A third of China’s urban population is at risk of city collapse

The land under half of the major cities chinese suffers moderate to severe subsidence, affecting almost a third of the population urban area of ​​the country, a phenomenon associated with human causes such as the extraction of groundwater or the weight of buildings.

This problem was analyzed by a group of Chinese researchers led by Peking University, who carried out a systematic assessment of land subsidence in 82 major cities between 2015 and 2022, and published their results in Science.

In recent decades, that country has experienced one of the most rapid and extensive urban expansions in human history, but “may be threatened by land subsidence,” writes the research team.

Of the urban land examined, a Four. Five% is sinking more than 3 millimeters a year and a 16% more than 10 millimeters, which affects a 29% and even 7% of the urban population, respectively.

The most affected points are Beijing and Tianjin, where the subsidence is ten millimeters per year or more, highlights an analysis article published by Science along with the study and signed by experts from the Virginia Polytechnic Institute (USA) and the University from East Anglia (United Kingdom).

Coastal cities like Tianjin are especially hard hit as land subsidence reinforces climate change and rising sea levels. Shanghai, China’s largest city, has sunk by up to three meters in the last century and continues to do so today.

The research also looks to the future and warns that by 2120, between 22 and 26% of the lands on the coast (of a 9 to eleven% of the coastal population) will have a relative elevation lower than sea level, due to the combined effect of the sinking of cities and the rise in sea level.

“Our results underscore the need to improve protective measures to mitigate potential damage from subsidence.”indicates the group of researchers, led by Jingyun Fang, from Peking University.

If subsidence is combined with sea level rise, “China’s urban area below sea level could triple by 2120, affecting between 55 and 128 million residents. “This could be catastrophic without a strong social response,” says the University of East Anglia, referring to its explanatory article.

Land subsidence poses a greater risk to roads, runways, building foundations, railway lines and pipelines, among other infrastructure.

The results of the study suggest that this subsidence process is associated with a series of anthropogenic factors. The main one would be the extraction of groundwater, which lowers the water table, along with the geology and the weight of the buildings.

The measurements were taken with an interferometric synthetic aperture radar (InSAR) on the Sentinel-1 satellite, which uses high-precision radar pulses to measure the change in distance between the satellite and the ground surface and can detect relatively small changes. In addition, terrestrial GPS data were used.

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