As sea levels climb, millions face an increased risk of flooding, but a new study highlights an even graver concern: land subsidence exacerbating this threat.
Researchers from the Technical University of Munich have raised alarms about cities around the world sinking towards sea level at a concerning pace.
The findings indicate that in certain coastal regions, the combination of land sinking and rising seas more than doubles the rate of sea level rise.
Moreover, the impact is most pronounced in major urban centers, which are not only the largest but also the most densely populated.
These heavily urbanized coastal zones experience a relative sea level rise of approximately six millimeters each year on average.
This rate is triple the global average for relative sea level rise, which is currently measured at 2.1 millimeters per year.
Likewise, land subsidence roughly doubles the absolute sea–level rise of 3.15 millimetres per year, which measures the actual increase in volume and height of the ocean.
Lead researcher Dr Julius Oelsmann, of Technical University of Munich, says that this can ‘significantly amplify the effects of climate–driven sea–level rise’.
Scientists have found that land subsidence is doubling the rate at which the water level rises in some of the world’s biggest cities, putting millions at risk of flooding. Pictured: Rate of uplift (red) and subsidence (blue) in coastal areas
Jakarta is the world’s fastest–sinking city, subsiding at a rate of 13.7 millimetres per year. This puts the megacity’s 42 million residents in extreme danger of flooding. Pictured: Flooding in Jakarta following heavy rain in 2024
As the climate gets hotter, melting glaciers and the expansion of warming water are gradually raising the level of the world’s oceans.
However, Dr Oelsmann and his co–authors warn that the surface of the sea only tells 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 says.
This is because a mixture of human activity and natural forces is combining to sink some of the world’s biggest cities into the ocean.
The biggest factors driving this subsidence are excessive groundwater and oil extraction, which remove underground resources that previously stabilised the surface.
Dr Oelsmann points out that the ‘sheer weight of cities’ is also driving urban areas below sea level.
As cities grow larger and taller, heavier buildings are constructed, which compacts the ground beneath and slowly sinks the city relative to its surroundings.
Combined with climate–change–driven sea level increase, urban areas are seeing the waterline rise a lot faster than the rest of the world.
In the UK, the US, and Europe, areas of the coastline are sinking into the sea due to land subsidence. This is leading to a significantly faster rate of relative sea level increase
Countries where relative sea levels are rising the fastest include Thailand, Bangladesh, Nigeria, Egypt, China, and Indonesia, where the ocean is getting seven to 10 millimetres higher each year.
The US, the Netherlands, and Italy also experience exceptionally rapid increases, with relative sea levels rising by around four to five millimetres per year.
Due to the strong influence of city size, many countries also featured intense ‘hot spots’ of subsidence.
The 42 million residents of Jakarta, Indonesia, the world’s most populous city, are in particular peril as the megacity slips towards the ocean at a rate of 13.7 millimetres per year.
This is closely followed by Tianjin, China, home to 13.8 million people, which is seeing 13.5 millimetres of subsidence per year.
Similarly, Bangkok, Lagos, and Alexandria are all experiencing well above average rates of subsidence with 8.5, 6.7, and four millimetres per year, respectively.
Even within cities, the different rates of subsidence meant that one neighbourhood might be falling towards the ocean while others rise out of it.
In Jakarta, for example, some parts of the city are sinking at a staggering 42 millimetres per year, while other regions actually see uplift.
Densely populated urban coastal regions are experiencing around six millimetres of relative sea level increase per year (red regions)
This means that millions of people in some of the world’s biggest cities are being put at risk of severe flooding.
Even if their homes don’t fall completely below sea level, every millimetre of relative sea level increase creates a bigger risk that storms or extreme weather will lead to severe floods.
This is especially concerning for Jakarta, where about 40 per cent of the city is below sea level.
Studies estimate that nearly half of the city could be inundated and uninhabitable by 2050 if sea levels continue to rise at the current rate.
These dense, urban regions stand in stark contrast to Scandinavia, where natural geological processes are gradually lifting the land out of the sea.
During the last Ice Age, these northern latitudes were weighed down with vast ice sheets that pushed the land into the ocean, much like coast megacities do today.
As these ice sheets have retreated, that weight was removed, and the land masses are still ‘rebounding’ towards a stable position.
That means, even as absolute sea levels increase, the relative sea level in Finland and Sweden is actually getting lower each year.
Unfortunately for the rest of the world, there are no geological processes to pull cities back out of the water.
However, the researchers point out that proper city planning can dramatically slow the rate of subsidence.
Co–author Professor Florian Seitz, of the Technical University of Munich, says: ‘In many large coastal cities, groundwater extraction is a major driver of land subsidence.
‘This means that local political and water–management decisions can make a significant difference.’
In Tokyo, Japan, for example, subsidence rates once exceeded 10 centimetres per year, reaching peaks of 24 centimetres per year in the worst areas.
However, through government intervention and the introduction of new water sources, those rates were dramatically reduced.
‘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,’ says Professor Seitz.
