China's megacities are experiencing significant subsidence, with nearly half of the country's major cities showing signs of sinking.
A recent study, which examined China's 82 largest cities, reported that over 30% of the hundreds of millions of people living in China's urban areas are at risk due to sinking ground levels. This trend could have severe consequences, especially as cities face rising sea levels and the threat of flooding.
Reasons for Subsidence
The study found that two primary factors contribute to the subsidence of China's megacities: the weight of buildings and the extraction of groundwater for drinking water.
University of East Anglia professor Robert Nicholls explained to the BBC that water extraction is likely the dominant cause of the problem. The continuous removal of groundwater creates voids, leading to the gradual collapse of the ground above.
Researchers from Peking University and South China Normal University used satellite radar measurements to track even minor changes in land elevation. Data from 2015 to 2022 revealed that 45% of urban areas in China are sinking by more than 3 millimeters per year, with 16% sinking by more than 10 millimeters per year.
Beijing and Tianjin in northern China are among the cities experiencing the fastest rates of sinking, with the largest proportion of people living in areas sinking by more than 10 millimeters annually.
The study warns that if current trends continue, large parts of these cities could be below sea level within a century, posing significant flood risks.
The Global Context
The problem of sinking cities extends beyond China. Large cities worldwide, particularly those near coastlines, are also experiencing land subsidence.
In Indonesia, the government is building a new capital city because Jakarta is sinking at an alarming rate of 20 centimeters per year. Other countries facing similar issues include Pakistan and Bangladesh, where urban areas are susceptible to subsidence due to water extraction and other factors.
Geophysicist Manoochehr Shirzaei from Virginia Tech emphasized that more research is needed to understand the causes and implications of sinking cities. He noted that studies on subsidence have often been overshadowed by those on rising sea levels, despite the potential for severe consequences.
