China’s cities are sinking – because there are too many people in them

There's a 'sinking belt'.

Dozens of Chinese cities are sinking – including Beijing – and hundreds of millions of people are going to be affected.

The sheer weight of buildings to house everyone living there, combined with groundwater extraction, is causing widespread subsidence.

Severe subsidence will affect around one-third of its urban population, and in the next century around a quarter of China’s coastal land will be below sea level, which could be catastrophic in the face of climate change.

In recent decades, China has experienced one of the most rapid and extensive urban expansions in human history.

But a study of 82 urban areas revealed that 45% are sinking, with 16% falling at a rate of 10mm a year or more – labelled China’s ‘sinking belt’.

Shanghai – China’s biggest city – has subsided up to three metres over the past century, and 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.

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.

The study, which used satellite radar and GPS data to measure how much the land is sinking, is published in the journal Science.

Professor Robert Nicholls from the University of East Anglia and Professor Manoochehr Shirzaei of Virginia Tech have commented on the study in an accompanying article.

Professor Nicholls 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.’

In the Japanese cities Osaka and Tokyo, after groundwater withdrawal was stopped in the 1970s, city subsidence ceased or was greatly reduced, showing this is an effective mitigation strategy.

The pair note that 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 awareness of subsidence in China is not new, the professors note the latest study highlights the need for a national response.

‘Many cities and areas worldwide are developing strategies for managing the risks of climate change and sea-level rise,’ said Professor Nicholls. ‘We need to learn from this experience to also address the threat of subsidence which is more common than currently recognised.’

Earlier this month, a map by Virginia Tech revealed dozens of US cities at risk of being submerged due to subsidence.