As China’s cities become taller, bigger and more modern, they face a major problem: the ground beneath them is sinking. A 2024 study1 found that nearly half of the land under the country’s major cities is subsiding at a “moderate” rate of more than 3 millimetres a year, and 16% is experiencing “rapid” sinking, meaning greater than 10 millimetres annually.
Many of these cities, such as Tianjin, Fuzhou and Ningbo, are located by the sea. The issue of land subsidence is so pressing that the study projected that one in ten residents of the country’s coastal cities will be living below sea level by 2120 if current trends continue.
The consequences are already showing. In 2023, nearly 4,000 people in Tianjin — a port city of more than 13 million residents — had to be evacuated from high-rise apartment buildings after the streets outside suddenly split apart. Scientists sent to investigate the site believe that the problem was caused by a ‘geological cavity’ some 1,300 metres under the ground, according to the city’s government. They pointed to the drilling of a geothermal well as a possible trigger, which might have caused subterranean water and soil loss, leading the land to give way.
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China’s plight provides a snapshot of a global crisis. Eight of the world’s ten largest cities are situated on the coast, including Shanghai, New York, India’s Mumbai and Lagos in Nigeria, and all of them are dealing with subsidence. The megacities that are rapidly expanding along Asia’s shores are among the fastest-sinking cities on the planet, according to a 2022 study2.
The highest subsidence rates have been recorded in Tianjin, Vietnam’s Ho Chi Minh City and Bangladesh’s Chittagong; parts of these cities were found to be subsiding at peak speeds of more than 50 millimetres per year. In North America, subsidence is also widespread. An analysis of 28 major US cities, including coastal cities such as Houston and New York, published this year3 estimated that at least one-fifth of all mapped urban areas are sinking, affecting some 34 million people.
Subsidence is not an issue that any one country can fight on its own. Some Chinese cities, such as Shanghai and Guangzhou, have modelled initiatives based on what’s worked in the Netherlands, for example, one of the lowest-lying countries in the world. Residents and institutions are encouraged by their local government to collect and reuse rainwater, such as by installing ‘green roofs’ covered with vegetation to hold rainwater, and building communal gardens to absorb or slow runoff.
China is also passing on its knowledge to other developing countries. In 2023, Shenzhen shared its experience of evacuating people from partly subsided buildings with policymakers from Tripoli in Lebanon, through a collaborative effort co-organized by the United Nations Development Programme to help the city better respond to natural disasters.
According to Liu Jianxin, a geophysicist at Central South University in Changsha, China, Chinese researchers are increasingly sharing data, writing papers and participating in workshops with international colleagues on the issue of subsidence. And, like Liu, many of them are based in inland cities that are also facing subsidence. “Tackling subsidence is a global effort,” Liu says.
Coastal challenges
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