Not Climate Change Alone: The Hidden Driver Accelerating Sea-Level Rise

When people talk about rising seas, the conversation almost always centres on melting glaciers and warming oceans. But there is another piece of the puzzle that rarely gets the attention it deserves: the land under many of the world's biggest coastal cities is actually sinking. And when you combine that with rising oceans, the result is flooding risk that is escalating far faster than most climate models suggest.

A team of researchers from the Technical University of Munich's German Geodetic Research Institute (DGFI-TUM), working with scientists from Tulane University, decided to dig into just how big this problem really is. What they found, published recently in Nature Communications, is striking: in densely populated coastal areas, sea levels relative to the land are climbing by about 6 millimetres a year on average. That is nearly double what you would expect from climate change alone, and about three times the global average once you consider the number of people who actually live along coastlines.

What is causing the ground to sink?

There is no single explanation; it is usually a mix of things. Cities that pump too much groundwater out of the earth are often the biggest offenders, but oil and gas drilling plays a role too, as does the natural settling of soft, sediment-rich soil in river deltas. Even the weight of skyscrapers and dense urban infrastructure can push land downward over time. Add in slower-moving forces like tectonic activity and the land's ongoing adjustment since the last Ice Age, and you get a genuinely complicated picture.

Julius Oelsmann, who led the study, put it simply: if we only watch the ocean, we are missing half the story. He says that in crowded coastal cities, it is often human activity — draining aquifers, pulling resources from underground — that is making the land drop faster. Pile the weight of a growing city on top of that, along with slow geological shifts, and you end up dramatically amplifying whatever sea-level rise climate change is already causing.

Where it is worst

Some countries are dealing with this far more than others. Thailand, Bangladesh, Nigeria, Egypt, China, and Indonesia all show population-weighted rates of roughly 7 to 10 millimetres a year. The U.S., the Netherlands, and Italy are not far behind, sitting around 4 to 5 millimetres annually.

Look at specific cities and the numbers get even more dramatic:

• Jakarta — averaging 13.7 mm a year, though some neighbourhoods are sinking by a staggering 42 mm annually
• Tianjin — 13.5 mm a year
• Bangkok — 8.5 mm a year
• Lagos — 6.7 mm a year
• Alexandria — 4 mm a year

Jakarta is a particularly good example of how uneven this can get: parts of the city are dropping fast while other pockets are actually holding steady or rising slightly.

Interestingly, not every coastline is losing the battle. In Sweden and Finland, the land is still rebounding from the weight of Ice Age glaciers that melted thousands of years ago, and it is rising faster than the sea — so relative sea levels there are actually falling.

The good news

Unlike global warming, subsidence is something local governments can actually do something about. Florian Seitz, who directs DGFI-TUM, points out that because groundwater overuse is usually the main driver in big cities, smarter water policy can make a real dent — things like tighter limits on pumping, or actively recharging aquifers.

Tokyo is proof this works. The city once sank by more than 10 centimetres a year, with some areas dropping nearly 24 centimetres annually at the worst of it. Government intervention and a switch to alternative water sources turned that around. Texas offers a similar success story: after heavy groundwater pumping caused serious subsidence around Houston and Galveston, the state created the Harris-Galveston Subsidence District back in 1975 to regulate water use, and it has made a real difference.

Why this matters

None of this is entirely new to scientists who study coastlines; hydrologists and geologists have flagged subsidence as a serious issue for years. But this research puts hard numbers behind something that is easy to overlook. Consider that climate-driven sea-level rise globally runs around 2 to 4 millimetres a year. In cities like Jakarta, or in Tokyo decades ago, subsidence has outpaced that figure by ten times or more.

That is a sobering reminder: if coastal cities only plan for rising oceans and ignore what is happening beneath their own streets, they are likely underestimating how bad flooding could get.