The big Chinese cities are sinking. Half of the surface of the country's largest cities, where three-quarters of its urban population is concentrated, loses more than 3 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 cause a quarter of the Asian giant's coastal lands to be below sea level within a century.
This progressive sinking—called subsidence—has been detailed by a large group of Chinese researchers in a study published this Thursday in the journal Science. Scientists have seen that the land sinks more or less quickly depending on the geological composition under the city. Human action, mainly through urbanization and the extraction of subsoil water, explains the decrease in land area.
The newly built surface sinks faster than that which has been built for years, and heavier buildings sink more slowly, being anchored to deeper layers of the soil. Furthermore, where groundwater levels are lower (mostly due to human causes), subsidence is greater.
Scientists warn that the loss of height in Chinese cities damages buildings and promotes flooding of coastal areas. For this reason, they call for collaboration between politicians, scientists and civil engineers to take measures to prevent the sinking and mitigate its consequences.
Perhaps the most direct and effective measure is to stop the extraction of subsoil water. Cities in Japan, the Netherlands or China itself are examples of success in which water table management has put a stop to a phenomenon that is global. 19% of the world's population lives in areas at risk of subsidence, according to a 2021 article with Spanish participation, but China, and Asia in general, are especially prone due to the geological composition of its subsoil. In Europe, however, the problem is less.
Although ground subsidence has been reported in many cities throughout the world, the study published today by Science is the first to analyze a large set of urban centers at the same time and with the same methodology. Scientists have 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 3 millimeters per year; 16% do so at more than 1 centimeter annually. These rates of decline affect 29 and 7% of the Chinese urban population respectively, and put at risk around 10% of the Asian giant's coastal population, which by 2120 will live below sea level, with the consequent increase in flood risk.
The analysis has been 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, allow tiny changes in the height of the earth to be measured. The same technology is used to measure ground deformation in volcanic eruptions and earthquakes.
“Both systems will help us develop better estimates of what is happening on broad scales, even at a global level,” Robert Nicholls, director of the Tyndall Center for Climate Change Research, who has not participated, points out in an email to La Vanguardia. in the study.
The expert points out, in an analysis of the topic also published by Science, that the method used by Chinese researchers will allow us to understand the causes of large-scale subsidence. For this reason, it will also be key to developing responses to the phenomenon, whose consequences largely depend on human behavior.
