China’s ‘sinking’ major coastal cities are at risk of flooding as sea levels rise, research warns

A quarter of China’s coastal area will sink below sea level within a century, putting hundreds of millions of people at risk of flooding, a new study has found.

The scientists behind the study said that changes in groundwater and the weight of buildings appeared to be linked to the phenomenon, and that “the key to tackling the subsidence of China’s cities could lie in the long-term, sustainable control of the groundwater extraction”.

The researchers from universities across China, including South China Normal University and Peking University, the Chinese Academy of Sciences and two universities in the United States, published their findings Friday in the peer-reviewed journal Science.

“We identified a significant risk of coastal flooding unless adequate protective measures are implemented and enforced,” they wrote.

“(China’s most populous city) Shanghai and its neighboring areas are actively pursuing long-term control of groundwater extraction, which likely explains the slow land subsidence observed there,” they said.

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They said collaboration between policymakers, the research community and civil engineers was needed to tackle the problem effectively.

According to the study, subsidence causes ground fissures, damages buildings and increases the risk of flooding. Disasters related to sinking land in China have injured or killed hundreds of people and cost direct economic losses of more than 7.5 billion yuan ($1 billion) in recent decades.

The team mapped the subsidence of cities between 2015 and 2022 on a national scale using a technique powered by the European Space Agency’s Sentinel-1 satellites to measure vertical land movements by using radar pulses to measure the change in distance between the satellite and the ground surface.

The scientists analyzed 82 major cities with a population of more than 2 million people, representing almost three-quarters of the country’s urban population of 920 million people.

They found that about a third of the population in those 82 cities lived in areas that were subsidence faster than 3 mm per year, while 7 percent lived in areas that were subsidence faster than 10 mm per year. Hotspots for the issue include Tianjin in northern China and Beijing.

The article found that 270 million Chinese now live on sinking land – equivalent to almost a third of Europe’s population – and that 67 million people live in regions of rapidly sinking land, roughly equivalent to the population of France.

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The researchers also found that 22 to 26 percent of China’s coastal area would fall below sea level by 2120 due to both land subsidence and rising sea levels. These areas are home to approximately 10 percent of the country’s coastal population.

They said coastal cities are at greater risk than inland cities due to rising sea levels. Compound flooding – due to the combination of sea level rise and subsidence of cities – poses a serious threat to the coastal population, according to the newspaper.

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