Sinking Towns: Several Major Cities in China Are Subsiding Due to Increasing Weight, Water Extraction

According to a new study, tens of millions of individuals who live in coastal cities in China could find their houses below sea level in 2120 due to subsidence and rising sea levels.

Sinking Cities in China

The study "A national-scale assessment of land subsidence in China's major cities'' notes that one in ten residents of the coastal cities of China could end up living below sea level in less than a century. This is mainly due to water extraction and the growing weight of their fast expansion.

There are some cities that are subsiding quite rapidly. In fact, one in six exceed 10 millimeters each year.

The rapid urbanization of the country in the past decades means that significantly more water is drawn up to meet the needs of people.

The country has seen a long history of handling subsiding land. In fact, both Tianjin and Shanghai have shown evidence of sinking as early as the 1920s. In the past century, Shanghai has sunk over three millimeters.

According to the researchers, the cities that face the worst problems are concentrated in five specific regions. These covers boast inland and coastal cities.

The list also includes cities in the largely land-locked southwestern area, such as Guiyang, Nanning, and Kunming. Such a discovery has surprised geographer Zhou Yuyu from the University of Hong Kong, as these areas are not as industrialized or densely populated compared to other areas in China.

Pingdingshan, which is one of China's largest coal areas, was also seen to subside at 109 millimeters per year.

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Examining Subsidence

Recently, China has also been seeing more evidence of sinking in several cities that have rapidly expanded in the past decades. To know more about the problem's scale, a research team from different universities in China examined 82 cities. This included all cities that have a population that exceeds two million.

The researchers made use of data of the Sentinel-1 satellites for measuring vertical land motions throughout the country. Examining the period from 2015 to 2022, the researchers were able to work out that roughly 45% of urban areas are seeing subsidence that is equivalent to over three millimeters each year. Moreover, roughly 16% of urban land is sinking faster than a rate of 10 millimeters a year. Scientists have described this rate as a rapid descent.

This means that around 67 million people are living in areas that are rapidly sinking.

There are various factors that affect the scale of the decline. These factors include building weight, geology, and groundwater loss, which the authors say is a major factor.

Essentially, this means water extraction near or underneath cities for the local population to use it. Such a phenomenon has already been observed in several other urban areas across the world, such as Delhi, Mexico City, and Houston.

For this case in China, the researchers were able to associate increasing subsidence levels with water extraction from more than 1,600 monitoring wells. Professor Robert Nicholls, who is from the University of East Anglia and who did not participate in the study, expresses that for him, water extraction could be the dominant contributor. Geologically speaking, there are several people in China who live in areas that have been sedimented fairly recently. Hence, when soils are drained or groundwater is taken, these end up subsiding.

Aside from this, urban transportation systems and coal and mineral mining also affect subsidence.

According to the researchers, land is sinking more rapidly compared to the rising sea levels. However, when put together, this would put hundreds of millions of individuals at risk for flooding.

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