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Half of China's cities are sinking due to humans and climate change

Land subsidence, often overlooked as a significant urban hazard, is gaining increased attention due to compelling new research led by an international team of scientists. 

By examining satellite data mapping land movement across China with unprecedented accuracy and consistency, the experts found that of the 82 Chinese cities analyzed, 45 percent are sinking.

Predicting future land subsidence 

While consistently measuring subsidence is highly important, this is just the starting point for more comprehensive investigations. 

In order to predict future subsidence, models should take into consideration all drivers, including human activities and climate change, particularly focusing on how they might change over time.

Such an approach is based on the assumption that subsidence is a complex issue influenced by both natural and anthropogenic factors.

Land subsidence across China

The scientists evaluated land subsidence across 82 cities from China, with a collective population of nearly 700 million people. The analysis revealed that 45% of these urban areas are sinking, with 16% experiencing a significant subsidence rate of more than 10mm per year. 

This phenomenon has broad implications, affecting roughly 270 million urban residents, nearly 70 million of whom are subject to rapid subsidence rates. Notable hotspots for this phenomenon include the densely populated cities of Beijing and Tianjin.

Land subsidence and sea-level rise 

The repercussions of land subsidence are particularly severe in coastal regions where the compounding effects of sinking land and sea-level rise pose substantial risks. For instance, the sinking of sea defenses was a major reason why Hurricane Katrina’s flooding brought such devastation and fatalities to New Orleans in 2005. 

Similarly, Shanghai has experienced up to three meters of subsidence over the past century and faces ongoing challenges. The researchers project that, combined with sea-level rise, the urban area in China below sea level could triple by 2120, potentially affecting between 55 to 128 million residents.

“Subsidence jeopardizes 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 Robert Nicholls, a climate scientist at the University of East Anglia, who was not involved in the study. Thus, there is an urgent need for robust urban planning and infrastructure resilience strategies to mitigate these risks.

The profound effect of human activity

The primary drivers of land subsidence in China include extensive groundwater withdrawal, urban development, and the natural geological conditions that predispose certain areas to sink. 

The profound effect of human activity is evident from historical precedents set by cities like Osaka and Tokyo, where substantial mitigation of subsidence was achieved by regulating groundwater extraction. 

In these cities, groundwater withdrawal was stopped in the 1970s and city subsidence has greatly reduced or even ceased, proving that this is a highly effective mitigation strategy.

Actionable strategies are urgently needed

In a commentary to the research article, Nicholls and Manoochehr Shirzaei, a researcher at Virginia Tech and the United Nations University for Water, Environment, and Health, Ontario call for a national, and indeed global, response to address this issue comprehensively. 

The experts advocate for a shift from mere measurement of subsidence to the development of actionable strategies that involve city planners and policymakers to tackle the underlying causes and manage the consequences effectively. 

This collaborative approach is crucial for enhancing the resilience of cities, particularly those in coastal and susceptible regions, to address the related challenges of subsidence and climate change.

Causes of land subsidence 

The most common cause of subsidence is the removal of groundwater from aquifers. When water is pumped out, the empty spaces in the aquifer collapse, causing the ground above to sink. Here are some other causes of land subsidence:

Natural compaction of sediments

Over time, sediment layers can naturally compact under their own weight, especially if the sediments are loose or contain a lot of water. This process can cause the ground to settle and lower.

Mining activities

The extraction of minerals, oil, and gas can lead to subsidence. When these resources are removed from the Earth, the voids left can cause the surface to collapse.

Drainage of organic soils

In areas with a high organic content in the soil, such as peat, drainage for agriculture or development can cause the soil to shrink and compress, leading to subsidence.

Thawing permafrost

In polar regions, permafrost thaw can lead to subsidence. When frozen ground thaws, it loses volume and stability, causing the ground above to sink.

Tectonic activity

Although less common, subsidence can also occur due to geological changes such as faulting or the gradual bending of the Earth’s crust.

Each of these factors can operate individually or in combination, depending on the geographical and geological conditions of an area.

Both the study and the commentary are published in the journal Science.


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