A new study published in the journal Science has revealed that nearly half of China's major cities are experiencing "moderate to severe" levels of subsidence, putting millions of people at risk of flooding, especially as sea levels continue to rise due to climate change. The research, conducted by a team led by Ao Zurui of the South China Normal University, analyzed nationwide satellite data and found that 45% of China's urban land is sinking faster than 3 millimeters per year, with 16% sinking at a rate of more than 10 mm per year.
The subsidence problem is driven not only by declining water tables but also by the sheer weight of the built environment in these rapidly urbanizing areas. With China's urban population already exceeding 900 million people, even a small portion of subsiding land could pose a substantial threat to urban life. The study estimates that subsidence already costs China more than 7.5 billion yuan ($1.04 billion) in annual losses, and within the next century, nearly a quarter of coastal land could be lower than sea levels, putting hundreds of millions of people at an even greater risk of inundation.
Robert Nicholls, a subsidence expert at the Tyndall Centre for Climate Change Research at the University of East Anglia, who was not involved in the study, emphasized the scale of the problem, stating, "It really brings home that this is for China a national problem and not a problem in just one or two places. And it is a microcosm of what is happening around the rest of the world."
The northern city of Tianjin, home to more than 15 million people, was identified as one of the worst-hit areas. Last year, 3,000 residents were evacuated after a "sudden geological disaster" that investigators blamed on water depletion and the construction of geothermal wells. Many of China's old coal districts have also suffered as a result of overmining, with authorities often forced to inject concrete into crumbling shafts to reinforce the land.
The subsidence problem is not limited to China, as a separate study published in February found that around 6.3 million square km (2.4 million square miles) of land across the globe is at risk. Indonesia, in particular, has been severely affected, with large parts of its capital, Jakarta, now below sea level.
Nicholls suggested that vulnerable cities could learn from Tokyo's example, which sank by about 5 m (16 feet) until it banned groundwater extraction in the 1970s. "Subsidence mitigation should be looked at very seriously, but you can't stop all of it so you are talking about adaptation and building dykes," he added.
Of the 44 major coastal cities suffering from subsidence, 30 were found to be in Asia, according to a 2022 Singapore study. Matt Wei, a geophysics expert at the University of Rhode Island, attributed the problem to "urbanisation and population growth - larger population density, more water extracted, (and) more subsidence."
This groundbreaking study is the first to use satellite data to systematically measure how much cities are sinking across China. Similar recent studies in Europe and the United States have also found significant subsidence in some cities, but not to the same extent as the widespread sinking present across China.
Measuring how much cities are sinking can help urban planners and government officials prevent future subsidence and even reverse some sinking that has already occurred. In places where groundwater removal is the main problem, cities can ban further pumping of groundwater from areas that are rapidly sinking. In some cases, it's even possible to pump water back into the ground to prop up land that had fallen.
