The 28 most populous cities in the United States are all at least partly sinking. That’s according to a new study from the Columbia University Climate School.
Researchers used U.S. census data from 2020 to pinpoint the country’s most populous cities, which, combined, are home to roughly 39 million people or 12% of the U.S. population. The list includes 11 coastal cities, eight riparian (sitting alongside rivers) and nine inland cities. Nearly half the locations on this list are among the country’s fastest-growing cities.
To create high-resolution maps of each city over time, researchers analyzed more than 2,500 satellite radar images between 2015 and 2021. They created roughly 400 precise images for each city that allowed them to compare vertical land movement over the study period. Previous studies have looked at sinking land but at a scale that made it difficult to identify specific areas and infrastructure at risk. The Columbia study is the first to provide fine-scale information that city planners can use to protect infrastructure at risk.
Natural processes can account for some of the land movement, but researchers found “most of the sinking land results from human-driven activities, with 80% of the subsidence associated with groundwater withdrawals,” the study authors write. Essentially, as groundwater is removed, the soil pressure changes and allows for more sinking.
Similarly, oil and gas extraction are also factors. Fossil fuel extraction and long-term dependence on groundwater combine to make Houston, Texas, the fastest-sinking city in the country; 42% of its land area is subsiding faster than 5 mm (0.2 inches) per year and roughly 12% of the city is sinking twice as fast as that. Several Texas cities top the list for the fastest subsidence rates in the country.
Importantly, the movement of land in cities is not uniform. Many areas are sinking, while, conversely, neighboring areas may be rising if aquifers are quickly filled. Such instability can wreak havoc on infrastructure.
From 1989 to 2000, some 225 buildings collapsed in the U.S.; researchers say roughly 2% were attributable to subsidence-related issues but another 30% were never classified, leading researchers to posit that subsidence may have played an unrecognized role in those collapses.
Climate change is expected to increase drought in parts of the U.S., which will lead to more groundwater extraction and exacerbate sinking for many cities, the researchers caution.
City planners must “acknowledge that land subsidence is not a write-off issue, but a critical amplifier of flood risk and infrastructure vulnerability in cities,” Leonard Ohenhen, the study’s lead author and a postdoctoral research scientist with the Columbia Climate School, told Mongabay in an email.
He said this information should be integrated into land use planning, including “investments in vulnerable areas, implementing zoning regulations in high-risk zones, and designing infrastructure such as roads, drainage systems, and coastal defenses with projections of future ground elevation changes in mind.”
Banner image: Flooding in Houston, courtesy of Daniel J. Martinez.
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