Sea levels are much higher than we thought. Real-world oceans are making a mockery of flood-risk forecasts based on crude global modeling. And to make matters worse, coastal lands almost everywhere are subsiding faster than anyone realized — often many times faster than the seas are rising. 

These findings come from two major new studies that are reshaping our understanding of the threats posed by rising tides and sinking land and underlining the imminent risk of inundation facing tens of millions of people in some of the world’s largest megacities, say researchers not involved in the studies.

“The impacts of sea level rise under climate change have been systematically underestimated,” concludes Matt Palmer, a specialist on sea level rise at the U.K. Met Office’s Hadley Centre for Climate Science. “We could see devastating impacts much earlier than predicted — particularly in the Global South.” 

“Taken jointly, these two papers paint a considerably more concerning picture than either would in isolation,” says Franck Ghomsi, an oceanographer at the University of Cape Town. “We are seeing an emerging body of research that rewrites the story of coastal vulnerability.” 

Around 80 million people are today living on land in coastal areas below sea level — almost twice previous estimates.

Katharina Seeger and Philip Minderhoud, geographers at Wageningen University & Research, in the Netherlands, say seas are on average almost 1 foot higher than standard estimates, which are based on global models that assume calm seas and ignore ocean currents and the effect of winds. Sea levels are not rising faster than thought, but the baseline for future rise is considerably higher in most places.

In many of the 385 cases the pair examined, previously accepted sea levels are 3 feet or more off — almost all of them too low. They conclude that around 80 million people are today living on land in coastal areas below sea level — almost twice previous estimates — dramatically increasing the numbers at risk as sea level rise accelerates in the coming decades. 

For many low-lying coastal areas, scientific forecasts of how soon they may flood as sea levels rise may be off by several decades, making planning to protect coastlines much more urgent than previously supposed by policymakers and funding bodies such as the World Bank that rely on scientific assessments of flooding risk. 

The other new study focused on the world’s river deltas. It has long been known that many deltas are sinking under the influence of groundwater pumping. But as Robert Nicholls, climate adaptation researcher at the University of East Anglia, notes, “There have been lots of different estimates.” Data were inconsistent and based on crude, delta-wide estimates. “Now at last we have a consistent data set, with high spatial resolution.” 

That data comes from Leonard Ohenhen, an earth system scientist at the University of California, Irvine, who used satellite-mounted radar to produce 3D maps of subsidence on 40 of the world’s biggest and most populous river deltas. He has found that subsidence afflicts more than half those deltas. Most startlingly, in 18 cases subsidence rates exceed those of rising tides — hence, more than doubling the effective yearly rise in local sea levels, and in some cases multiplying it tenfold.  

This again puts tens of millions of people once thought safe from rising tides this century in imminent harm’s way, including those living on the deltas of the Nile in Egypt, the Mekong in Vietnam, the Mahanadi in India, and the Yellow River in China. If the current rate of subsidence persists, these areas will be flooded much sooner than thought.

Researchers say many deltas and other low-lying coastal areas not included in the two studies are at greater risk than believed and urgently require detailed investigation of both actual sea levels and the rate of land subsidence.

Data from tidal gauges shows that actual sea levels worldwide are on average 9.4 to 10.6 inches higher than predicted by models.

The two studies, conducted largely independently, show glaring gaps in past research tracking the severity of climate change impacts. Seeger says peer-reviewed studies and reports of the Intergovernmental Panel on Climate Change are almost all based on faulty methodology. 

One problem is that more than 90 percent of local studies estimating current sea levels and future rises cut-and-paste the results of mathematical models of the “geoid,” the shape of the Earth as calculated from the planet’s rotation and gravitational fields. They deliver approximate sea levels, but by assuming calm and uniform oceans, they result in significant local errors, says Seeger. Very few studies use actual measured data, even when it is available. 

In particular, Seeger says, this approach “ignores ocean dynamics,” such as currents, water expanding from unually high temperatures, and prevailing winds piling up water along shorelines. Her painstakingly assembled real-world data from tidal gauges shows that actual sea levels worldwide are on average 9.4 to 10.6 inches higher than predicted by geoid models. 

This discrepancy exceeds total global sea level rise since the start of the 20th century, notes Jonathan Bamber, a glaciologist at the University of Bristol. 

Higher than expected sea levels turn out to be greatest “in the Global South, where ocean dynamics tend to be stronger,” says Minderhoud. Tides along the coasts of Southeast Asia are 3 feet or more higher than geoid modeling predicts. Just a few places have lower levels — notably around Antarctica and the northern Mediterranean. 

“Our corrected calculations reveal that up to 37 percent more area and up to 68 percent more people will fall below sea level following [3.3 feet] of sea level rise,” says Seeger.  That represents an additional area at risk of inundation the size of the United Kingdom, occupied by 132 million people, equivalent to the population of Mexico. 

Seeger says the “methodological blind spot” that she and Minderhoud have uncovered went undetected for so long in part because geoid estimates are most accurate in Europe and the eastern seaboard of North America, where the majority of published researchers are based. 

The fact that current sea levels are higher than supposed does not directly alter global projections of future rise. But it raises the baseline from which future rises will occur in most places, and the evidence of strong local variations in sea levels suggests that future rises in some places will be higher than in others. 

Parts of Shanghai have subsided by more than 6 feet, Bangkok by more than 5 feet, and Osaka and Tianjin by around 10 feet.

Minderhoud says the prevalence of geoid data in the scientific literature has led to complacency, by predicting that many heavily populated coastal regions have more leeway before flooding thresholds are crossed than now appears likely. 

A case in point is the Mekong Delta in Vietnam, where Minderhoud began his own investigations into the reliability of sea level data in the scientific literature a decade ago. Published data predicted that “the land would start to become inundated if sea levels were to rise by [5 to 6.6 feet],” he says. “But I could see that the surface water level was already in many places… much higher.” Without urgent protection, the land was on the brink of becoming inundated. 

Many deltas face a similar harsh confrontation with reality, because they are also sinking into the rising ocean. Some deltas subside naturally, if rivers do not supply enough sediment to replenish what is lost to erosion by the ocean. But “natural subsidence is rarely more than [0.1 inches] a year,” says Nicholls. That is a fraction of current rates, which mostly have an anthropogenic origin, he says. 

Parts of Shanghai, a megacity of 25 million people on the Yangtze Delta, have subsided by more than 6 feet, Bangkok by more than 5 feet, and Osaka and Tianjin by around 10 feet, says Nicholls.

Much of Jakarta, the megacity capital of Indonesia, has sunk by up to 13 feet since 1970, and it continues downward 10 times faster than seas are rising in the adjacent bay. The threat of widespread submersion was a primary reason for the Indonesian government’s 2019 decision to move its capital to Borneo. 

Ohenhen’s analysis confirms that subsidence of deltas is usually caused by pumping groundwater to fill city faucets and supply industry and agriculture. The dried-out subsurface loses volume, causing widespread sinking at the surface. 

Jakarta is just the largest of many coastal cities and farming areas threatened by groundwater withdrawal on Java, the world’s most populous island. Semarang, a booming coastal Javan city of 2 million people pumps so much water that subsidence reaches between 20 and 50 times sea level rise, according to analysis by Ohenhen. Floods washed through the city in October 2025 and again in February this year. Waterlogged residents repeatedly rebuild their houses on ever higher stilts to avoid inundation. But whole neighborhoods are disappearing permanently beneath the waves. 

A second cause of subsidence is dams and levees on rivers. These structures cut off sediment supplies that maintain deltas. Thanks to a total of more than 20 large dams on its main stem, China’s Yellow River is no longer so yellow from sediment. Ohenhen found that, as a consequence of the reduced sediment, its delta has been subsiding up to 10 times faster than the Yellow Sea, into which it empties, is projected to rise. 

In parts of the world, authorities are acting to protect against rising tides without realizing how high sea levels already are.

Sometimes subsidence has a range of causes, says Nicholls. The Nile Delta, which contains two-thirds of Egypt’s farmland, has been starved of sediment by the High Aswan dam, which was constructed in the 1960s, and undermined from beneath by groundwater pumping to irrigate crops. The new analysis found rates of subsidence were “much bigger than previously reported, with millions of people in harm’s way.” Nicholls says. It is the dominant cause of a retreat in the delta coastline that in places exceeds 300 feet per year, engulfing farmland, threatening the city of Alexandria, and poisoning palm trees as salty water infiltrates underground water. 

The new findings point to urgent problems for governments and aid agencies. In many parts of the world, authorities are acting to protect communities against rising tides without realizing either how high current sea levels already are or the extent of land subsidence. 

In Java, the Indonesian government and NGOs have been restoring coastal mangroves as a natural way to help hold back tides that have swamped whole villages. But as the work has progressed, it has emerged that groundwater pumping in Semarang and elsewhere is making this work much less impactful.

Still, the often-dominant role of land subsidence as a cause of coastal flooding is potentially good news. For, while rising sea levels can only be halted with global climate action, subsidence can be stopped quickly by local action, such as ending groundwater pumping, says Scott Jasechko, a hydrologist at the University of California Santa Barbara, and author of a new global study of groundwater recovery. 

Parts of Tokyo sunk by up to 15 feet between 1920 and 1960, as it became the world’s most populous metropolitan area. But then the city largely banned groundwater pumping, and land levels have been stable ever since, says Nicholls. 

Elsewhere, the operation of dams can be altered, and levees removed, to restore sediment supplies to deltas. After Hurricane Katrina flooded New Orleans in 2005, the U.S. Army Corps of Engineers began turning the tides in the Mississippi Delta by diverting sediment-rich water to restore protective coastal marshes.

Still, the threats loom large, especially in Southeast Asia and Africa, where Ghomsi, of the University of Cape Town, sums up what faces countries with few resources to repel the tides: “The sea level baseline is higher than assumed; the land is sinking in many critical regions; the sea is rising faster; and extreme events now come on top, producing impacts that are greater than the sum of their parts.”