AGI – 45% of China’s urban areas are sinking, with 16% sinking at a rate of 10 mm per year or more. This was revealed by a study conducted by scientists from the University of East Anglia and Virginia Tech, published in Science. Robert Nicholls, of UEA’s Tyndall Center for Climate Change Research, and Manoochehr Shirzaei, of Virginia Tech and the United Nations University for Water, Environment and Health, in Ontario, highlighted how land subsidence is an overlooked danger in cities and, therefore, how important is a new research paper that analyzes satellite data that accurately and consistently maps ground movements in China. The study looked at 82 cities with a combined population of nearly 700 million people. Nationwide, an estimated 270 million urban residents have been affected, with nearly 70 million experiencing rapid declines of 10mm per year or more. Hot spots include Beijing and Tianjin.
According to scientists, the constant measurement of subsidence is a great achievement, but it is only the beginning in the search for solutions. Predicting future subsidence requires models that take into account all causes, including human activities and climate change, and how these may change over time. Coastal cities like Tianjin are particularly hard hit, as sinking land reinforces climate change and rising sea levels. The collapse of sea defenses is one reason why Hurricane Katrina brought so much devastation and deaths to New Orleans in 2005. Shanghai, China’s largest city, has sunk as much as 10 feet over the last century and continues to lower even today. If subsidence is combined with sea level rise, China’s urban area below sea level could triple by 2120, affecting 55 to 128 million residents. This situation could be catastrophic without a strong response from society.
“Subsidence puts the structural integrity of buildings and infrastructure at risk and exacerbates the impact of climate change in terms of flooding, particularly in coastal cities where it reinforces sea level rise,” said Nicholls, who did not participated in the study, but whose research focuses on sea level rise, coastal erosion and flooding, and how communities can adapt to these changes. Subsidence is mainly caused by human action in cities. Groundwater withdrawal, which lowers the water table, is considered the most important factor in subsidence, along with geology and the weight of buildings. In Osaka and Tokyo, groundwater abstraction was stopped in the 1970s and city subsidence has ceased or is significantly reduced, proving this to be an effective mitigation strategy. Traffic vibrations and tunnels are also potentially a contributing factor at a local level: in Beijing there is a subsidence of 45 mm per year near subways and highways. Natural ground movements up or down also occur, but are typically much smaller than human-induced changes.
Although human-induced subsidence was already known in China before this study, Nicholls and Shirzaei believe these new findings reinforce the need for a nationwide response. This problem occurs in sensitive cities outside of China and is a widespread phenomenon throughout the world. The researchers called on the scientific community to move from measurement to understanding the implications and supporting responses. New satellite measurements are providing detailed new data on subsidence, but methods for using this information to work with urban planners to address these issues need further development. Affected coastal cities in China and in general need special attention. “Many cities and areas around the world are developing strategies to manage the risks of climate change and sea level rise,” Nicholls said. We must learn from this experience to also address the threat of subsidence, which is more common than we think,” concluded Nicholls.
