Millions of people in China are at risk of flooding as the country’s major cities collapse. It is “moderate to severe” in nearly half of the megacities.
The risk increases as sea levels rise, according to a study of satellite data from across the country released on Friday and cited by Reuters.
The authors of the paper, published in the journal Science, found that 45% of urban land in China is sinking at a rate faster than 3 mm per year, and 16% at more than 10 mm per year, which is due not only to the reduction of subsoil water, but also the burden of construction.
Given the fact that China’s urban population already exceeds 900 million people, “even if a small part of the land sinks, it can become a significant threat to urban life,” said the team of researchers from South China University.
Land subsidence is already costing China more than 7.5 billion yuan ($1.04 billion) in annual losses, and over the next century nearly a quarter of coastal land may actually be below sea level, which will put hundreds of millions of people at even greater risk of flooding.
The northern city of Tianjin, home to more than 15 million people, has been identified as one of the worst affected. Last year, 3,000 of its residents were evacuated after a “sudden geological catastrophe” that investigators blamed on water depletion as well as the construction of geothermal wells.
Many of China’s old coal areas have also suffered as a result of over-mining, with authorities often forced to inject concrete into collapsing shafts to shore up the ground.
The problem is not limited to China. A separate study published in February said about 6.3 million square kilometers of land worldwide were at risk. Among the worst affected countries is Indonesia, where much of the capital, Jakarta, is below sea level. Experts give the example of Tokyo, which sank by about 5 meters until it banned groundwater extraction in the 1970s.
