In the north of Jakarta there are mosques whose entrance now lies below street level. Not because the street was raised, but because the building went down. Over four decades the ground there has subsided by as much as four metres.
Coastal cities and climate change are usually discussed in one direction: the sea is rising. For Jakarta that is true, and it is still the smaller half of the story. Global sea level rises in millimetres per year. Jakarta's ground sinks in centimetres. An order of magnitude separates the two movements, and it decides what the city has to talk about.
Three centimetres that change everything
A survey by the Indonesian research agency BRIN puts average subsidence at about 3.5 centimetres a year. In the badly affected north of the city the figure is around 4.9 centimetres, and at individual hotspots in the north-west up to 20 centimetres a year have been measured. About 40 percent of North Jakarta now lies below sea level. What that means day to day is visible in buildings like those mosques: the city is not growing up around them, the ground is sinking away beneath them.
The cause is well known and unspectacular: groundwater. Because the public water supply never reached large parts of the city, households and businesses drill their own wells. When water is pulled out of the sediment layers, the subsoil loses volume and the ground above settles. Added to that is the weight of the city itself, its towers, roads and reclaimed land, resting on soft alluvial soil. The two reinforce each other: where a lot is built, a lot of water is needed, and where a lot of water is taken out, what has been built presses down all the deeper.
The sea rises in millimetres. Jakarta sinks in centimetres.
— TerraTally · Climate desk
The difference between rising water and sinking land is not academic. It decides what a city can do at all. Jakarta cannot influence global sea level, since those decisions are taken elsewhere. It can influence groundwater extraction, with means an administration actually has: a water pipe that really arrives, and rules for the wells.
Tokyo showed exactly that. After severe subsidence in the 20th century, extraction was regulated and replaced by piped water, and the sinking largely came to a halt. The ground already lost does not come back, which is the price of waiting. But the process can be stopped, and that is precisely what separates it from sea level.
Indonesia has also chosen a second route and is moving the seat of government to the island of Borneo. That does not solve Jakarta's problem. Millions of people live in the metropolitan region and will stay, because their work, their families and their homes are there. A new capital relieves the administration, not the ground.
Jakarta is therefore not a special case but a preview. Coastal cities from Ho Chi Minh City to Houston are sinking for similar reasons, wherever soft subsoil, fast growth and private wells come together. Sea level is the slow, global problem. The ground under your own city is the fast, local one, and it is the only one a city holds in its own hands.