As China’s cities grow, they also sink.
An estimated 16 percent of China’s major cities are losing more than 10 millimeters of elevation per year, and nearly half are losing more than 3 millimeters a year, according to a new study Published in Science.
These amounts may seem small, but they add up quickly. Research shows that a quarter of China’s urban coastal land may be below sea level in 100 years due to the combined effects of land subsidence and sea level rise.
“This is a national problem,” said Robert Nicholls, a climate scientist and civil engineer at the University of East Anglia who reviewed the paper. Dr Nicholls added that to his knowledge the study was the first to use state-of-the-art radar data from satellites to measure subsidence in many urban areas simultaneously.
The study found that the subsidence in these cities was caused in part by the weight of buildings and infrastructure. Pumping water from urban aquifers also plays a role, as does oil drilling and coal mining, all of which leave empty spaces underground where soil and rock can compact or collapse.
Beijing is one of the fastest sinking places in the country.The same is true around Tianjin. I went there last year. Thousands of residents were evacuated After emerging from the high-rise apartment building, the street outside suddenly fell into pieces. Within these cities, subsidence is uneven. When adjacent land settles at different rates, anything built on that land is at risk of damage.
Other countries, Including the United Statesthere are similar problems.
“Land subsidence is a neglected problem that occurs almost everywhere,” said Manoochehr Shirzaei, a geophysicist at Virginia Tech. Land subsidence in coastal cities in the United States Use a similar approach. Dr. Shirzaei also reviewed new research on Chinese cities.
“I think most of the adaptation strategies we have and the resilience plans we have to deal with climate change are inaccurate because they don’t take land subsidence into account,” he said. “It hasn’t been studied in the same way that sea level rise has been studied. “
The new study compared the extent of land surface changes in 82 major cities, accounting for three-quarters of China’s urban population, between 2015 and 2022, based on satellite radar measurements. The researchers compared these measurements with data on potential influencing factors, such as the weight of buildings in these cities and changes in groundwater levels beneath the buildings.
The researchers also combined subsidence measurements with sea-level rise projections to determine which cities may eventually fall below sea level. One caveat to these findings is that they assume subsidence rates will remain constant for the next 100 years, but these rates may change with human activity.
Currently, about 6% of the land in China’s coastal cities has a relative altitude below sea level. The study found that this proportion could rise to 26% if global mean sea levels rise by 0.87 meters, or just under 3 feet, by 2120 (the higher of two common scenarios considered by the researchers).
Being below sea level doesn’t mean a city is doomed. Much of the Netherlands is below sea level and sinking, but the country has undertaken extensive engineering to prevent flooding in some places and areas. to adapt to others.
The key to minimizing damage is to limit groundwater extraction, the researchers wrote. Shanghai has already adopted 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 even fight subsidence by injecting water into depleted aquifers through a process called managed recharge.
Dr Nicholls said it would be difficult to completely stop subsidence. “You have to live with the rest.” That mostly means adapting to rising sea levels in coastal areas, he said; not just rising sea levels caused by climate change, but also the effects of sinking land.
