America's Largest Cities, Including San Francisco, Are Quietly Sinking

The slow sinking of the nation’s biggest metropolitan areas — including parts of San Francisco — poses a growing hazard with vast socioeconomic consequences, researchers said in a new study.

Because the sinking areas are usually in the densest parts of cities, as many as 34 million people could be affected and 29,000 buildings could be at high risk of damage, according to the study published this week in the journal Nature Cities.

Using satellite data from 2015 to 2021, the authors looked at 28 U.S. cities with populations over 600,000 and found that in every one, at least 20% of urban areas are sinking — but in 25 of the cities, at least two-thirds of their area is subsiding. This phenomenon, often caused by overpumping groundwater, can increase flood potential.

“Settlement can allow cracks to propagate across structures, causing warping of roads and pavement, which can strain infrastructure,” said lead author Leonard Ohenhen, a postdoctoral researcher at the Columbia Climate School’s Lamont-Doherty Earth Observatory.

In San Francisco, groundwater pumping isn’t a problem. Because much of the city’s fringes are built on filled-in land and the region is influenced by tectonic activity, the authors suggest that soil is compacting over time in areas like Treasure Island and Islais Creek.

The authors found that parts of the city subsided around 5 millimeters a year. On average, San Francisco sank a millimeter annually, Los Angeles 0.7 millimeters per year and San Diego 1.1 millimeters per year. Houston led all major U.S. cities, with about 20 millimeters of sinking annually.

In some cities, including parts of San José, however, land is lifting slightly, potentially because of groundwater recharge.

While millimeters’ worth of land movement may seem tiny, subsidence can stress infrastructure over time, making it unsafe in the decades to come.

“The sky is not falling,” Ohenhen said. “This does not mean that you have to leave your homes right now. But in places like San Francisco, where you have tectonic forces and earthquakes, you’re already weakening the foundations of the buildings.”

Ohenhen said the extra pressure from seismic activity can “lead eventually to such high-end catastrophes like a building collapse or more prominent structural failure.”

The good news is that with this knowledge, cities can prepare for a future that involves sinking land.

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

A separate study from this year, also using satellite imagery, found that land along San Francisco Bay in San Rafael, Corte Madera, Foster City and Alameda’s Bay Farm Island is subsiding more than 0.4 inches, about 10 millimeters, a year. When considering the subsidence rate, local sea levels could rise by more than double the regional estimate by 2050.

“Areas might be affected by the rising water much sooner than we anticipate, and that brings by itself increased flooding and tidal inundation,” said Marin Govorcin, the study’s lead author and a NASA remote sensing scientist in Southern California.

According to the study, infrastructure such as San Francisco International Airport could see nearly a foot of sea level rise by 2050.

“What this says is that our situation in relation to flooding is worse than we thought,” said Kristina Hill, a UC Berkeley professor and a leading sea level rise scientist who was not involved in the study. “The region’s economy depends on the airports, so we’ll need to continue to focus on how to adapt those airports to these new conditions.”