Land is sinking underneath millions of peoples' feet in China's major cities due to human activities, putting the country's coastal areas more at risk of flooding and rising sea levels, new research shows.
Nearly half of China's urban areas comprising 29% of the country's population are sinking faster than 3 millimeters (about 0.12 inches) per year, according to the study published Thursday in the journal Science. That's 270 million people living on sinking land.
Meanwhile, 67 million people are living on land that is sinking faster than 10 millimeters (0.4 inches) each year.
China's rampant groundwater extraction is one of the primary factors for subsidence, according to the researchers, from Peking University in Beijing and other Chinese institutions.
Cities have been pumping water from underground aquifers faster than it can be replenished, a situation exacerbated by climate change-fueled drought. Excessive pumping lowers the water table and causes the overlying land to sink.
The land is also sinking due to the growing weight of cities themselves. Soil can compact naturally from the weight of sediments accumulating over time and from heavy buildings pressing down on the ground, causing the land to steadily sink.
Land subsidence, a gradual settling or sudden sinking of the Earth's surface, isn't just a problem in China. In the US, dozens of coastal cities, including New York City, are sinking. In the Netherlands, 25% of its lands have sunk lower than sea levels. And in Mexico City, likely the world's fastest subsiding city, land is sinking at the speed of up to 50 centimeters, or nearly 20 inches, a year.
The impact of sinking is typically worse along the coasts, where the sea level is rising at the same time. This combination exposes more land, people and property to destructive flooding.
The study suggests roughly a quarter of China's coasts will be lower than sea level because of subsidence and projected sea level rise, priming the area for colossal damage and putting lives at risk. Major population centers like Tianjin, Shanghai and areas around Guangzhou are significantly exposed to both issues, the study found.
But some coastal areas in China have already built physical protection from the growing risk of inundation, and the study does not take those protections into account. In Shanghai, for example, Shengli Tao, co-author of the study and professor at Peking University, said the city has built "impressive" dike systems that are meters tall.
"Such massive coastal dike systems will largely reduce the risk of being inundated even given both land subsidence and sea level rise," Tao told CNN. "I am not aware of other countries that have built such massive dike systems."
Keep reading about the sinking cities.
More on environmental challenges faced by the country:
