flood risks," Ohenhen warned. "This subsidence can produce stresses on infrastructure that will go past their safety limit."
The researchers emphasize that continued population growth, increased water usage, and climate-induced droughts in some areas will likely worsen subsidence in the future. This convergence of factors creates a perfect storm, amplifying the existing vulnerabilities and accelerating the rate of land sinking.
Despite the alarming findings, the researchers remain optimistic that the problem can be addressed. Ohenhen stressed the importance of moving beyond simply recognizing the problem and actively pursuing solutions. "As opposed to just saying it’s a problem, we can respond, address, mitigate, adapt," said Ohenhen. "We have to move to solutions." He believes that the detailed data provided by the study offers a crucial foundation for developing effective strategies to combat subsidence and protect urban communities.
The study, published on May 8 in the British journal Nature Cities, provides a comprehensive assessment of land subsidence in major US cities and underscores the urgent need for proactive measures to mitigate its impacts. By understanding the causes, identifying the warning signs, and implementing sustainable water management practices, cities can safeguard their infrastructure, protect their residents, and ensure a more resilient future.
