Almost half of China's major cities are sinking, putting millions of locals at risk of flooding, according to a new study published in the journal Science this week.
The study found that 45 percent of China's urban land was sinking faster than 3 mm a year, while 16 percent was sinking at a rate of more than 10 mm a year.
The study authors looked at 82 Chinese cities with populations of more than 2 million and used radar pulses from satellites to identify any changes in the distance between the satellite and the ground.
They then measured how the cities' elevations had changed between 2015 and 2022.
They found that China's largest city, Shanghai, was continuing to subside despite already sinking around 3 m over the past 100 years.
Cities such as Beijing and Tianjin were also particularly affected.
Causes
There are a number of factors leading to the subsidence, but Robert Nicholls, a professor of climate adaptation at the University of East Anglia, who was not involved in the research, told the BBC that he believed that water extraction was "probably the dominant reason."
"In China there are lots of people living in areas that have been fairly recently sedimented, geologically speaking. So when you take out groundwater or you drain the soils, they tend to subside," he said.
The paper also suggested that the resolution to the issue "could lie in the long-term, sustained control of groundwater extraction."
In a comment article also published in Science, Nicholls warned that "subsidence jeopardises the structural integrity of buildings and critical infrastructure and exacerbates the impacts of climate change in terms of flooding, particularly in coastal cities where it reinforces sea-level rise."
Subsidence already costs China over 7.5 billion yuan, which is around $1.05 billion, a year, Reuters reported.
The problem is not confined to China, however.
Another paper published in February said that around 6.3 million square km, or around 2.4 million square miles, of land around the world was at risk of subsidence, with Indonesia one of the worst affected countries.
Parts of the US have also suffered from the problem, with more than 17,000 square miles of land across 45 states directly affected, according to the United States Geological Survey (USGS).
According to the USGS, more than 80 percent of the identified cases of subsidence in the US occurred due to the exploitation of underground water.
"The increasing development of land and water resources threatens to exacerbate existing land-subsidence problems and initiate new ones," it adds.
This article was originally published by Business Insider.
More from Business Insider: