How 'sinking cities' can address subsidence challenges

In an era where climate change dominates headlines, an often silent but significant concern is urban land subsidence, or sinking cities. New York City in the United States, for example, is currently sinking under the weight of its buildings. According to the US Geographical Survey, about 1.68 trillion pounds of concrete, glass and steel is built on the city's 777sq km of land. Such weight can cause the ground to compress and subside. Rising sea levels and intensifying storms have also made the city more vulnerable to flooding, which can exacerbate subsidence.

Coastal cities are at the greatest risk of land subsidence, although landlocked cities like Mexico City and Mexico face similar challenges. Notably, in a recent study of 99 coastal cities, 33 were found to be sinking five times faster than sea level rise. This issue is particularly severe in parts of Asia, where cities like Manila in the Philippines, Karachi in Pakistan and Tianjin in China are sinking 10 to 20 times faster than the rising sea level.

Nearly half of China's major cities are sinking under the weight of their infrastructure and due to groundwater extraction. Recent research showed that 45% of 82 cities in the country are subsiding by more than 3 millimetres annually, which could potentially impact 29% of the country's urban population. By 2120, between 22% and 26% of coastal land in China could be lower than sea level.

The inhabitants of sinking cities are likely to face severe and escalating hardship. Subsidence can result in significant land loss, water insecurity, infrastructure damage and displacement of people. In Indonesia, for example, the government has relocated its capital from Jakarta to the island of Borneo due to such concerns.

Stopping these cities, and many others around the world, from sinking will require a re-evaluation of water use and infrastructure, as well as conscious and intelligent efforts to build resilience into urban planning and design.

The phenomenon of urban land subsidence can go undetected for years. It is driven by complex and interconnected causes that are both natural and human-induced.

Excessive groundwater extraction is a leading cause of urban land subsidence – it is projected to impact 19% of the global population by 2040. As cities grow, demand for freshwater for domestic and industrial use increases. In areas where surface water resources are limited or polluted, groundwater becomes the primary water source and over-pumping of aquifers can cause the ground above to compact and sink.

The weight of a city’s infrastructure in relation to its soil type can also cause subsidence. As urban areas become denser and more structures are built, the cumulative weight can also gradually increase the rate of land subsidence.

Natural geological processes such as tectonic movements or seismic activity can also affect sediment compaction, contributing to subsidence. And while climate change is not a direct cause of subsidence, related problems like rising sea levels can exacerbate it by increasing the risk of flooding in low-lying areas. Extreme weather events can also accelerate erosion and sediment displacement, further destabilizing the ground and creating the dual challenges of sinking land and encroaching waters.

International communities can harness centuries of knowledge, expertise, skills and technology to create pathways for mitigation and adaptation for sinking cities. However, addressing urban land subsidence will require a comprehensive and multifaceted strategy that should include the following five actions:

Engaging directly with affected cities and communities while gathering quantitative and qualitative data can help determine the severity, scale and cost of the issue while also helping to raise awareness.

Sinking cities also have broader socio-economic implications, given the significant “water divide” that already exists. In 2022, 2.2 billion people lacked access to safely managed drinking water services, while daily water use per capita can range from 2,000 litres to 50 litres, depending on the country. Urban land subsidence can contaminate freshwater with seawater, exacerbate water scarcity and further contribute to water inequity.

To reduce excessive groundwater consumption and address existing infrastructure problems, sinking cities need accurate, real-time information for monitoring and predicting water leakages, land subsidence and groundwater levels. Artificially recharging aquifers by injecting them with surface water or treated wastewater could also help restore groundwater levels.

By evaluating the water supply chain, cities can identify alternative business models to meet local and industrial water demands without over-draining aquifers. Solutions may include recycling wastewater, harvesting rainwater, desalination plants and locating water services outside of impacted areas.

The cost-effectiveness of these solutions and the return on investment should be assessed for long-term operational and economic sustainability, while aiming to maximize social and environmental benefits.

Governments, the private sector, civil society, and academia can all play crucial roles in a multifaceted approach to saving sinking cities. For example, these groups can work together to identify current best practices and innovations for land subsidence mitigation and adaptation. This can involve increasing funding and investments in infrastructure and urban planning and management, as well as creating funds dedicated to water conservation.

Public-private partnerships can also be instrumental in funding and implementing large-scale infrastructure projects, such as redesigning drainage systems, building flood defences in vulnerable coastal cities and reinforcing buildings to withstand sinking.

The private sector must reevaluate and determine best practices that consider the long-term impacts of assets on land subsidence, particularly in cities located near the coast. This should include using materials and techniques that minimize the impact of built assets on vulnerable soils. It should also involve reducing intensive land use and dependency on groundwater for industrial purposes.

Water conservation education and advocacy are crucial for reducing groundwater extraction and encouraging individuals and industries to consider their water footprint. By raising awareness and promoting more sustainable water management practices, communities and policymakers can collaborate to mitigate subsidence and protect vulnerable urban areas.

The issue of sinking cities demands urgent and sustained attention. Although the problem is multifaceted, involving both natural processes and human activities, viable solutions are within reach. By reducing groundwater extraction, enhancing urban infrastructure and fostering collaboration between governments and the private sector, cities can mitigate subsidence risks and adapt effectively.

As the impacts of climate change continue to unfold, addressing subsidence will be crucial to ensuring the resilience of cities worldwide.