The nation's biggest cities are sinking, according to data from a new study.
Known scientifically as land "subsidence," the most common cause of the sinking is "massive ongoing groundwater extraction," say the study authors, though other forces are at work in some places. The cities include not just those on the coasts, where sea level rise is a concern, but many in the interior.
In every city studied, at least 20% of the urban area is sinking - and in 25 of 28 cities, at least 65% is sinking.
It's a unique study: "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," study lead author Leonard Ohenhen of the Lamont-Doherty Earth Observatory told USA TODAY on May 7.
He added that the study "offers critical information for urban planning, infrastructure adaptation, and hazard preparedness."
The nation's fastest-sinking city is Houston, with more than 40% of its area dropping more than 5 millimeters (about 1/5 inch) per year, and 12% sinking at twice that rate.
Some localized spots are going down as much as 5 centimeters (2 inches) per year, according to the study.
Two other Texas cities, Fort Worth and Dallas, are not far behind.
Subsidence is often called an "invisible threat" because it can go unnoticed for long periods, Ohenhen said. "Here, (in the study), we are making it less invisible."
When land shifts downward, even just a little bit, the structural integrity of buildings, roads, bridges, and dams can be profoundly impacted, said Ohenhen, a geosciences graduate student.
He told USA TODAY that "some early warning signs (of subsidence) that are often overlooked 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."
Generally, according to a statement from Lamont-Doherty, it happens as water is withdrawn from aquifers made up of fine-grained sediments; unless the aquifer is replenished, the pore spaces formerly occupied by water can eventually collapse, leading to compaction below, and sinkage at the surface.
In Texas, the problem is exacerbated by pumping of oil and gas, the study says.
On the map, three cities (San Jose, Memphis, and Jacksonville) have green dots, indicating "uplift". Why is this? "The uplift we detect is an average across the city and is small in these cities: 0.1 to 0.3 millimeters per year, which is almost negligible," he said. "We can just say 0 millimeters per year, so on average stable. It does not mean the entire city is rising uniformly."
"A lot of small changes will build up over time, magnifying weak spots within urban systems and heighten flood risks," said Ohenhen. "This subsidence can produce stresses on infrastructure that will go past their safety limit," he added.
The researchers say that continued population growth and water usage combined with climate-induced droughts in some areas will likely worsen subsidence in the future.
"As opposed to just saying it's a problem, we can respond, address, mitigate, adapt," said Ohenhen. "We have to move to solutions."
The study was published May 8 in the British journal Nature Cities.