While China's cities are growing, they are also sinking.
An estimated 16 percent of the country's major cities are losing more than 10 millimeters a year, and nearly half are losing more than 3 millimeters a year, according to a new study published in the journal Science.
These amounts may seem small, but they add up quickly. In 100 years, a combination of land subsidence and sea level rise could result in a quarter of China's urban coastlines being underwater, according to the study.
“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. Nichols added that, to his knowledge, this study is the first to use state-of-the-art radar data from satellites to measure subsidence in many urban areas at once.
Studies have found that the subsidence of these cities is partly due to the weight of buildings and infrastructure. Pumping water from underground aquifers in cities also plays a role, as does oil drilling and coal mining, and all of these activities leave empty space underground, where soil and rock can become compacted or collapse. There is a possibility.
Beijing is one of the fastest sinking places in the country. The same was true in nearby Tianjin, where thousands of residents were evacuated from high-rise apartment buildings last year when streets outside suddenly split. Within these cities, subsidence is uneven. If adjacent land sinks at different rates, anything built on top of that land is at risk of damage.
Other countries, including the United States, have similar problems.
“Subsidence is an overlooked problem that exists almost everywhere,” says Manuchel Shirzai, a geophysicist at Virginia Tech who uses similar methods to study subsidence in American coastal cities. To tell. Dr. Shirzaei also reviewed new research on Chinese cities by Zurui Ao of South China Normal University, Xiaomei Hu of Peking University, Shengli Tao, and colleagues.
“I think most of the adaptation strategies and resilience plans we have to combat climate change are inaccurate simply because they don’t include subsidence,” he said. Stated. “For example, it has not been studied in the same way that sea level rise has been studied.”
The new study was based on satellite radar measurements showing how much the ground surface in 82 major cities, representing three-quarters of China's urban population, rose and fell between 2015 and 2022. The researchers compared these measurements with data on potential contributing factors. , changes in the weight of buildings in these cities and in the groundwater levels beneath them.
The researchers also combined subsidence measurements with predictions of sea level rise to understand which cities could be submerged below sea level. One caveat about these findings is that although constant subsidence rates are assumed over the next 100 years, these rates may change with human activity.
Currently, about 6 percent of the land in China's coastal cities has a relative elevation below sea level. That rate could rise to 26 percent if global average sea levels rose by 0.87 meters, or just under 3 feet, by 2120 (the higher of two common scenarios considered by the researchers). was found in this study.
Just because a city is below sea level doesn't mean it's automatically doomed. Much of the Netherlands is below sea level and sinking, but the country is undergoing extensive construction to prevent flooding in some places and cope with flooding in 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 many years has proven successful in stabilizing land subsidence in Tokyo and Osaka.
In some places, land subsidence is being combated by injecting water into depleted aquifers, a process called managed recharge.
Dr Nichols said it would be difficult to stop land subsidence completely. “We have to live with what's left.” This mainly means adapting to rising sea levels in coastal areas, he said. In addition to sea level rise due to climate change, land subsidence may also have an impact.
