Sinking cities are urban environments that are in danger of disappearing due to their rapidly changing landscapes. The largest contributors to these cities becoming unlivable are the combined effects of climate change (manifested through sea level rise, intensifying storms, and storm surge), land subsidence, and accelerated urbanization. Many of the world's largest and most rapidly growing cities are located along rivers and coasts, exposing them to natural disasters. As countries continue to invest people, assets, and infrastructure into these cities, the loss potential in these areas also increases. Sinking cities must overcome substantial barriers to properly prepare for today's dynamic environmental climate.
Background and history
Development
The vast majority of sinking cities are located in coastal lowlands. These areas are particularly vulnerable to climate related hazards, but since ancient times, have also been preferred areas for human settlement. Soil fertility, availability of fresh water from rivers, accessibility due to flat topographical relief, and sea and waterways allowing for trade routes, have long made coastal plains valuable agricultural and economic resources. Throughout history, these areas have continued to develop, and today, are some of the most densely populated regions in the world.
Causes
The growing physical risks to many coastal cities stem from a combination of factors relating to rapid urbanization, climate change, and land subsidence. Many of these natural hazards are largely anthropogenic in origin. In many cases, the fundamental aspects that lead to sinking cities become tightly interwoven, and over time, are increasingly difficult to resolve.
Urbanization
For the first time in human history the majority of people live in urban areas. The United Nations estimates that approximately 68% of the world's population will be living in urban areas by 2050. Urbanization has vast implications including the urban planning, geography, sociology, architecture, economics, and public health of a region. The rate at which urbanization occurs is also important. Slower rates of urbanization allow city planners time to make thoughtful planning decisions. Once cities reach maturity, it can take decades for local governments to develop, fund, and execute major infrastructure projects to alleviate the issues brought on by rapid urbanization.
In particular, some regions in Asia are currently experiencing unprecedented urban growth. Currently, the Asian urban population is increasing by 140,000 per day and is expected to nearly double from 1.25 billion in 2006 to 2.4 billion by 2030. The more troubling fact is that much of this growth is taking place along the coasts. In China, population growth in urban coastal locations was three times the national growth rate. Rapid increases in population growth challenge the carrying capacity of these urban environments often leading to mismanagement of natural resources. For sinking cities, the most common result has been over-extraction of groundwater ultimately resulting in land subsidence.
Climate change
Low-lying cities are especially prone to the most devastating effects of climate change. The risks posed by climate change will continue to grow into the next century, even if a dramatic reduction in greenhouse gas emissions is achieved, due to the built-in momentum from previous emissions. Moreover, recent reports by the United Nations have shown that climate change may be accelerating rather than slowing down. The 2019 Emissions Gap Report confirmed that GHG emissions continue to rise, despite highly publicized political commitments. The report goes on to emphasize that countries must increase their Intended Nationally Determined Contributions threefold to remain below the 2 °C goal and more than fivefold to achieve the 1.5 °C goal.
Coastal cities will bear the largest impacts of climate change due to their proximity to the sea. Storm surges and high tides could combine with sea level rise and land subsidence to further increase flooding in many regions. Oftentimes even recently completed infrastructure projects have not properly accounted for the rapidly changing climate. Asia's coastal megacities are particularly at risk as certain cities' flood protection measures have been cited as inadequate even for 30-year flood events.