As sea levels continue to rise and threaten coastal areas, cities around the world are also sinking. This can happen because the huge populations of massive cities pump a lot of groundwater, which is the case for Jakarta, home to more than ten million people, as well as Mexico City, where over 22 million people live.

Now scientists have shown that US cities are sinking too. Every one of the 28 largest cities (by population) is sinking to some degree. This not only includes coastal cities like New York or Miami, but also Dallas and Las Vegas. In some cases, different areas of the city are sinking at wildly different rates. This movement could strain and stress architecture and infrastructure. The findings have been reported in Nature Cities.

This study used satellite data to create grids and then map the movement of land down to the milliliter within the grids. Of the 28 cities, at least two-thirds of the land area is sinking, and these areas are home to about 34 million people.

Houston is sinking fastest, although at very different rates depending on the neighborhood. Over 40% of the city is moving downwards by about 5 millimeters (0.2 inches) every year, and 12% is sinking two times faster than that. Parts of Fort Worth and Dallas are also sinking almost as fast as Houston. There are localized hotspots of sinking in other cities like Las Vegas, San Francisco, and Washington, D.C., as well as parts of New York near LaGuardia Airport.

The investigators also analyzed the extraction of groundwater alongside this data, and found that the removal of groundwater is to blame for almost all subsidence – 80% of it. Aquifers that hold water deep underground are being tapped, and not replenished fast enough. If all of the water is taken from an aquifer, the remaining hole can simply collapse, and the ground compacts there. The extraction of oil and natural gas in Texas is also making the problem worse.

“As cities continue to grow, we will see more cities expand into subsiding regions,” said lead study author Leonard Ohenhen, a postdoctoral researcher at the Columbia Climate School’s Lamont-Doherty Earth Observatory. “Over time, this subsidence can produce stresses on infrastructure that will go past their safety limit.”

Although population growth and water usage, along with stresses introduced by climate change will likely make subsidence worse in the coming years, natural forces are also at work.

An ice sheet that once sat on top of North America’s interior until 20,000 years ago caused some bulging of the land at the edges. Some are still subsiding now, at rates of 1 to 3 millimeters each year, because of this. Cities including New York, Philadelphia, Nashville, Indianapolis, Denver, Chicago, and Portland are affected by this phenomenon.

The weight of buildings is also not negligible in places like New York City; they may be worsening subsidence there. Construction in Miami can also disturb the ground in ways that may promote subsidence.

In a few cases, there is also uplift along with subsidence. Examples include Jacksonville, Memphis, and San Jose, when uplift may be compensating for whatever subsidence is happening. Water also seems to be the cause of this uplift, as rivers and aquifers can fill up if there is enough precipitation.

One major problem may be when a city is moving up and down at varying rates.These uneven movements could potentially cause infrastructure to tilt, they suggested. Very small changes could damage infrastructure as well.

The research suggested that about one in 45 San Antonio buildings are at high risk from subsidence; while one in 71 Austin buildings; one in 143 Fort Worth buildings; and one in 167 Memphis buildings are at high risk. While very few buildings have been confirmed to have collapsed due to subsidence, the cause for about 30% of bulding collapses is unknown. The researchers suggested that subsidence may be involved in more collapses than we know.

“As opposed to just saying it’s a problem, we can respond, address, mitigate, adapt,” said Ohenhen. “We have to move to solutions.”

Sources: Columbia Climate School, Nature Cities