HQ Team
April 20, 2024: Forty-five percent of 82 urban areas in China, with a total population of about 700 million, are sinking — with 16% falling at 10mm a year or more, a study finds.
Researchers from the University of East Anglia and Virginia Tech analyzed satellite data that “accurately and consistently” mapped land movement across China.
“Land subsidence is overlooked as a hazard in cities,” Prof Robert Nicholls of the Tyndall Centre for Climate Change Research and Prof Manoochehr Shirzaei of Virginia Tech and United Nations University for Water, Environment and Health, Ontario, said in a UEA statement.
China’s 270 million urban residents were estimated to be affected, with nearly 70 million experiencing rapid subsidence of 10mm a year or more in places including Beijing and Tianjin.
Katrina flooding
Coastal cities such as Tianjin were being affected as sinking land reinforces climate change and sea-level rise.
The sinking of sea defences is one reason why Hurricane Katrina’s flooding brought such devastation and death toll to New Orleans in 2005, according to the study.
Shanghai – China’s biggest city – has subsided up to 3m over the past century and continues to subside today. When subsidence is combined with sea-level rise, the urban area in China below sea level could triple in size by 2120, affecting 55 to 128 million residents.
This could be catastrophic without a strong societal response, the researchers said.
“Subsidence jeopardises the structural integrity of buildings and critical infrastructure and exacerbates the impacts of climate change in terms of flooding, particularly in coastal cities where it reinforces sea-level rise,” said Prof Nicholls, who was not involved in the study, but whose research focusses on sea-level rise, coastal erosion and flooding, and how communities can adapt to these changes.
Human cause
The subsidence was mainly caused by human action in the cities. Groundwater withdrawal, which lowered the water table is considered the most important driver of subsidence, combined with the geology and weight of buildings.
In Osaka and Tokyo, groundwater withdrawal was stopped in the 1970s and city subsidence has ceased or greatly reduced showing this is an effective mitigation strategy.
Traffic vibration and tunnelling were potentially a local contributing factor – Beijing has sinking of 45mm a year near subways and highways.
The natural upward or downward land movement also occurs but is generally much smaller than human-induced changes, the researchers stated.
While human-induced subsidence was known in China, Prof Nicholls and Shirzaei said these new results reinforce the need for a national response.
Worldwide problem
This problem happens in susceptible cities outside China and is a widespread problem across the world, they said.
The research community must move from measurement to understanding implications and supporting responses, they said.
The new satellite measurements were delivering detailed subsidence data but the methods to use this information to work with city planners to address these problems need much more development. Affected coastal cities in China need particular attention.
“Many cities and areas worldwide are developing strategies for managing the risks of climate change and sea-level rise,” said Prof Nicholls.
“We need to learn from this experience to also address the threat of subsidence which is more common than currently recognised.”
Predicting future subsidence requires models that consider all drivers, including human activities and climate change, and how they might change with time, he said.
