America’s Sinking Cities: A Growing Threat to Infrastructure

A groundbreaking study published in Nature Cities has revealed a concerning trend across the United States: major urban centers are gradually sinking. This phenomenon, known as subsidence, isn’t limited to coastal cities traditionally vulnerable to rising sea levels. The research indicates that all 28 U.S. cities with populations exceeding 600,000 are experiencing some degree of land subsidence, posing a significant and escalating threat to infrastructure in rapidly growing urban areas.

The study, conducted by researchers using advanced satellite data analysis, provides a comprehensive overview of vertical land movements in the nation’s largest cities. Their findings demonstrate that subsidence is widespread, impacting cities both near and far from the ocean. The primary culprit identified is groundwater extraction, a practice that has far-reaching consequences for the stability of urban landscapes.

Leonard Ohenhen, the lead author of the study and a postdoctoral researcher at Columbia Climate School’s Lamont-Doherty Earth Observatory, emphasized the growing risk associated with this trend. As cities continue to expand, they are increasingly encroaching upon areas prone to subsidence. This ongoing land movement, even seemingly minor changes, can exert undue stress on infrastructure, eventually exceeding safety limits and compromising the structural integrity of buildings, roads, and other essential systems.

The researchers meticulously analyzed satellite data, breaking down the urban landscapes into systematic grids of 90 square feet (28 square meters) to map vertical land movement. While the sinking of coastal cities like New Orleans is widely recognized, the study highlights that even inland municipalities such as Denver, Oklahoma City, and Fort Worth are not immune to subsidence.

The comprehensive data analysis revealed that in a staggering 25 out of the 28 cities examined, at least two-thirds of the land is sinking. Houston stands out as the fastest-sinking metropolis, with over 40% of its land area dropping more than 0.20 inches (5 millimeters) per year. Alarmingly, 12% of Houston’s land is subsiding at twice that rate, signifying a particularly severe level of instability.

Other major cities, including New York, Las Vegas, Washington D.C., and San Francisco, also exhibit localized zones of rapid subsidence, despite their overall stability. These variations in sinking rates contribute to what researchers term "differential motion," where some areas sink at different speeds, while others might even experience uplift. This uneven movement is particularly dangerous because it can induce tilting and other structural stresses in buildings, jeopardizing their long-term stability.

While only a small fraction (around 1%) of the documented areas are experiencing differential motion severe enough to immediately impact infrastructure, these zones tend to be concentrated in the most densely populated urban areas. The study estimates that approximately 29,000 buildings are located within these high-risk zones. The data further reveals that in San Antonio, one in 45 buildings is at high risk; in Austin, one in 71; in Fort Worth, one in 143; and in Memphis, one in 167. These figures underscore the urgency of conducting more detailed assessments to determine the specific risks faced by individual buildings.

The researchers emphasize that subsidence-induced infrastructure damage can occur even with seemingly minor changes in land motion, unlike flood-related hazards where risks are only triggered when land elevation drops below a critical threshold. It’s important to note that sinking land is generally more susceptible to flooding, exacerbating the existing risks.

By analyzing the correlation between groundwater extraction and vertical land movement, the researchers concluded that this practice is responsible for approximately 80% of the documented subsidence. When groundwater is extracted from aquifers – the underground layers of porous rock that hold water – the pores in the rock can collapse, causing the land surface above to sink. This phenomenon is expected to worsen in some areas due to factors such as population growth, which increases water demand, and climate-driven droughts, which further depletes groundwater reserves.

However, groundwater extraction is not the only factor at play. Some areas are still undergoing post-glacial rebound, a process of land adjustment following the retreat of massive ice age glaciers. The immense weight of buildings, particularly in densely populated cities like New York, may also contribute to land movement.

The researchers hope that their study will serve as a catalyst for implementing mitigation efforts to address the issue of subsidence. These efforts could include land raising, retrofitting existing buildings to withstand ground movement, and updating construction codes to account for subsidence risks.

Ohenhen emphasized that the study’s primary goal is to move beyond simply identifying the problem and toward developing solutions. He called for proactive measures to address, mitigate, and adapt to the challenges posed by sinking cities.

The findings of this study should serve as a wake-up call for policymakers, urban planners, and engineers. A comprehensive and coordinated approach is needed to mitigate the risks associated with land subsidence and ensure the long-term sustainability and resilience of America’s cities. Ignoring this growing threat could lead to devastating consequences for infrastructure, economies, and the safety and well-being of urban populations. Further studies are needed to gather localized data and to propose specific mitigation measures.