Index – Science – China’s big cities are being swallowed up by the earth

A new study has revealed that China’s big cities are sinking dangerously due to groundwater extraction and the weight of buildings. Rapid urbanization could threaten the housing of millions of people in the next hundred years. Among the affected cities are Beijing and Tianjin, the largest coastal city in northern China.

As sea levels rise, by 2120 at least 22-26 percent of land near the sea will be at sea level, putting at least 10 percent of the people who live here at risk of housing. This could affect up to 100 million people. According to the researchers, the land subsidence is faster than the sea level rise, but the two together represent a huge danger.

The new study, published in Science, relies on soil measurements carried out between 2015 and 2022. During these seven years, land subsidence was probed in every Chinese city with more than 2 million inhabitants. Of the 82 cities examined, 45 percent sink by more than 3 millimeters per year, 16 percent by more than 10 millimeters per year.

Subsidence also has natural causes, such as geological location and depth of bedrock, which affect the load-bearing capacity of the soil. The researchers discovered that groundwater loss leaves empty pore space in the crust, which compresses due to the weight on it.

Groundwater changes are mainly due to human activity, transport and deep drilling for hydrocarbon mining are all to blame for soil changes, but precipitation is only 12 percent responsible for subsidence.

It’s dangerous elsewhere

The most important thing would be the long-term, permanent control of groundwater extraction, because destabilized, cracked soil increases the risk of floods. Currently, six percent of the country’s territory is below sea level, and the only way to stop the sinking is to slow down groundwater extraction. A good precedent for this is Tokyo, where it succeeded in slowing down the decline in the 1970s. At that time, there was a lot of subsidence in the region around the port, but the authorities started to bring piped water from elsewhere and made it a law that residents should not use local well water. This stopped the rapid dive.

Buildings and infrastructure are not only at risk in China, the structural integrity of New York, Baltimore and Charleston are also threatened by subsidence. NASA satellite images from late February clearly show land movement on the East Coast. According to satellite data, between 2007 and 2020, the ground under New York and Norfolk is sinking an average of 1-2 millimeters per year. But several counties in Delaware, Maryland, South Carolina and Georgia had double or triple that, especially in coastal cities where sea-level rise is also adding to human activity.

Subsidence near Maryland and around Virginia is likely due to groundwater withdrawal and pumping of water back into aquifers. By the way, this is done to prevent the infiltration of salt water. The high subsidence rate of the coasts of Georgia, South and North Carolina is probably influenced by the presence of dams, which block the passage of sediment traveling in rivers. This sediment would fill coastal areas.

NASA researchers say the sinking land will see salty seawater seep into farmland and freshwater supplies, but could also affect wildlife habitat and disrupt marshland ecosystems.

(Cover photo: Locals look at a sunken building in Ziyuan County, China in 2015. Photo: Getty Images)

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