Sinking cities: how China is moving subsidence research forward

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.

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

Coastal cities are particularly susceptible to sinking because of their natural conditions. For one, they are often built on river deltas or coastal plains, where sediments compact over time, which leads to subsidence, says Ding Xiaoli, a geodesist at the Hong Kong Polytechnic University, who measures and monitors Earth’s shape and size. Some coastal cities, such as Tokyo, also sit on earthquake-prone areas, where tectonic activity can contribute to subsidence.

But the growth of coastal cities itself — nearly one-third of the global population in 2018, or more than 2 billion, lived within 50 kilometres from the shore — is also greatly exacerbating the issue.

In China, the main cause of subsidence linked to human activity is the excessive extraction of groundwater as cities expand, says Zhang Yonghong, a researcher at the Chinese Academy of Surveying and Mapping in Beijing. The practice lowers the underground water level, which causes the surrounding soil to compact and the land to sink. “The building of infrastructure, such as subways, can also cause parts of a city to subside,” Zhang says.

Disruption to the natural recharging of groundwater owing to paved urban surfaces can worsen the situation, says Yu Kongjian, a landscape architect at Peking University in Beijing. “Land subsidence is one of the most profound manifestations of ecological mismanagement in urban regions,” he says.

Cities are also getting heavier as they grow, says Zhao Qing, a geodesist at East China Normal University in Shanghai. Ongoing research by her team suggests that the increasing weight of buildings is one of the three factors — together with dropping groundwater levels and the nature of the soil — that are causing Shanghai and other cities in the Yangtze River Delta region to subside.

None of the above is unique to China, and some of these issues — such as groundwater extraction and building weight — often affect cities inland, too. But on the coast, these factors are driving the land downwards in areas that also face accelerated sea-level rise owing to climate change.

The speed at which sea levels are rising has more than doubled over the past three decades⁴, increasing from around 2.1 millimetres per year in 1993 to around 4.5 millimetres per year in 2023. By the end of this century, the global average sea level is projected to be up to 0.55 metres higher than the 1995–2014 average, even if the world limits global warming to 1.5 degrees above pre-industrial levels, as outlined in the Paris Agreement.

A 2022 study⁵ that assessed 99 coastal cities around the world found that most of them saw parts of their land subside faster than the sea level was rising. At their current downward trajectories, they would be challenged by flooding much sooner than the timelines projected by sea-level models, the authors said.

One of the cities that has been struck by this double whammy is Jakarta, the current capital of Indonesia, an archipelagic country on the frontline of fighting sea-level rise. More than 40% of the city is already below sea level — where risks of flooding, storm surges, infrastructure damage and economic and life losses are particularly high — and by the middle of the century up to 95% of its coastal areas might be submerged. The grim prospect was a major factor in Indonesia deciding to move its capital to a different island by 2028.

Since 2014, Indonesia has been building a 32-kilometre ‘Giant Sea Wall’ in a bid to shield Jakarta from flooding — a project that is expected to cost US$50 billion. But sea walls have shallow foundations, so they follow the movement of the land, says Pietro Teatini, a hydrologist at the University of Padova in Italy, and chair of the UNESCO Land Subsidence International Initiative (LaSII), a working group that aims to spread knowledge on subsidence around the world and help developing countries to better address the issue. “If the land subsides, the wall subsides, too,” he says.

Successful measures

Shanghai — the first city in China to identify subsidence — also faces the compounding effect of sinking ground and rising sea level. It sank by 1.69 metres from 1921 to 1965 as a result of excessive groundwater pumping. The city’s government has carried out a series of measures to tackle the issue over the past 60 years, mainly by restricting groundwater usage and replenishing groundwater supply using water from the Yangtze River, according to Ye Shujun, a hydrogeologist at Nanjing University in China. Those methods have reduced the speed at which Shanghai is sinking to within 6 millimetres a year, said Ye, who shared the city’s experience at a webinar organized by LaSII in early 2025.

A lot of work has been done in China to address subsidence from scientific and technical points of view, says Teatini. “One of the most important” steps for tackling subsidence is the artificial recharging of groundwater, he says, which has proved effective in Shanghai. But he warns that the method is expensive, so it might be hard to introduce to poorer countries.

Teatini points to the advanced equipment that Chinese cities use, such as extensometers, which measure how materials change under stress. Researchers drill down , sometimes to almost 1,000 metres, to place the devices at the bottom of Earth’s water-bearing layers, known as the aquifer system, to monitor which layers are compacting. “In Italy, we have three or four [extensometers] spread all over the country. In Shanghai, there are about 50,” says Teatini, who works with Chinese researchers regularly.

Some of Teatini’s long-term collaborators come from the Capital Normal University in Beijing, which has forged a formal partnership with the University of Padova. They have teamed up in a series of studies, ranging from assessing the causes of Beijing’s subsidence to developing models for predicting the problem.

To Teatini, China’s action to reduce land subsidence has been “very, very effective”. One of its achievements comes through the South–North Water Transfer Project, a colossal infrastructure project launched by the government to divert water from the south of the country to the water-stressed north. Although the main goal of the project is to balance water supplies, it has played a crucial role in mitigating long-term subsidence of the North China Plain by reducing the extraction of groundwater. Before the project went into operation in 2014, Beijing, situated in the country’s arid north, had heavily relied on groundwater and suffered severe subsidence of up to 159 millimetres a year as a result.