China's major cities are sinking

A new study suggests that 270 million people live in sinking lands

A Earth Human activities are sinking under the feet of millions of people in China's major cities, putting the country's coastal areas at risk of flooding and rising sea levels, new research says.

Nearly half of China's urban areas, which cover 29% of the country's population, are sinking at a rate of more than 3 millimeters a year, according to a study published Thursday. Journal of Science. 270 million people live on sinking land.

Meanwhile, 67 million people live in land that is sinking rapidly, at 10 millimeters per year.

According to researchers, widespread extraction of groundwater in China is one of the main factors in the decline. Cities are pumping water from groundwater aquifers at a faster rate than they can replenish, a situation exacerbated by climate change-induced droughts. Overpumping lowers the water table and weakens the ground that covers it.

The land is sinking as the weight of cities increases. The weight of sediments that build up over time and heavy buildings pressing down on the ground cause the land to continue to sink, causing soil to naturally compact.

Land subsidence is not a problem unique to China. In America, Dozens of coastal townsincluding the city New York, they sink. In the Netherlands, 25% of its land is submerged below sea level. And on Mexico CityProbably the world's fastest sinking city, the Earth is sinking at a rate of 50 centimeters per year.

The impact of this event is generally worse along coastlines, while sea levels rise. This combination exposes more land, people and property to devastating floods.

A quarter of China's coastlines could be below sea level, the study suggests, due to sea-level rise. Areas around Tianjin, Shanghai and Guangzhou are significantly exposed to both problems, the study found.

But some coastal areas in China have already built physical defenses against the risk of flooding, and the study did not take these defenses into account. In Shanghai, for example, Shengli Dao, study co-author and professor at Peking University, said the city has developed “impressive” meter-high dike systems.

“These massive coastal dam systems will greatly reduce flood risk, even taking into account land subsidence and sea level rise,” Tao told CNN International. “I don't know of any other country that has built such massive dam systems.”

The study was “scientifically sound” and did a “good job” of highlighting that subsidence is not a “coastal problem,” said Leonard Ohenhen, a postdoctoral researcher at Virginia Tech in the US. Land in America.

“Most urban cities are affected by land subsidence, but we focused our attention on coastal cities because of sea level rise,” Ohenhen, who was not involved in the study, told CNN International. “However, most urban cities experience land subsidence at rates comparable to or greater than land subsidence in coastal cities.”

Tao said the Chinese government has been dealing with land subsidence in a number of ways, including enforcing stricter laws to control groundwater abstraction in recent years. Shanghai and surrounding areas have restricted groundwater discharge, which has slowed the region's rate of decline. Decades ago, the fallout from Shanghai was a significant problem.

China is also pumping water north from the Yangtze River in southern China – including to Beijing, which is suffering from water shortages. The study concluded that the project avoids the need for excessive groundwater abstraction and has put an end to land subsidence in Beijing.

“I believe the Chinese government's efforts will solve the problem of land subsidence,” Tao said. “But I recommend that groundwater abstractions in large cities should be regularly controlled and dike systems should be regularly maintained in coastal areas.”