China: why almost half of the country’s big cities are starting to sink

Nearly half of China’s major cities are sinking due to water extraction and the increasing weight of its rapid expansion, researchers say.

Some cities are sinking rapidly: one in six exceeds 10 mm per year.

China’s rapid urbanization in recent decades means much more water is now being extracted to meet people’s needs, scientists say.

In coastal cities, this sinking threatens millions of people with floods as sea level rises.

China has a long history of dealing with land subsidence, and both Shanghai and Tianjin showed evidence that they were sinking in the 1990s. 1920.

In the last century, this city has sunk more than 3 meters.

In more modern times, the country is seeing widespread evidence of subsidence in many of the cities that have expanded rapidly in recent decades.

To understand the magnitude of the problem, a team of researchers from several Chinese universities examined 82 cities, all with a population of more than 2 million.

The researchers used data from satellites Sentinel-1 to measure the vertical movements of the earth throughout the country.

By analyzing the period between 2015 and 2022, the team was able to determine that the 45% of urban areas are sinking more than 3 mm per year.

Around 16% of urban land is sinking at more than 10mm a year, which scientists describe as a rapid decline.

In other words, it means that 67 million people live in rapidly sinking areas.

Researchers say the cities facing the worst problems are concentrated in the five regions highlighted on the map below.

The magnitude of the decline is influenced by a number of factors, including the geology and the weight of buildings.

But an important element, according to the authors, is the groundwater loss.

Basically, this means the extraction of water from under or near cities for use by local people.

This has already been seen in several major urban areas around the world, such as Houston, Mexico City and DelhYo.

In China, the research team was able to associate water withdrawal from more than 1,600 monitoring wells with increasing levels of subsidence.

“I think water extraction is, in my opinion, probably the dominant reason,” says Professor Robert Nicholls of the University of East Anglia in the United Kingdom, who was not involved in the research.

“In China there are a lot of people who live in areas whose sediments, from a geological point of view, have been deposited recently. So when you extract groundwater or drain the soil, it tends to sink.”

Other factors that are influencing include the systems of urban transport and the extraction of minerals and coal.

In the northern Pingdingshan region, one of the country’s largest coalfields, land is sinking at an extremely rapid rate of 109 mm per year.

The study’s authors say that a major threat in the future is the exposure of urban populations to flooding, due to a combination of subsidence and sea level rise caused by climate change.

In 2020, about 6% of China had a relative elevation below sea level. Within 100 years, this figure could rise to 26% of the country in a medium to high carbon emissions scenario.

Researchers say the land is sinking faster than the seas are rising, but together they would put hundreds of millions at risk of flooding.

However, studies show that there are effective strategies that can combat the phenomenon.

Subsidence problems have affected other major urban centers in Asia in the past, including Osaka and Tokyo in Japan.

“Tokyo sank around the port area, up to five meters in the 20th century,” explains Professor Nicholls.

“But in the 1970s, they used good water pipes from other areas and they also had a law that said well water would not be usedand that essentially stopped the sinking.”

The study was published in the journal Science.

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