China's sinking cities: A looming crisis for over 100 million people

China is facing a serious problem. Land subsidence threatens more than half of its largest cities. Scientists warn that the issue could affect over 100 million people within a hundred years.

Researchers from the University of East Anglia and Virginia Tech warn that nearly half of China's 82 largest cities are at risk of sinking due to rapid land subsidence. Their analysis, published in the journal "Science", indicates that this problem could affect between 55 and 128 million people by 2120 if appropriate measures are not taken.

The researchers emphasize that the problem of land subsidence in Chinese cities has often been overlooked and underestimated, even though it poses a significant threat. Their publication explains that land subsidence is a common process that can sometimes be dramatic. Potentially, as much as 19% of the world's population is at high risk associated with this process.

Subsidence results from a combination of natural and anthropogenic factors, with groundwater withdrawal due to human activity playing a significant role.

To understand the scale of the problem, scientists analyzed satellite data mapping land subsidence across China. They included 82 cities, home to nearly 700 million people collectively. Their analysis shows that 45% of the urban areas examined are sinking (affecting 270 million people), and the subsidence rate in 16% is 10 millimetres per year or more (where about 70 million people live). The worst situation is in Beijing and Tianjin.

Coastal cities, such as Tianjin, are particularly vulnerable to subsidence. This process goes hand in hand with rising sea levels caused by climate change. The effects of this phenomenon can be observed, for example, in New Orleans, where in 2005, Hurricane Katrina caused flooding of over 80% of the city area. It was one of the greatest natural disasters in United States history, with the earlier subsidence of southern Louisiana terrain considered a primary reason for such extensive damage.

In Beijing, land subsidence is facilitated by vibrations caused by traffic and tunnelling; neighbourhoods located near subway lines and highways are sinking even at a rate of about 45 millimetres per year. The largest city in China – Shanghai, is also threatened by subsidence. Over the course of a century, the ground level has lowered by approximately 3 metres, and the process continues.

Experts believe that this pace of land subsidence, combined with the predicted rise in sea levels, will cause the area of Chinese cities located below sea level by 2120 to triple, threatening between 55 and 128 million residents. Without decisive action by the authorities, this could have catastrophic consequences.

Professor Robert Nicholls from the University of East Anglia says land subsidence threatens the structural integrity of buildings and critical infrastructure and exacerbates the effects of climate change, such as floods, especially in coastal cities vulnerable to rising sea levels.

The specialist believes that salvation could come from a change in groundwater use policy. In Osaka and Tokyo, where groundwater withdrawal was stopped in the 1970s, the subsidence process either halted or significantly reduced.

Scientists add that the issue of land sinking affects not only Chinese cities but the entire world. Therefore, they call on the scientific community to move from measurements and analyses to a deeper understanding of this process's consequences and develop solutions in collaboration with urban planners.