Big Chinese cities are sinking

All major Chinese cities, to a greater or lesser extent, are sinking. The subsidence process has been taking place for years, in some cases up to a century. But the most recent measurements, obtained from satellite, show an accelerated rate of sinking of several millimeters per year, even up to two centimeters per year in some cases. Among the causes, in addition to natural geological ones, there are several anthropogenic ones, especially the abuse of aquifers. Although subsidence processes are occurring throughout the planet, in China their pace is very pronounced and appears related to the accelerated urbanization of recent decades. Around 300 million Chinese urbanites are watching the earth sink beneath their feet.

A few years ago, a group of scientists led by Spanish experts put on the map hundreds of subsidence events that were occurring on the planet. Many of them appeared located in China, in the most populated areas of the enormous country. Now, Chinese researchers have used a complex tool reminiscent of LIDAR (which has given archaeologists so much joy discovering lost cities) to detect changes in the elevation of the terrain in the 82 cities with more than two million inhabitants. The InSAR (Interferometric Synthetic Aperture Radar) system mounted on the Sentinel-1 satellite allows detecting variations in altitude of millimeters for each pixel of terrain, which is equivalent to grids of 40 x 40 meters of surface. The authors of this research have recorded the changes that have been occurring since 2015, a year after the satellite entered service. Their results have just been published in

.

44.7% of the area of ​​all large Chinese cities has been sinking at least at a rate of 3 millimeters per year, the threshold at which InSAR measurements are reliable. This means that it affects a third of the urban population, about 270 million people. There is a significant 15.8% of the territory that is sinking even faster, above 10 mm/year (one centimeter), with almost 70 million inhabitants. And 5% of the urban surface does it 2.2 centimeters annually. In global terms and based on its population, the list of most affected cities is headed by Tianjin, the fifth most populated city, with more than 15 million. Put on the map, the cities that are sinking the most are concentrated in the entire eastern fringe of the country and in the south, those that have led the modernization of China that began with the long march of Deng Xiaoping and accelerated in recent decades.

The specific causes that trigger the sinking are human and the first is the abuse of aquifers. The explanation is very simple, when water is removed above its replacement rate, the subsoil, as if it were Swiss cheese, is filled with cavities that cannot support the weight from above. The study shows the correlation between the state of 1,619 aquifers with the millimeters that the land subsides. The second factor that stands out is the vertical design of the new cities that, with their skyscrapers, grow more upward than horizontally. They found that the more recent the construction and the average height, the greater the degree of subsidence. Highways and all the traffic they support also sink the ground. In Beijing, for example, areas near roads are being lowered by 45 mm per year. There are other more local phenomena such as obtaining hydrocarbons through

or mining

The industrial city of Pingdingshan, a city in the country's main coal region, is falling at a rate of more than 10 centimeters a year.

“If an entire area sinks under an infrastructure there is no angular distortion, the problem is when the subsidence is not uniform”

Roberto Tomás, professor at the University of Alicante, expert in subsidence and civil engineering

“Venice is sinking at a rate of 1.6 millimeters a year,” recalls Roberto Tomás, professor at the University of Alicante and expert in subsidence and civil engineering. “Meanwhile, Lorca and the Guadalentín valley had been sinking 100 mm (now 80 mm) annually due to the withdrawal of water,” he adds. This area of ​​the Region of Murcia is the most extreme case in Europe caused by the exploitation of aquifers. With these two examples, Tomás wants to show that a subsidence rate of 3 millimeters does not have to be worrying. Another thing is 10 millimeters. “It's one centimeter a year, 10 in a decade,” he remembers. Although the global map of subsidence in which Tomás intervened showed the global nature of the phenomenon, it does coincide with several factors that accentuate it in the case of Chinese cities: “Soft sediments, expansion of urbanization, with cities created from nothing , with all its people, all its infrastructure, its water needs...", highlights Tomás.

The Spanish researcher, member of a special UNESCO commission for subsidence processes, believes it is important to take into account “differential settlement”: not how much the soil sinks, but whether it does so unevenly. “If an entire area sinks under an infrastructure there is no angular distortion, the problem is when the subsidence is not uniform,” he develops. And he gives two examples that he has studied well, Lorca and Murcia. The first is the urban area that sinks the most in all of Europe, “but there is no damage to the buildings because it is a uniform subsidence,” says Tomás. The opposite happens in the capital, in Murcia. In the 1990s he participated in the preparation of a report in which they detected 150 buildings with serious damage that cost, at the exchange rate in pesetas, 150 million euros. And all because in the Murcian capital it is sinking unevenly.

“Favorable geological environments (deltas and floodplains) are found widely in China and even more so in South, Southeast and East Asia”

Robert J. Nicholls, director of the Tyndall Center for Climate Change Research at the University of East Anglia, United Kingdom

The director of the Tyndall Center for Climate Change Research at the University of East Anglia (United Kingdom), Robert J. Nicholls, maintains that subsidence is not a global phenomenon in the strict sense. “Subsidence doesn't happen everywhere; It can only occur where the geology favors it,” he says. Regarding the specificity of the Chinese case, he adds that “favorable geological environments (deltas and floodplains) are widely found in China and even more so in South, Southeast and East Asia.” To which is added, he continues, "in this area, including China, the pressure of cities and the extraction of groundwater develops, although there are other reasons why subsidence may occur."

The authors of the study on Chinese cities projected the sinking of coastal cities within 100 years. More than half of the 82 included in their research are next to the sea or a few kilometers from it. And in these cases two problems will come together, the subsidence of the land part and the expected rise in sea level due to climate change. Its results are very dependent on what is done to stop both processes. In the case of the first, the most effective measure, which is already used in the Murcian valleys, was introduced by two Japanese cities in the 1970s. Osaka and the capital, Tokyo, had been sinking throughout the 20th century due to excessive exploitation. of its aquifers. After a decade of replenishing more water than they removed, both cities stopped their sinking. In China, if anthropogenic causes are not combated and in the worst expected climate scenario, a quarter of coastal urban areas will be like Venice or New Orleans, being below sea level to a greater or lesser extent by the year 2120.