Nearly half of China's major cities are reported to be experiencing "moderate to severe" levels of subsidence, posing significant flood risks as sea levels continue to rise, according to a study based on nationwide satellite data released on Friday.

Published in the journal Science, the study reveals that 45% of China's urban land is sinking at rates exceeding 3 millimeters per year, with 16% experiencing subsidence of over 10 mm per year. This phenomenon is attributed not only to declining water tables but also to the immense weight of urban infrastructure.

With China's urban population surpassing 900 million, the researchers led by Ao Zurui from South China Normal University highlight the substantial threat to urban life posed by even a small portion of subsiding land.

Subsidence already imposes annual losses exceeding 7.5 billion yuan ($1.04 billion) on China, and projections suggest that within the next century, nearly a quarter of coastal land could be below sea level, placing hundreds of millions of people at heightened risk of flooding.

Robert Nicholls from the Tyndall Centre for Climate Change Research at the University of East Anglia emphasizes that subsidence is a nationwide issue in China and mirrors global trends. Tianjin, a northern city with a population exceeding 15 million, is notably affected, as evidenced by a recent evacuation of 3,000 residents due to a "sudden geological disaster" attributed to water depletion and geothermal well construction.

Additionally, China's historical coal districts have suffered from overmining, necessitating reinforcement measures such as injecting concrete into crumbling shafts.

This problem extends beyond China, with a separate study published in February highlighting approximately 6.3 million square kilometers (2.4 million square miles) of global land at risk of subsidence. Indonesia, particularly Jakarta, faces significant challenges, with parts of the capital now situated below sea level.

Nicholls underscores the importance of subsidence mitigation efforts, drawing attention to Tokyo's successful measures, which included banning groundwater extraction in the 1970s. While complete prevention may be unattainable, adaptation strategies such as building dykes are essential.

According to a 2022 study in Singapore, of the 44 major coastal cities grappling with subsidence, 30 are located in Asia. Matt Wei, a geophysics expert at the University of Rhode Island, attributes this trend to urbanization, population growth, and increased water extraction.