The great Chinese cities are collapsing. Half of the surface of the country’s largest cities, where three quarters of its urban population are concentrated, loses more than three millimeters of height per year. In some regions, the rate exceeds one centimeter per year. This phenomenon, together with rising oceans due to climate change, will mean that a quarter of the Asian giant’s coastal lands will be below sea level within a century.
This progressive collapse has been detailed by a large group of Chinese researchers in a study published this Thursday by Science magazine. Scientists have seen that the lands sink more or less quickly depending on the geological composition under the city. Human action, mainly through urbanization and the extraction of water from the subsoil, has just explained the decrease in land.
Newly built surfaces sink faster than those that have been built for years, and heavier buildings sink more slowly because they are anchored to deeper layers of the soil. In addition, where groundwater levels are lower (mostly due to human causes), subsidence is greater.
Scientists warn that the loss of height of Chinese cities damages buildings and favors flooding in coastal areas. That is why they call for collaboration between politicians, scientists and civil engineers, to take measures to prevent the collapse and mitigate its consequences.
Perhaps the most direct and effective measure is to stop the extraction of groundwater. The cities of Japan, the Netherlands and China themselves are examples of success in which phreatic management has put a stop to a phenomenon that is global.
The scientists collected data between 2015 and 2022 for 82 Chinese cities: all those with more than two million inhabitants, all provincial capitals and the main industrial centers of the country.
45% of the surface of these cities is sinking at a rate of more than three millimeters a year; 16%, at more than 1 centimeter per year. These rates of decline affect 29% and 7% of China’s urban population, respectively, and put at risk nearly 10% of the Asian giant’s coastal population, which by 2120 will live below sea level, with consequent increase in flood risk.
The analysis was carried out from space, thanks to the Sentinel-1 satellite of the European Space Agency (ESA), which is equipped with a device called InSAR. The device emits very precise radar waves, which, combined with GPS technology, make it possible to measure minute changes in the height of the earth. The same technology is used to measure ground deformation in volcanic eruptions and earthquakes.
“The two systems will help us develop better estimates of what is happening on large scales, even on a global scale,” Robert Nicholls, director of the Tyndall Center for Climate Change Research, said in an email to La Vanguardia, which does not participated in the study.
