The ground beneath many Chinese cities is steadily sinking – with an increasing risk of flooding in the metropolises on the country’s coast as a result. Almost half of China’s urban areas – 45 percent – are sinking at a rate of more than three millimeters per year, a research team reports in the journal Science. In 16 percent it is even more than ten millimeters per year. The megacity Beijing is one of the cities severely affected by subsidence.
It is also known from Shanghai that areas of the city have sunk by up to three meters in the last century. The team led by Zurui Ao from South China Normal University in Foshan now evaluated satellite measurements from 82 major Chinese cities with 74 percent of the country’s urban population between 2015 and 2022. Around a third (29 percent) of the population in these cities is affected by subsidence of more than three millimeters. As of 2020, according to the researchers, a total of 920 million people lived in urban areas in China – an estimated 270 million of them lived on sinking land.
Risk of flooding
The researchers warn that ground subsidence could have long-term consequences, especially in densely populated coastal regions. Due to the combination of ground subsidence and sea level rise, around a quarter of the areas there are at risk of sinking below sea level in the next hundred years. This means a significant flood risk for large population groups. There would also be direct damage to buildings and foundations, infrastructure and sewage systems.
Soil subsidence has been linked to a number of factors, including groundwater extraction and the weight of buildings. In recent decades, China has experienced one of the fastest and most extensive urban expansions in human history, the researchers said. Cases of ground subsidence are already increasingly being reported in large cities. Subsidence-related disasters in China have already caused hundreds of deaths and injuries every year as well as immense economic damage in recent decades.
Protection is possible
“The results underline the need to strengthen protective measures and strictly control groundwater extraction,” the study said. Long-term controls and more sustainable water management, as is already being successfully practiced in the Japanese cities of Tokyo and Osaka, could also help to stabilize the subsidence rate in China.
Land subsidence is a phenomenon that causes significant problems not only in China but also in many other parts of the world. It is often caused by human activities such as overexploitation of groundwater, intensive development or oil and gas extraction. But natural geological factors also play a role. For example, cities such as Venice, Italy and Mexico City, Mexico are known for their problems with ground subsidence.
In some areas of the world there are opposing processes – for example through the ongoing uplift of land areas freed from the weight of the ice sheet of the last ice age.
