28 of the largest cities in the US are sinking – and the climate crisis is exacerbating the risk

All 28 of the most populous cities in the United States are sinking, according to a new study published in the journal Nature CitiesThe researchers, led by Dr. Leonard Ohanhan of Columbia University's Climate Research Institute, used millimeter-resolution satellite data and discovered that most of these urban areas are in the process of land subsidence – a slow but dangerous process that endangers infrastructure, buildings, and even increases the vulnerability of cities to flooding, especially in densely populated areas.

The main cause of subsidence is over-pumping of groundwater – a well-known phenomenon but one that has worsened in recent years. When water is pumped from underground reservoirs composed of thin sediments, microscopic cavities that were filled with water often collapse. This collapse leads to compaction of the soil layers and subsidence of the surface. In places like Texas, oil and gas extraction also contributes to this. According to the study, about 80% of subsidence results from direct human activity related to the use of water resources.

The climate crisis is exacerbating the decline

The effects of the climate crisis are exacerbating this phenomenon. Prolonged droughts – a result of climate change – lead to a decrease in precipitation and a loss of available natural water sources. The result: increased pumping of groundwater to meet urban and agricultural demand. This creates a harmful cycle – less available water, more pumping, more subsidence. Of course, the combination of rising sea levels and the subsidence of cities also worsens the situation.

The situation is particularly severe in the city of Houston, where over 40% of the city is sinking by more than 5 mm per year, and in some areas, subsidence of up to 5 cm per year has been measured. Other cities such as Dallas, Fort Worth, San Francisco, New York, Las Vegas, and Washington, D.C. are also suffering from subsidence at a significant rate. In total, about 34 million residents in the United States currently live in subsiding areas.

In addition to human factors, natural forces also affect the land. Until about 20,000 years ago, a huge ice sheet covered large parts of North America. The weight of the ice caused the land to compress, and in the areas south of it, "swellings" were formed in the land - a kind of geological balloons. Even after the ice retreats, the land continues to slowly sink - a process called Isostatic subsidenceCities such as New York, Chicago, Indianapolis, Philadelphia, and Portland are affected by this precipitation at a rate of 1–3 mm per year.

Previous research has even suggested that the sheer weight of buildings – for example, the million buildings in New York City – could contribute to subsidence. New findings from Miami indicate the effects of new construction on the stability of the ground in the area.

Another serious problem identified in the study is Differential settlement – That is, different areas within the same city sink at different rates, and sometimes certain parts even rise, for example due to the recharge of water reservoirs near rivers. These uneven movements can cause slopes, deformations and cracks in infrastructure, even when the changes in elevation are relatively small. In contrast to the risk of flooding resulting from significant subsidence, structural damage can also be caused by subsidence of just a few millimeters, if it occurs unevenly.

According to the researchers, only about 1% of the city's land area is in areas of differential subsidence, but these are dense urban centers, containing about 29,000 buildings. The most dangerous cities in this regard are San Antonio, Austin, Fort Worth, and Memphis.

Here too, there is a close connection to the climate crisis. Phenomena such as population growth in urban areas – sometimes as a result of internal climate migration – increase the demand for water and construction. The land is not “sufficient” to respond to changes at such a rapid pace, and the result is a loss of urban resilience – literally.

Finally, the researchers call for action. Solutions include innovative urban planning that takes into account land subsidence, improving drainage systems, creating green catchments such as artificial wetlands, adjusting building and maintenance standards in at-risk areas, and even banning new construction in areas prone to subsidence. “Rather than just saying it’s a problem, it’s time to respond, deal with, mitigate, and adapt,” Ohanen concludes. “We need to move from concern to action.”

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