45% of China's urban land is rapidly sinking due to manmade development

A perfect storm is brewing for China's most densely populated areas due to rising sea levels and subsiding land that has been accelerated beyond normal fluctuations. Scientists have sounded the alarm that, without intervention, urban areas below sea level could triple in size by 2120, impacting up to 128 million people.

It's a grave warning for a nation that has rapidly urbanized like nowhere else on the planet. In 1980, those living in cities and surrounds accounted for around 20% of the population; in 2023, it was more than 65%. Now, it's expected to reach 80% in the next decade. The growth has exceeded earlier modeling, which forecast it would not reach the current numbers until well after 2030.

In population ecology, every species population within its environment has a carrying capacity – the maximum number of individuals that can be sustained before a 'tipping point' results in a decline to below that threshold. Resources – food, shelter, for example – are limiting factors that ultimately control population size. While humans defy many laws of nature, in this case our ability, through wealth and industry, or both, to generate more resources for a given environment's sustained growth, we have been able to stretch our carrying capacity well beyond what would otherwise be unsustainable.

Though us long-living humans may not experience the 'boom-bust' population cycle seen in many species such as rodents and locusts, this latest research by an international team of scientists thrusts our carrying capacity into sharp focus – and, much like in nature, a demand on resources required to support rapidly increasing numbers is at the center of it.

A double threat – rapid industrialization and development to support the population coupled with the negative environmental impact of that, like rising sea levels through anthropogenic climate change – is what urban China is now facing, according to this new study.

In fact, the study suggests that the two-fold impact, without intervention, could see the urban areas of China below sea level tripled by 2120, impacting up to 128 million people.

In the research, scientists looked at subsidence data for 82 Chinese cities, which some 700 million people call home. What they found was that in 45% of these urban areas, the land beneath them showed distinct signs of subsidence. Of that percentage, 16% – including Beijing – were sinking at 10 mm or more each year.

For coastal cities such as Tianjin, even what sounds like a small amount of subsidence makes these areas far more vulnerable to the growing threat of sea-level rise. The researchers also highlighted China's largest city, Shanghai, which has subsided three meters (10 ft), and continues to sink.

"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 Professor Robert Nicholls, from the Tyndall Centre for Climate Change Research at the University of East Anglia (UAE). While not involved in the Chinese study, Nicholls' research expertise is sea-level rise, erosion and flooding – and how we can respond to these challenges.

Subsidence – largely caused by human activity in urban areas – is not a new phenomenon in China, or in many other parts of the world. But this study shows just how much accelerated development is impacting cities, and calls for scientists to work on responses to mitigate the threat.

"Subsidence leads to ground fissures, damages buildings and civil infrastructure, and increases the risk of floods," the researchers noted. "Over the past decades, subsidence-related disasters in China have already incurred an annual direct economic loss of more than 7.5 billion yuan (US$1.04 billion), accompanied by hundreds of fatalities or injuries per year."

Accelerated subsidence is largely caused by the population above it. Weight of buildings (though, the researchers note, often the heavier buildings are not the culprit, due to the depth of their anchoring beneath the surface), transport networks and groundwater removal are all big factors in the changing landscape.

"Urban transportation systems, for example, cause subsoil and track bed compaction through repeated dynamic loading and traffic vibration, potentially contributing to subsidence," the scientists noted. "Such is the case in mega cities such as Beijing, where regions around the subways and highways are sinking with a minimum speed of –45 mm/year."

The extraction of groundwater has long plagued Beijing, a rare "megacity" that relies on this source of water for its population. Government intervention has slowed its depletion, after 2016 research triggered large-scale measures to ensure groundwater sustainability.

In their analysis, the researchers found that groundwater loss, which "decreases pore pressure and leads to subsurface compaction," is predominantly caused by humans. Just 12% could be linked to natural rainfall fluctuations. Mining and (with some grim irony) land reclamation were also found to be contributing to this 'perfect storm' in urban China.

"Land subsidence also occurs in hydrocarbon extraction areas experiencing a reduction of fluid pressure and consequent compaction," said the researchers. "In China’s Daqing city, subsidence up to –31 mm/year occurred around the oil field rather than directly within it, possibly because of the extraction of water from areas near the oil field and its subsequent injection into the oil field."

However, the researchers note that it's not too late to halt the progress of this dire timeline – with the study calling on industry, and the science and engineering communities, to come up with subsidence-forward plans and policies to slow the sinking cities.

"We provide comprehensive maps of city subsidence across China, aiding in the precise identification of sinking regions and the formulation of mitigation measures," the team noted in the paper. "Effectively addressing the challenge of city subsidence ultimately demands collaborative and coordinated endeavors from stakeholders across various tiers, including policy-makers, the research community, and civil engineers."

And, with interventions that can help absorb the many pressures put on resources in growing cities, the populations of these urban areas may be able to live on the right side of that all-important carrying capacity threshold in the long-term.

The research was published in the journal Science.

Source: University of East Anglia via EurekAlert