Satellite observations show that coastal cities around the world are sinking by several centimetres per year on average. Researchers report in the April 16 issue of Geophysical Research Letters that the combination of subsiding land and rising seas puts these coastal regions at greater risk of flooding than previously thought.
Matt Wei, an earth scientist at the University of Rhode Island in Narragansett, and his colleagues looked at 99 coastal cities across six continents. “We tried to strike a balance between population and location,” he says. While cities have previously been measured for subsidence, previous research has tended to focus on just one city or region. Wei claims that this investigation is unique. “It’s one of the first to truly use data from around the world.”
Wei and his colleagues used data from two European satellites collected between 2015 and 2020. Microwave signals are sent to Earth by instruments onboard, which then record the waves that return. The team determined the height of the ground with millimetre accuracy by measuring the timing and intensity of those reflected waves. The researchers were able to track how the ground deformed over time because each satellite flies over the same part of the planet every 12 days.
The team discovered that Asian cities with the highest subsidence rates — up to five centimetres per year — are mostly in Tianjin, China; Karachi, Pakistan; and Manila, Philippines. Furthermore, one-third of the cities studied, or 33, are sinking by more than a centimetre per year in some areas.
That’s a concerning trend, according to Daro Solano-Rojas, an earth scientist at Mexico City’s National Autonomous University who was not involved in the study. These cities are being affected by a double whammy: the soil is sinking at the same time as sea levels are increasing owing to climate change (SN: 8/15/18). “Knowing that portion of the problem is critical,” adds Solano-Rojas.
Wei and his colleagues believe that individuals are to blame for much of the subsidence. When the researchers examined Google Earth photos of quickly sinking locations within cities, they found predominantly residential or commercial sectors. The scientists determined that this is a sign that groundwater exploitation is to blame. As water is drained out of aquifers, landscapes tend to sink (SN: 10/22/12).
However, there is cause to be optimistic. Cities like Shanghai and Jakarta, Indonesia, used to sink by more than 10 centimetres each year on average. However, sinking in such areas has halted recently, probably as a result of new government laws restricting groundwater exploitation.