As China’s cities grow, they also sink.
An estimated 16 percent of the nation’s major cities are losing more than 10 millimeters of elevation per year, and nearly half are losing more than 3 millimeters per year, according to a new study. was published in the journal Science.
These amounts may seem small, but they add up quickly. In 100 years, a quarter of China’s urban coastal land could be below sea level due to a combination of subsidence and sea-level rise, according to the study.
“It’s a national problem,” said Robert Nicholls, a climate scientist and civil engineer at the University of East Anglia who reviewed the work. Dr Nicholls added that, to his knowledge, this study is the first to measure subsidence in multiple urban areas simultaneously using state-of-the-art radar data from satellites.
The subsidence in these cities is caused in part by the sheer weight of buildings and infrastructure, according to the study. Pumping water from aquifers beneath cities also plays a role, as do oil drilling and coal mining, all activities that leave empty space underground where soil and rock can compress or collapse.
Beijing is one of the fastest sinking parts of the country. So is nearby Tianjin, where last year thousands of residents were evacuated from high-rise apartment buildings after the streets outside suddenly split. Within these cities, the immersion is uneven. When pieces of land next to each other subside at different rates, anything built on that land is at risk of damage.
Other countries, including the United States, have similar problems.
“Soil subsidence is a neglected problem that exists almost everywhere,” said Manoochehr Shirzaei, a geophysicist at Virginia Tech who has studied subsidence in American coastal cities using similar methods. Dr. Shirzaei also reviewed the new study on Chinese cities by Zurui Ao of South China Normal University, Xiaomei Hu and Shengli Tao of Peking University and their colleagues.
“I think the majority of the adaptation strategies that we have, and the resilience plans to combat climate change, are inaccurate, just because they didn’t include land subsidence,” he said. “It hasn’t been studied the way, for example, sea level rise has been studied.”
The new study was based on satellite radar measurements of how much the land surface in 82 major cities, representing three-quarters of China’s urban population, moved up or down between 2015 and 2022. The researchers compared these measurements with data on potential factors such as the weight of buildings in these cities and the change in groundwater levels beneath them.
The researchers also combined the subsidence measurements with projections of sea level rise to figure out which cities could end up below sea level. A caveat with these findings is that they assumed a constant rate of subsidence over the next 100 years, but those rates may change along with human activity.
About 6 percent of the land in China’s coastal cities currently has a relative elevation below sea level. If global average sea levels rise by 0.87 meters, or just under 3 feet, by 2120 (the higher of the two commonly used scenarios the researchers looked at), that figure could rise to 26 percent, according to this study.
Being below sea level doesn’t mean a city is automatically doomed. Much of the Netherlands is below sea level and sinking, but the country is extensively designed to prevent flooding in places and accommodate it in others.
The key to minimizing the damage is limiting groundwater extraction, the researchers wrote. Shanghai is already taking this approach and is sinking more slowly than other Chinese cities. In Japan, groundwater management over the years has proven successful in stabilizing subsidence in Tokyo and Osaka.
Some places are even combating sedimentation by injecting water into depleted aquifers in a process called managed recharge.
It is difficult to stop sedimentation completely, Dr Nicholls said. “You have to live with what’s left.” Mainly, he said, this means adapting to sea-level rise in coastal areas. not only sea level rise caused by climate change, but also the effects of land subsidence.