A startling 45% of China’s urban regions are currently sinking, with 16% experiencing a downward shift of 10 mm or more per year, as revealed by a comprehensive study conducted by scientists from the University of East Anglia and Virginia Tech, recently published in Science. Robert Nicholls from the Tyndall Centre for Climate Change Research at UEA and Manoochehr Shirzaei from Virginia Tech and the United Nations University for Water, Environment, and Health in Ontario emphasized the underestimated risk of soil subsidence in urban areas, underscoring the critical need for new research to meticulously map ground movements in China using satellite data. The study encompassed 82 cities with an aggregate population of nearly 700 million individuals. Nationally, it’s estimated that around 270 million urban residents have been affected, with almost 70 million witnessing a rapid subsidence rate of 10 mm or more annually. Notable hotspots include Beijing and Tianjin.

However, while the continuous measurement of subsidence marks a significant stride, it merely initiates the quest for solutions. Forecasting future subsidence necessitates comprehensive models that factor in all causative agents, encompassing human activities and climate variations, and their evolving dynamics over time. Coastal municipalities like Tianjin face particularly acute challenges, as land subsidence compounds the effects of climate change and rising sea levels. The subsidence of coastal defenses exemplifies why Hurricane Katrina wrought such havoc and loss of life in New Orleans in 2005. Shanghai, China’s sprawling metropolis, has sunk by up to 3 meters over the last century and continues to subside presently. When coupled with rising sea levels, the urban areas of China situated below sea level could triple by 2120, imperiling anywhere from 55 to 128 million residents. Such a scenario underscores the urgent need for a robust societal response.

“Subsidence not only undermines the structural integrity of buildings and infrastructure but also exacerbates the ramifications of climate change, particularly in terms of flooding, especially pronounced in coastal cities where it compounds the effects of sea-level rise,” emphasized Nicholls, whose research focuses on sea-level rise, coastal erosion, flooding, and strategies for community adaptation. Subsidence primarily stems from human interventions within urban landscapes. Groundwater extraction, which lowers the water table, ranks as the primary catalyst for subsidence, alongside geological factors and the weight of built structures. The cessation of groundwater extraction in Osaka and Tokyo in the 1970s led to a cessation or substantial reduction in city subsidence, demonstrating the efficacy of mitigation strategies. Moreover, local factors such as traffic-induced vibrations and subterranean infrastructure, notably observed in Beijing where subsidence near subways and highways reaches 45 mm annually, also contribute to localized subsidence. While natural terrain movements occur, they typically pale in comparison to human-induced alterations.

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