For decades, the story of coastal risk has been dominated by climate change and rising seas. But a major new global study suggests that, for hundreds of millions of people living on river deltas – including those living in cities such as New Orleans and Bangkok – an even more immediate threat is unfolding beneath their feet.
Across much of the world, the land itself is sinking – and in many places, it’s sinking faster than the ocean is rising.
Using satellite radar to track tiny changes in the Earth’s surface, scientists have discovered that more than half of the world’s river-delta regions – the low-lying land where major rivers meet the sea – are now sinking. In many of the most densely populated deltas on the planet, this gradual subsidence, rather than rising seas alone, is now the dominant driver of flood risk.
“It’s a real call to arms,” Prof Robert Nicholls, a coastal scientist at the University of Southampton and co-author of the study, told BBC Science Focus. “Before this, nobody had a global view of delta subsidence. This study shows how widespread the problem is and really brings home the need to respond to it.”
The findings were published in Nature.
A global problem hiding in plain sight
River deltas occupy just 1 per cent of Earth’s land area, but they support between 350 and 500 million people, including some of the world’s largest cities and most productive farmland. They are economic powerhouses, ecological hotspots and vital food baskets. They are also, by their nature, fragile.
Deltas are built from loose, water-logged sediments deposited over thousands of years. Even without human interference, these sediments slowly compact under their own weight, causing gradual sinking.
Historically, that natural sinking was balanced by regular floods that replenished the land with fresh sediment. But modern development has changed that balance.
In the new study, researchers analysed satellite measurements across 40 major river deltas between 2014 and 2023, creating the first high-resolution, delta-wide global picture of how fast land is rising or falling.
The findings were stark. At least 35 per cent of delta land is currently sinking, and in most deltas, more than half of the surface is subsiding.
In 18 of the 40 river deltas studied, on average, the ground is sinking faster than local sea levels are rising. And in almost every delta, scientists found hotspots where the land is dropping more quickly than both regional and global sea-level rise.
Across Asia, Africa, Europe and the Americas, the same pattern repeats: relative sea level is rising not just because oceans are expanding, but because the ground is dropping away.
“From a risk point of view, it doesn’t really matter whether the sea level rises or the land sinks,” Nicholls said. “The effect at that point is identical, but the responses aren’t necessarily the same.”
What’s driving the sinking?
The study examined three major human-linked drivers of land sinking: groundwater extraction, reduced sediment supply and urban expansion. Of these, groundwater pumping stood out as the strongest overall predictor.
When water is pumped out from underground reservoirs, the soft sediments around them begin to collapse and squeeze together. Crucially, this process is largely irreversible: once sediments compress, they do not spring back even when water returns.
In 10 of the 40 deltas studied, groundwater loss was the dominant factor driving subsidence. In many others, it combined with reduced river sediment – caused by dams and flood defences – and the sheer weight of expanding cities built on soft soils.
Together, these pressures have turned what was once a slow geological process into a fast-moving environmental crisis.
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The US case: the Mississippi Delta
In the US, the most striking example is the Mississippi River Delta, home to New Orleans and large stretches of coastal Louisiana.
The new analysis confirms that subsidence remains widespread across the delta, with more than 90 per cent of the area sinking, and average land loss of around 3.3mm per year, with much faster rates in some localised zones.
That may sound small, but over decades it adds up, especially when combined with rising seas and the risks posed by powerful hurricanes.
The Mississippi Delta has lost thousands of square kilometres of coastal wetlands over the last century, with an astonishing football field-sized area lost to open water every 100 minutes.
A shortage of fresh sediment is a major part of the problem. Levees and dams stop rivers from flooding and laying down new material that would naturally help rebuild the land. At the same time, drainage systems, oil and gas extraction, and decades of pumping groundwater have further squeezed and weakened already fragile soils.
While some deltas are putting up resistance, one solution is simply to move away. Nicholls pointed to New Orleans’ population, which has been falling since the 1960s, as an example of this approach.
“In the US, people are perhaps more used to the idea of retreat,” he said, noting that mobility is often higher and land-use change more politically acceptable than in parts of Europe or Asia, where long-term protection tends to be the default response.
A warning for megacities
While North America faces serious risks, the most extreme sinking rates are concentrated in parts of South and Southeast Asia, where dense populations depend heavily on groundwater for agriculture, industry and drinking water.
In deltas such as the Mekong (Vietnam), Ganges–Brahmaputra (Bangladesh and India), Chao Phraya (Thailand) and Yellow River (China), large areas are sinking faster than the current global sea-level rise – in some places by more than a centimetre per year.
Megacities, including Bangkok, Dhaka, Shanghai and parts of Jakarta, are built on these sinking foundations.
The good news is that, unlike global sea-level rise, which will continue for centuries, human-driven subsidence can respond quickly to policy changes. Tokyo offers one of the clearest success stories.
During the mid-20th century, parts of Tokyo sank by more than four metres as factories pumped huge volumes of groundwater.
But strict controls on extraction and investment in alternative water supplies caused subsidence rates to fall rapidly.
“The authorities provided a good alternative water supply and put in laws to stop groundwater use,” Nicholls said. “And almost overnight, it actually stabilised.”
There are other tools too. In agricultural parts of deltas, managed flooding could help replenish soils. “We tend to treat sediment like a pollutant,” Nicholls said.
But when rivers spill over their banks, they deliver the very material that once built deltas in the first place. “Some people talk about it as brown gold.”
Urban areas can also be protected through hard engineering, such as sea walls, levees and storm-surge barriers.
“Measures to address subsidence are complementary to adapting to rising seas, and dealing with subsidence makes us less vulnerable to rising seas,” Nicholls said.
A shift in how we think about coastal risk
The study’s authors argue that subsidence has been dangerously under-represented in global climate-risk planning, partly because it is viewed as a local problem rather than a planetary one.
But local does not mean small. Even under worst-case climate scenarios, sinking land will remain the dominant driver of relative sea-level rise in many deltas for decades.
For many regions, financial and institutional barriers make large-scale intervention difficult, yet delaying action only makes future adaptation more expensive and disruptive.
Once land has sunk, lifting cities is not an option, and communities may face hard choices about retreat and relocation.
As Nicholls put it, “The first key thing is to recognise there’s a problem.”
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