As China’s cities grow, they are also sinking.
According to a new study, an estimated 16% of the country’s major cities are losing more than 10 mm of elevation per year, and nearly half are losing more than 3 mm per year. published in Science magazine.
These amounts may seem small but they add up quickly. According to research, in the next 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.
“It 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, this study is the first to measure subsidence across multiple urban areas at once using modern radar data from satellites.
Research shows that subsidence in these cities is partly due to the weight of buildings and infrastructure. 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 compact or fall.
Beijing is one of the places in the country that is sinking the fastest. So did nearby Tianjin, where last year Thousands of people had to evacuate from high-rise apartment buildings after the streets outside suddenly separated. Within these cities, sinking is uneven. When adjacent pieces of land sink at different rates, anything built on that piece of land is at risk of damage.
Other countries, including the United Statesencountered the same problem.
“Land subsidence is an overlooked problem that exists almost everywhere,” said Manoochehr Shirzaei, a geophysicist at Virginia Tech who did the research. Subsidence in US coastal cities using similar methods. Dr. Shirzaei also reviewed new research on Chinese cities by Zurui Ao of South China Normal University, Xiaomei Hu and Shengli Tao of Peking University, and their colleagues.
“I believe that the majority of adaptation strategies we have and resilience plans to combat climate change are inaccurate, simply because they do not include land subsidence,” he said. “It hasn’t been studied in the way, for example, that sea level rise has been studied.”
The new study is based on satellite radar measurements of how land surface changes in 82 major cities, accounting for three-quarters of China’s urban population, have increased or decreased between 2015 and 2015. 2022. Researchers compared these measurements with data on potential contributing factors. , like the weight of the buildings in these cities and the change in water table beneath them.
The researchers also combined their subsidence measurements with projections of sea level rise to work out which cities could lie below sea level. One thing to note with these findings is that they assume a constant rate of subsidence over the next 100 years, but this rate could change with human activity.
About 6% of land in China’s coastal cities is now relatively below sea level. This study shows that if global average sea levels rise by 0.87 meters, or a little less than 3 feet, by 2120 (the higher of the two scenarios commonly considered by researchers), the That could increase to 26%.
Being below sea level doesn’t mean a city is automatically doomed. Much of the Netherlands is below sea level and is sinking, but the country has been extensively designed to prevent flooding in many places and to contain it in others.
The key to minimizing damage is to limit groundwater extraction, the researchers write. Shanghai has 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 combat subsidence by pumping water into depleted aquifers in a process called managed recharge.
Dr Nicholls said it was difficult to completely prevent subsidence. “You have to live with what’s left.” This mainly means adapting to rising sea levels in coastal areas, he said; not only sea level rise due to climate change but also the effects of land subsidence.
