As a result of the subsidence, coastlines are even more vulnerable to rising seas.
|Manila, Philippines, is one of the world's fastest sinking cities, with some areas subsiding at a rate of up to 1.5 cm per year.
GETTY IMAGES/MATTEO COLOMBO/DIGITALVISION
Satellite data show that coastal communities throughout the world are sinking by several centimeters each 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 communities at higher danger of flooding than previously assumed.
Matt Wei, an earth scientist at the University of Rhode Island in Narragansett, and his colleagues looked at 99 coastal communities across six continents. "We wanted to strike a balance between population and location," he explains. While cities have previously been measured for subsidence, past 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 incorporate data from around the world."
Wei and his colleagues used data from a pair of European satellites collected primarily between 2015 and 2020. Microwave signals are sent to Earth by instruments on board, which then record the waves that return. The scientists measured the height of the ground with millimeter accuracy by measuring the timing and intensity of those reflected waves. The researchers were able to track how the ground distorted over time since each satellite flies over the same section of the planet every 12 days.
The researchers discovered that Asian cities with the highest subsidence rates — up to five centimeters per year — are predominantly Tianjin, China; Karachi, Pakistan; and Manila, Philippines. Furthermore, one-third of the cities studied, or 34, are sinking by more than a centimeter every year in some areas.
That's a concerning tendency, 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.
From 2015 to 2020, satellite measurements of ground height in and around coastal communities demonstrate how quickly many are sinking. The ground is settling by more than 10 millimeters per year in portions of some cities, including these four. Positive numbers indicate that the ground is sinking and moving away from a satellite, whereas negative values indicate that the ground is rising. The insets depict satellite imagery of locations with the most subsidence, indicating that those areas are being developed for residential or industrial purposes. The Global Navigation Satellite System stations used in the analysis are indicated by magenta triangles and circles.
Rates at which the ground level is changing in four cities
Wei and his colleagues believe that people are to blame for much of the subsidence. When the researchers examined Google Earth imagery of fast sinking locations within cities, they found predominantly residential or commercial areas. The scientists determined that this is a sign that groundwater extraction is to blame. As water is drained out of aquifers, landscapes tend to sink (SN: 10/22/12).
However, there is reason to be optimistic. Jakarta, for example, was sinking at a rate of roughly 30 cm per year in the past. However, subsidence there and elsewhere has halted recently, probably as a result of new government laws limiting groundwater exploitation.