Land under: China’s land area is sinking in many places, satellite data shows. This affects almost half of all large cities and around a third of the country’s urban population, as a research team reports in “Science”.
In the coming decades, about a quarter of China’s coastal areas will be below sea level. This increases the risk of flooding for millions of residents. But what is behind land subsidence and how can it be prevented?
China has massively expanded its cities in recent decades. They grew both in height and width and recorded an increase in population. However, rapid urbanization could now be put on hold. There is increasing evidence that the land area under Chinese cities is decreasing, especially among large cities. However, little is known about the extent of this land subsidence.
A team led by Zurui Ao from the South China Normal University in Foshan has now investigated this. To do this, the researchers evaluated measurement data from a satellite radar and GPS data for 82 of China’s largest cities. These metropolises are important industrial centers and home to a total of around 700 million people. The data precisely documented land subsidence between 2015 and 2022 by reflecting the distance of the satellite to the Earth’s surface with millimeter precision.
The evaluations showed that 45 percent of the urban areas examined are sinking quite quickly – by more than three millimeters per year. Around 16 percent of cities are actually sinking by ten millimeters or more every year. 270 million and 70 million people live in these areas – 29 percent and seven percent of China’s urban population, respectively, the team reports.
The declining hotspots include the metropolises of Beijing and Tianjin. China’s largest city, Shanghai, has already sunk by three meters in the last century.
But how does this land subsidence come about? Ao and his colleagues attribute these primarily to human activities, as their extent is significantly greater than natural ground subsidence caused by geological activities. These include, for example, the buildings themselves, whose weight pushes the land masses downward. But other factors such as the extraction of groundwater or the vibrations of traffic have probably also led to the sinking of the soil.
In itself, the decline would not be dramatic. But it endangers the stability of buildings and infrastructure. In coastal regions, land subsidence also means that cities like Tianjin and Shanghai will be below sea level in the future, as Ao and his colleagues have calculated. This effect is intensified by the predicted further rise in sea level due to climate change.
Overall, according to the analyses, around a quarter of the coastal areas in China will be below sea level in the future. This increases the risk of flooding for residents of the cities there.
In order to protect coastal cities, countermeasures must be taken to stop further land subsidence, the researchers write. The Japanese cities of Tokyo and Osaka could serve as a model here, as they are sinking significantly less or not at all since groundwater is no longer extracted there. “A major challenge is moving from measuring subsidence to thinking systematically about its effects,” write Robert Nicholls of the University of East Anglia and Manoochehr Shirzael of Virginia Tech in a commentary on the study.
The project is looking for urban planning solutions to counter subsidence in China that can also be applied to similarly subsiding regions in other countries – such as the Mekong Delta or the east coast of the USA. “Ideally, this will serve as a guide for immediate and long-term strategic actions, analogous to the strategies developed for coastal areas threatened by sea level rise.” (Science, 2024; doi: 10.1126/science.adl4366)
Quelle: American Association for the Advancement of Science (AAAS), University of East Anglia
By Claudia Krapp
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The original for this article “Why China’s gigantic cities are sinking further and further” comes from scinexx.
