China's Major Cities Experiencing Subsidence Risk: Study

A study of nationwide satellite data released on Friday revealed that nearly half of China's major cities are experiencing "moderate to severe" levels of subsidence, putting millions of people at risk of flooding, especially as sea levels rise.

In the paper published by the journal Science, it was found that 45% of China's urban land is sinking faster than 3 millimetres per year, with 16% sinking at more than 10 mm per year, as reported by Hindustan Times.

China's Urban Subsidence Risk

This subsidence is driven not only by declining water tables but also by the sheer weight of the built environment. The team of researchers led by Ao Zurui of the South China Normal University highlighted that with China's urban population already exceeding 900 million people, "even a small portion of subsiding land in China could therefore translate into a substantial threat to urban life."

According to the study, subsidence already costs China more than 7.5 billion yuan ($1.04 billion) in annual losses, and within the next century, nearly a quarter of coastal land could actually be lower than sea levels, putting hundreds of millions of people at an even greater risk of inundation, as reported by Hindustan Times citing Reuters.

"It really brings home that this is a national problem for China and not limited to just one or two places," remarked Robert Nicholls at the Tyndall Centre for Climate Change Research at the University of East Anglia. "And it is a microcosm of what is happening around the rest of the world."

The northern city of Tianjin, housing more than 15 million people, was identified as one of the worst-hit areas. Last year, 3,000 residents were evacuated after a "sudden geological disaster" attributed to water depletion and the construction of geothermal wells. Many of China's old coal districts have also suffered due to overmining, with authorities often required to inject concrete into crumbling shafts to reinforce the land, as reported by Hindustan Times.

The issue is not unique to China. A separate study published in February revealed that around 6.3 million square km (2.4 million square miles) of land across the globe is at risk of subsidence. Among the most affected countries is Indonesia, where large parts of the capital Jakarta are now below sea level.

According to Nicholls, vulnerable cities can learn from Tokyo's experience, which sank by about 5 m (16 feet) until groundwater extraction was banned in the 1970s. "Subsidence mitigation should be taken very seriously, but it's not possible to stop all of it, so we are talking about adaptation and building dykes," he added.

Of the 44 major coastal cities affected by subsidence, 30 are in Asia, according to a 2022 Singapore study. "It is a problem of urbanization and population growth - larger population density, more water extracted, (and) more subsidence," explained Matt Wei, a geophysics expert at the University of Rhode Island. ($1 = 7.2382 yuan), as per media reports.