The Biggest US Cities are Sinking

Imagine you’ve built a sandcastle on a nice, flat stretch of beach. You’ve kept it well away from the waves. Over the course of the day, though, the tide recedes. It pulls water out of the layers of sand beneath your castle. Gradually, the earth below your creation starts to settle and sink. The sands above start to shift. How long might your sandcastle last?

City planners across the US are asking themselves similar questions. A new study on subsidence, or the gradual sinking of an area over time, shows that as many as 34 million Americans live in sinking cities. Just like a sandcastle, coastal cities are at risk. But so are those in the Heartland.

“While often considered ... a coastal hazard due to relative sea-level rise, subsidence also threatens inland urban areas, causing increased flood risks, structural damage and transportation disruptions,” researchers wrote in the study. They looked at satellite images taken between 2015 and 2021. They found that all 28 of the largest cities in the US are sinking. Houston, Texas, is dropping the fastest at around 5 millimeters per year.

The most common cause of subsidence is human use of groundwater. Water found in the soil and rock beneath a city helps absorb the pressure of the tons of buildings, streets, and cars above. When we draw that water out of the ground, the dirt settles into the space left behind. That drags down everything above it.

To help avoid damage in the future, the study's authors suggest cities start planning now. “This problem,” they warned, “is always only going to increase as we progress into the future.”

The study was published May 8 in the journal Nature Cities.

Reflect: If you could help a city stay safe from problems like sinking or flooding, what would you do to protect it?

Photo of Houston, Texas, from Unsplash courtesy of Kevin Hernandez.