China’s ‘sinking’ big coastal cities at risk of floods as sea levels rise, study warns

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

The scientists behind the research said groundwater changes and building weight appeared to be linked to the phenomenon, and “the key to addressing China’s city subsidence could lie in the long-term, sustained control of groundwater extraction”.

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

“We found a considerable risk of coastal inundation unless adequate protective measures are implemented and maintained,” they wrote.

“[China’s most populous city] Shanghai and its neighbouring areas have been actively pursuing long-term control of groundwater extraction, which likely explains the slow subsidence observed there,” they said.

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They said collaborations between policymakers, the research community and civil engineers were needed to effectively address the issue.

Subsidence causes ground fissures, damages buildings and heightens the risk of floods, according to the study. Disasters related to sinking land in China have injured or killed hundreds of people and cost an annual direct economic loss of more than 7.5 billion yuan (US$1 billion) in the past decades.

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

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

They found that around one-third of the population in those 82 cities lived in regions subsiding faster than 3mm a year, while 7 per cent lived in areas subsiding faster than 10mm a year. Hotspots for the issue include Tianjin in northern China and Beijing.

The paper determined that 270 million Chinese people now lived on sinking land – equivalent to nearly one-third of the population of Europe – and 67 million people were living in regions with rapidly sinking land, or roughly equivalent to the population of France.

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The researchers also found that 22 to 26 per cent of China’s coastal land would fall below sea level by 2120 because of both subsidence and rising seas. These areas host around 10 per cent of the country’s coastal population.

They said coastal cities were at higher risk than inland cities because of rising sea levels. Compound floods – from the combination of sea level rise and city subsidence – pose a serious threat to coastal populations, according to the paper.

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