China’s sinking cities indicate global-scale problem, Virginia Tech researcher says
A third of China’s urban population at risk of city sinking, new satellite data shows.
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NEW SATELLITE TOOLS ALLOW RESEARCHERS TO GENERATE ACCURATE, HIGH-RESOLUTION MEASUREMENTS OF SINKING LAND, WHICH REPRESENTS A GROWING HAZARD ALL OVER THE WORLD.
view moreCREDIT: ILLUSTRATION COURTESY OF MANOOCHEHR SHIRZAEI.
Sinking land is overlooked as a hazard in urban areas globally, according to scientists from Virginia Tech and the University of East Anglia in the United Kingdom.
In an invited perspective article for the journal Science, Virginia Tech’s Manoochehr Shirzaei collaborated with Robert Nicholls of the University of East Anglia to highlight the importance of recent research analyzing how and why land is sinking — including a study published in the same issue that focused on sinking Chinese cities.
Results from the accompanying research study showed that of the 82 Chinese cities analyzed, 45 percent are sinking. Nearly 270 million urban residents may be affected with hard-hit urban areas such as Beijing and Tianjin sinking at a rate of 10 millimeters a year or more. Land sinking, or subsidence, results in increased risk to roadways, runways, building foundations, rail lines, and pipelines.
The phenomenon isn’t limited to China, said Shirzaei.
“Land is sinking almost everywhere,” said Shirzaei, who was not involved in the China-focused study but whose recent research using satellite-monitoring techniques shed light on the growing dangers of sinking land along the U.S. East Coast. “If we don’t account for it in adaption and resilience plans now, we may be looking at widespread destruction of infrastructure in the next few decades.”
Shirzaei and Nicholls expounded on this concept in the perspective article, focusing on three major points.
Advances in satellite monitoring revealed the extent of land sinking for the first time
The technique used to map consistent large-scale measurements of sinking land in China relied on space-based radar. Over the past decade, advances in satellite imaging technology granted researchers like Shirzaei the ability to measure millimeter-scale changes in land level over days to years.
“This is a relatively new technique,” said Shirzaei. “We didn’t have the data before. Now we have it, so we can use it — not only to see the problem, but to fix the problem.”
Land sinking is just an observation – more research is needed
While consistently measuring the sinking of urban land will provide a baseline to work from, predicting future subsidence requires models that consider all drivers, including human activities and climate change and how they might change with time.
Land sinking is mainly caused by human activity, but it can also be addressed with human activity
Land sinking is mainly caused by human action in the cities. Groundwater withdrawal, which lowers the water table, is considered the most important driver of subsidence, combined with geology and weight of buildings. Recharging the aquifer and reducing pumping can immediately mitigate land sinking.
Shirzaei and Nicholls called for the research community to move from measurement to understanding implications and supporting responses.
JOURNAL
Science
ARTICLE PUBLICATION DATE
18-Apr-2024
A third of China’s urban population at risk of city sinking, new satellite data shows
UNIVERSITY OF EAST ANGLIA
Land subsidence is overlooked as a hazard in cities, according to scientists from the University of East Anglia (UEA) and Virginia Tech.
Writing in the journal Science, Prof Robert Nicholls of the Tyndall Centre for Climate Change Research at UEA and Prof Manoochehr Shirzaei of Virginia Tech and United Nations University for Water, Environment and Health, Ontario, highlight the importance of a new research paper analysing satellite data that accurately and consistently maps land movement across China.
While they say in their comment article that consistently measuring subsidence is a great achievement, they argue it is only the start of finding solutions. Predicting future subsidence requires models that consider all drivers, including human activities and climate change, and how they might change with time.
The research paper, published in the same issue, considers 82 cities with a collective population of nearly 700 million people. The results show that 45% of the urban areas that were analysed are sinking, with 16% falling at a rate of 10mm a year or more.
Nationally, roughly 270 million urban residents are estimated to be affected, with nearly 70 million experiencing rapid subsidence of 10mm a year or more. Hotspots include Beijing and Tianjin.
Coastal cities such as Tianjin are especially 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.
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.
"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.
The subsidence is mainly caused by human action in the cities. Groundwater withdrawal, that lowers the water table is considered the most important driver of subsidence, combined with 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 is potentially also a local contributing factor - Beijing has sinking of 45mm a year near subways and highways. Natural upward or downward land movement also occurs but is generally much smaller than human induced changes.
While human-induced subsidence was known in China before this study, Profs Nicholls and Shirzaei say these new results reinforce the need for a national response. This problem happens in susceptible cities outside China and is a widespread problem across the world.
They call for the research community to move from measurement to understanding implications and supporting responses. The new satellite measurements are delivering new detailed subsidence data but the methods to use this information to work with city planners to address these problems needs much more development. Affected coastal cities in China and more widely 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.”
JOURNAL
Science
METHOD OF RESEARCH
Commentary/editorial
ARTICLE TITLE
Earth’s sinking surface: China’s major cities show considerable subsidence from human activities
ARTICLE PUBLICATION DATE
18-Apr-2024
China’s major cities show considerable subsidence from human activities
The land under nearly half of China’s major cities is undergoing moderate to severe subsidence, affecting roughly one-third of the nation’s urban population, according to a systematic national-scale satellite assessment. The findings suggest that within the next century, 22 to 26% of China’s coastal land will have a relative elevation lower than sea level, putting hundreds of millions of people at elevated risk of flooding due to sea-level rise. Over the last several decades, China has experienced one of the most rapid and extensive urban expansions in human history. This massive wave of urbanization may be threatened by land subsidence – a gradual sinking of an area of land. To date, instances of subsidence have been increasingly reported in major Chinese cities. However, a comprehensive understanding of the scale and speed of subsidence in China’s cities remains unclear. Using measurements from the spaceborne Sentinal-1 Interferometric Synthetic Aperture Radar (InSAR) and ground-based GPS data, Zurui Ao and colleagues performed a national-scale scale evaluation of land subsidence in 82 of China’s major cities from 2015 to 2022. InSAR uses highly precise radar pulses to measure the change in distance between the satellite and the grounds surface and can detect even relatively small changes in elevation on the order of millimeters per year. Ao et al. found that 45% of the studied urban land area is subsiding faster than 3 millimeters per year (mm/year), and as much as 16% is subsiding at a rate of 10 mm/year or more. These sinking lands contain 29% and 7% of China’s urban population, respectively. According to the authors, this subsidence is associated with a range of anthropogenic factors, including groundwater extraction and the weight of the built environment. The findings suggest that due to this subsidence and projected sea-level rise, roughly a quarter of China’s coastal lands will have an elevation lower than sea level, presenting a considerable risk of flooding for large populations unless adequate protective measures to mitigate city subsidence are implemented and maintained. “One major challenge is to move from measuring subsidence to thinking systematically about its implications,” write Robert Nicholls and Manoochehr Shirzael in a related Perspective. “Ideally, this will guide immediate and long-term strategic actions, analogous to strategies that have emerged for coastal areas threatened by sea level rise.”
JOURNAL
Science
ARTICLE TITLE
A national-scale assessment of land subsidence in China’s major cities
ARTICLE PUBLICATION DATE
19-Apr-2024
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