Sea-Level Risk May Be Far Higher Than Assumed

A major blind spot in coastal risk mapping

For years, sea-level rise has been framed mainly as a problem of future climate change. But a new study published in Nature suggests that a large part of the problem may begin much earlier: with how coastal risk is measured in the first place.

The paper, by Katharina Seeger and Philip S. J. Minderhoud, argues that most coastal hazard assessments are not properly aligning land elevation with actual local sea level. That may sound technical. In practice, it means many global and regional flood-risk studies may be starting from the wrong baseline.

Their conclusion is striking. In the vast majority of assessed studies, coastal sea level was effectively assumed to be lower than it really is relative to the land. When corrected, the amount of land and the number of people projected to fall below sea level rises sharply.

This matters far beyond academic mapping. Coastal exposure estimates help shape adaptation plans, public investment, infrastructure timelines, insurance logic, climate finance priorities, and international risk narratives. If the baseline is wrong, every downstream decision can be distorted.

What the study found

The researchers reviewed 385 peer-reviewed studies published between 2009 and 2025 on sea-level rise, coastal flooding, exposure, vulnerability, and related coastal hazards.

Their core finding was not just that many studies contain uncertainty, but that the underlying methodology is often flawed in a systematic way.

They found that:

  • more than 99% of evaluated studies handled sea-level and land elevation data inadequately
  • around 90% effectively assumed coastal sea level using geoid models rather than measured local sea level
  • only 0.3% of studies fully documented and correctly aligned elevation and sea-level data

That methodological gap has large consequences. On average, the study found that measured coastal sea level is higher than the baseline assumed in many assessments. Globally, the difference averages about 0.24 to 0.27 meters depending on the geoid model used. In some regions, especially in the Global South, the mismatch exceeds 1 meter.

That is not a rounding error. In low-lying deltas and coastal plains, a few decimeters can radically change exposure maps.

The overlooked technical issue with huge real-world consequences

At the heart of the paper is a geospatial problem: coastal elevation is often measured relative to one vertical reference system, while sea level is represented using another. If the two are not properly converted and aligned, the resulting hazard map can misjudge how high the land actually sits above the sea.

Many studies rely on digital elevation models, or DEMs, which are essential for estimating which places are near or below sea level. But a DEM is only as useful as the reference system behind it.

The problem identified in the paper is that many coastal hazard assessments treat the geoid as though it were the same as actual local mean sea level. It is not.

A geoid is a gravity-based model of Earth’s shape. Actual sea surface height is also shaped by currents, winds, tides, salinity, temperature, and large-scale ocean circulation. The gap between these is known as mean dynamic topography.

In some coastal regions, especially where gravity data is sparse or ocean dynamics are strong, that gap can be substantial. So when assessments skip the conversion from geoid-based elevation to measured local sea level, they can understate exposure from the start.

Why this matters more in the Global South

One of the most important dimensions of the study is geographic inequality.

The mismatch between assumed and measured sea level is not evenly distributed. In data-rich regions of the Global North, such as eastern North America and parts of Europe, geoid models tend to approximate local sea level more closely. But in many parts of the Global South, especially Southeast Asia, Pacific island regions, parts of Africa, Latin America, and the wider Indo-Pacific, the mismatch is much larger.

That means the places already facing some of the highest coastal vulnerability may also be the places where hazard assessments are most likely to underestimate the problem.

This is not only a technical issue. It is also a knowledge infrastructure issue.

If global methods are developed and validated primarily in better-instrumented regions, then exported worldwide without enough correction, they can reproduce geographic bias. The study hints at this larger pattern: methodologies that appear reliable in the Global North may not transfer cleanly to data-sparse coastal regions elsewhere.

For a sustainability audience, that matters because climate risk is never only about physical exposure. It is also about who gets measured accurately, whose vulnerabilities are legible to institutions, and whose risks remain statistically softened.

The exposure numbers rise sharply when sea level is corrected

The study tested what happens when coastal elevation is properly aligned to measured mean sea level rather than assumed geoid-based baselines.

Using a hypothetical 1 meter of relative sea-level rise, the authors found that:

  • land area falling below sea level increases by about 31% to 37%
  • population falling below sea level increases by about 48% to 68%
  • estimated exposed population rises to roughly 77 million to 132 million people

Those are global estimates across multiple elevation and population datasets.

The regional results are even more revealing. In Southeast Asia, the increase in estimated exposure is especially dramatic, with area and population projections rising by as much as the mid-90% range in some analyses.

This is exactly the kind of systems-level correction that changes how adaptation timelines are understood. A coast believed to have more time may actually be closer to critical thresholds already.

Sea-level rise is only part of the story

The paper also reinforces a broader point that sustainability coverage often misses: coastal risk is not driven by global sea-level rise alone.

Relative sea-level rise is shaped by at least two moving parts:

  1. the ocean surface getting higher
  2. the land surface getting lower

That second component includes land subsidence, which can be caused by natural processes but is often worsened by groundwater extraction, sediment starvation, urbanization, and other human pressures.

This is especially important in major delta regions. Some of the world’s most densely populated and agriculturally productive coastal zones are also subsiding. So even if global mean sea-level rise were moderate, local relative sea-level rise could still be severe.

The study’s message is that if assessments already begin with an understated sea-level baseline, then adding subsidence on top of that creates even more risk of delay and under-preparation.

Why this study matters for climate adaptation

The real importance of this research is not that it adds another alarming sea-level statistic. It is that it challenges the reliability of the tools used to guide adaptation itself.

Governments, city planners, donors, development banks, insurers, and international agencies often depend on coastal hazard assessments to prioritize investments. Those assessments influence:

  • where seawalls are built
  • when settlements are protected, relocated, or redesigned
  • which ports and roads are treated as urgent
  • how climate losses are estimated
  • where adaptation finance is directed

If sea-level exposure has been systematically underestimated, then adaptation may be sequenced too slowly, funded too weakly, or targeted too narrowly.

This is especially consequential for places that already have limited institutional capacity and weaker access to high-resolution elevation data.

What this means for the IPCC and global climate policy

The paper goes further than critiquing individual studies. It also investigates how these flawed assessments have circulated into larger climate reporting systems.

The authors found that dozens of evaluated studies were cited in the latest IPCC assessment cycle. Some of those studies likely carried the same vertical reference problems identified in the review.

That does not invalidate the broader reality of sea-level risk. But it does suggest that some widely cited exposure estimates may be conservative because they are built on inconsistent geospatial foundations.

For climate policy, this has several implications.

First, adaptation needs may be more immediate than current narratives suggest.
Second, loss-and-damage discussions may be relying on incomplete exposure baselines.
Third, future major assessments may need stronger review standards for geospatial methodology, not just emissions pathways and climate model outputs.

In other words, better sea-level science is not enough if the elevation side of the equation is mishandled.

A hidden standards problem in environmental research

One of the paper’s strongest contributions is that it frames the issue as a community-wide standards problem.

The authors argue that geodetic transformations are often left to end users who may not specialize in vertical reference systems. That creates predictable room for omission, incomplete documentation, and accidental error.

This is a familiar sustainability problem in another form: the quality of decisions depends on invisible technical standards that most readers never see.

The study calls for:

  • better documentation of vertical datums in publications
  • clearer peer-review expectations
  • data-provider support through better pre-aligned products
  • more careful use of sea-level references and elevation models in hazard studies

That may sound procedural, but it is foundational. In climate risk work, methodology is policy.

What coastal planners and sustainability readers should take from this

This paper does not mean every coastline is suddenly doomed. It does mean many coastlines may be closer to serious thresholds than previously estimated.

A useful takeaway is not panic, but recalibration.

The practical lesson is that coastal adaptation should not rely too heavily on single exposure maps, outdated elevation assumptions, or simplified global models used without local validation. The most credible coastal planning will increasingly need to combine:

  • accurate local elevation data
  • current sea-level references
  • subsidence measurements
  • transparent vertical datum handling
  • repeated updates as coastlines and land surfaces change

For sustainability policy, this also reinforces a broader principle: resilience depends on measurement quality. When the baseline is wrong, resilience planning becomes fragile.

The bigger pattern: climate risk is often underestimated by system design

This study fits a larger pattern in environmental governance.

Many risks are underestimated not because scientists are ignoring them, but because institutions inherit default assumptions, uneven datasets, and methods that travel poorly across regions. The result is not dramatic error everywhere. It is systematic softness around the edges of risk, especially where data is sparse and vulnerability is already high.

In that sense, the paper is about more than sea level. It is about how technical conventions can quietly shape the world’s understanding of planetary risk.

That is why this research matters for sustainabilityawakening.com’s audience. It shows that the climate challenge is not only about more heat, more water, or more extremes. It is also about whether our measurement systems are calibrated to the real world people are living in.

Conclusion

The most important message from this Nature study is deceptively simple: many coastal hazard assessments may be comparing land to the wrong sea level.

That mistake, repeated across the literature, can make coastal exposure appear smaller than it really is. Once corrected, millions more people and far more land fall into at-risk categories under rising seas.

For planners, policymakers, and sustainability readers, the implication is clear. Coastal adaptation is not only a race against future sea-level rise. It is also a race to improve the accuracy of today’s risk maps.

If the world is underestimating where the shoreline risk line already lies, then some adaptation deadlines may be arriving sooner than expected.

FAQ

What is the main finding of the new Nature sea-level study?

The study finds that most coastal hazard assessments do not properly align land elevation with actual local sea level, which leads to widespread underestimation of coastal exposure.

Why are many coastal hazard assessments wrong?

Many studies assume geoid-based elevation baselines represent local mean sea level. But actual sea level differs from geoid models because of ocean dynamics and regional data limitations.

How much sea-level risk may be underestimated?

Globally, the study suggests that with 1 meter of relative sea-level rise, 31% to 37% more land and 48% to 68% more people may fall below sea level than many assessments estimate.

Which regions are most affected by this problem?

The largest discrepancies appear in parts of the Global South, especially Southeast Asia, Pacific regions, parts of Africa, Latin America, and other data-sparse coastal areas.

What is relative sea-level rise?

Relative sea-level rise reflects both rising ocean levels and sinking land. It is often more relevant for local coastal risk than global mean sea-level rise alone.

Why does this matter for climate adaptation?

Adaptation planning depends on accurate exposure estimates. If risk maps understate present or future sea-level threat, governments may act too late or invest too little.

Does this change the overall science on sea-level rise?

It does not overturn sea-level science. Instead, it suggests many impact assessments may be conservative because they start from an inaccurate local elevation-to-sea-level relationship.

Internal Linking Opportunities

  • What is relative sea-level rise and why it matters more than global averages
  • Why land subsidence is accelerating coastal climate risk
  • How climate adaptation planning fails when baseline data is wrong
  • Sea-level rise in major deltas: Mekong, Nile, Ganges-Brahmaputra, Mississippi
  • The low-elevation coastal zone and why population counts vary so much
  • Climate finance and the challenge of measuring coastal vulnerability

What is the problem with many coastal hazard assessments?
Many coastal hazard assessments compare land elevation to geoid-based sea-level assumptions instead of measured local mean sea level. This can underestimate how much land and how many people are at risk from sea-level rise and coastal flooding.

Why is local sea level different from geoid models?
Local sea level differs from geoid models because sea surface height is influenced by currents, winds, tides, salinity, and temperature, not just Earth’s gravity and rotation.

How much could coastal exposure be underestimated?
A new Nature study suggests that with 1 meter of relative sea-level rise, proper sea-level referencing could increase estimated exposed land by 31% to 37% and exposed population by 48% to 68%.

America’s Vanishing Groundwater: The Invisible Crisis Beneath Our Feet

Fly over Kansas, Arizona, or California, and you will see a miracle: perfectly circular fields of lush green corn and alfalfa thriving in the middle of an arid desert.

But this miracle is an illusion.

Beneath the soil, a catastrophe is unfolding. These crops are not being watered by rain, but by ancient aquifers—underground oceans formed over thousands of years. Today, America is pumping this water to the surface faster than nature can replace it.

According to recent investigations, including a landmark analysis by the New York Times, the United States is systematically depleting its groundwater reserves, leading to a crisis that threatens the nation’s food supply, its economy, and the very ground we stand on.

The Scale of the Drain

Groundwater provides nearly 45% of the water used for irrigation in the US and supplies drinking water to nearly half the population.

However, we are treating this resource like an infinite bank account.

  • The Deficit: In many parts of the High Plains and the Southwest, aquifers are being drained at 10 to 50 times the rate of natural recharge.
  • The “Fossil” Water: Much of this water is “fossil water,” trapped underground during the last Ice Age. Once it is gone, it is gone forever.

Ground Zero: The Ogallala Aquifer

The massive Ogallala Aquifer stretches across eight states, from South Dakota to Texas. It is the engine of American agriculture, supporting the “Breadbasket of the World.”

But the engine is running out of fuel.

  • In parts of the Texas Panhandle and Kansas, the water table has dropped by over 150 feet.
  • Farmers who once pumped 1,000 gallons a minute are now struggling to pump 300.
  • As the water drops, the cost of electricity to pump it from the depths skyrockets, pushing small family farms into bankruptcy.

The Physical Consequence: Sinking Land

When you suck the water out of a sponge, it shrinks. The Earth does the same. This phenomenon is called Land Subsidence.

In California’s San Joaquin Valley, the ground has sunk by as much as 28 feet in the last century. This isn’t just a geological curiosity; it destroys infrastructure.

  • Cracked Canals: The very canals meant to deliver surface water are buckling as the ground sinks.
  • Damaged Roads: Highways and bridges are warping.
  • Aquifer Compaction: The most dangerous part is “compaction.” Once the clay layers in an aquifer collapse, they can never hold water again. We are permanently destroying the storage tank.

Why Is This Happening? The Regulatory “Wild West”

Unlike surface water (rivers/lakes), which is heavily regulated, groundwater in the US is often governed by the archaic “Rule of Capture.”

In many states, if you own the land, you can pump as much water as you want, regardless of how it affects your neighbor. This creates a “Race to the Bottom.” If you don’t pump the water, your neighbor will. This lack of federal oversight has allowed industrial agriculture to drain aquifers unchecked for decades.

The Link to Climate Change

Climate change acts as a “threat multiplier” for groundwater depletion.

  1. Hotter Droughts: As surface rivers (like the Colorado River) dry up due to aridification, farmers are forced to pump more groundwater to survive.
  2. Thirsty Air: Higher temperatures increase evaporation from soil and crops, increasing the water demand.

Is There a Solution?

The situation is dire, but not hopeless. Solutions exist, but they require political will.

1. Managed Aquifer Recharge (MAR)

Instead of letting storm water run into the ocean, we can divert floodwaters onto porous landscapes to soak back into the ground. (See our guide on Can We Refill the Earth?)

2. Crop Switching

We are currently growing thirsty crops like almonds, alfalfa, and cotton in the desert. Shifting to drought-resistant crops or high-value, low-water produce is essential.

3. Ending the “Open Loop”

States like Arizona and California are finally beginning to pass laws limiting groundwater extraction, though implementation is slow.

Conclusion: The End of the Free Lunch

The era of unlimited, cheap water in America is over. We have spent the last century living beyond our hydrological means. To save our future, we must make the invisible crisis visible and treat groundwater not as a commodity to be exploited, but as a strategic national reserve to be protected.

Further Reading:

5 Major Cities Sinking Fast: The Hidden Cost of Groundwater Depletion

We often perceive the ground beneath our feet as the ultimate symbol of stability. However, for millions living in burgeoning metropolises, this stability is a geological illusion. The phenomenon of Groundwater Depletion Sinking Cities is no longer a fringe scientific concern; it is an active structural crisis threatening the survival of major global hubs.

While atmospheric climate change and rising sea levels dominate international headlines, a more immediate “quiet crisis” is unfolding in the lithosphere. Land subsidence—the gradual settling or sudden sinking of the Earth’s surface—is fundamentally altering the topography of our world. This occurs when we extract groundwater from underground aquifers at rates that far outpace natural recharge.

As the water is removed, the structural integrity of the soil’s pore spaces collapses, leading to a permanent “deflation” of the land. This is not merely an engineering inconvenience; it is a systemic failure of our relationship with the hydrological cycle. To understand the gravity of this situation, we must examine the specific regions where the earth is literally giving way.

Table of Contents

land subsidence, groundwater depletion, urban planning, climate change, water management, geological hazards, Jakarta, Mexico City, Groundwater Depletion Sinking Cities,

Structural damage from subsidence costs billions annually in infrastructure repairs and lost productivity.

The Geo-Economics of a Sinking World

The crisis of land subsidence is inextricably linked to the “Invisible Crisis Beneath Our Feet” (America’s Vanishing Groundwater: The Invisible Crisis Beneath Our Feet). When we analyze why cities sink, we find a common denominator: the prioritization of short-term economic expansion over long-term geological stability.

Aquifers act as a hydraulic support system for the layers of earth above them. When the water is pumped out, primarily for industrial agriculture or megacity consumption, the pressure drops. This reduction in “pore pressure” causes the sediment to compact. Once this compaction occurs, the storage capacity of the aquifer is often lost forever.

This is a classic example of ecological debt. We are borrowing water from the past (ancient aquifers) to fuel a present that the future cannot sustain. The following five regions represent the “canaries in the coal mine” for this global hydrological mismanagement.

1. Jakarta, Indonesia: The World’s Fastest Sinking Capital

Jakarta stands as the definitive warning for coastal urban centers. Parts of this megacity are submerging by as much as 25 centimeters per year. This rate is so aggressive that the Indonesian government has officially commenced the transition of its national capital to “Nusantara” on the island of Borneo.

  • The Systemic Cause: A lack of piped water infrastructure forces millions of residents and commercial developers to rely on illegal, deep-well groundwater extraction.
  • The Ecological Consequence: Roughly 40% of the city now sits below sea level. This creates a “bathtub effect” where monsoon rains cannot drain into the sea, leading to perpetual, catastrophic flooding.

The collapse of Jakarta is a direct result of failing to bridge the gap between urban density and resource limits. It is a stark reminder that when Groundwater Depletion Sinking Cities occurs at this scale, the only remaining policy option may be total abandonment.

2. Mexico City: A Legacy of Ancient Hydrology

Built upon the soft, clay-rich sediments of a former lakebed (Lake Texcoco), Mexico City is a case study in geological vulnerability. Over the last century, some sections of the city have subsided by more than 10 meters.

Unlike Jakarta, which faces the sea, Mexico City’s struggle is internal. The city relies on the underlying aquifer for approximately 70% of its water needs. As the clay dries and shrinks, the very foundations of the city’s history—its colonial cathedrals and ancient ruins—are tilting and cracking.

The uneven nature of this sinking creates “differential subsidence.” This exerts massive mechanical stress on subway lines, sewage pipes, and electrical grids. Furthermore, researchers are investigating how this crustal deformation relates to seismic vulnerability, a topic explored in depth in our report on how removing water triggers earthquakes (Does Groundwater Extraction Cause Earthquakes? The Science Explained).

land subsidence, groundwater depletion, urban planning, climate change, water management, geological hazards, Jakarta, Mexico City, Groundwater Depletion Sinking Cities,

In agricultural heartlands, the sinking land is a permanent scar of over-irrigation.

3. San Joaquin Valley, California: The High Cost of Food Security

Subsidence is not exclusively an urban nightmare. In California’s San Joaquin Valley, the earth is sinking due to the demands of global food systems. This region produces a significant portion of the United States’ produce, but the cost is etched into the very elevation of the valley floor.

Since the 1920s, some areas have dropped by nearly 9 meters (28 feet). During periods of extreme drought, farmers are forced to pump groundwater to keep high-value crops like almonds and grapes alive. This creates a vicious cycle: the more the land sinks, the less water the underground “bank” can hold for the next drought.

This regional collapse has reached a critical mass, prompting the state to implement the Sustainable Groundwater Management Act (SGMA). For more on the political hurdles facing these regulations, see our analysis on “Powerful Players Block Change“.

Comparative Subsidence Rates and Primary Drivers

RegionPeak Subsidence Rate (cm/year)Primary DriverMajor Infrastructure Risk
Jakarta25 cmDomestic/Commercial WellsSea Wall Failure
Mexico City40 cmAquifer Over-extractionSewer/Water Line Ruptures
San Joaquin Valley15-20 cmIndustrial AgricultureAqueduct Damage
Beijing11 cmIndustrial/Urban DemandHigh-Speed Rail Alignment
Tehran25 cmIllegal Agriculture WellsGiant Fissures/Sinkholes

4. Beijing, China: Thirst of the Megacity

Beijing’s rapid ascent as a global economic powerhouse has come at a steep hydrological price. The city’s Chaoyang district, home to skyscrapers and international business hubs, is sinking at a rate of 11 centimeters per year.

The “Why” here is a combination of massive population growth and the heavy water requirements of industrial cooling and manufacturing. The Chinese government has attempted to mitigate this through the South-to-North Water Diversion Project—one of the largest engineering feats in human history. However, even this massive influx of surface water has not yet fully halted the subsidence, as the deep aquifers take decades, or even centuries, to stabilize.

5. Tehran, Iran: A Landscape of Fissures

Tehran is currently experiencing some of the highest subsidence rates on the planet. This is not just a gradual lowering of the land; it is a violent fracturing of the landscape. In the plains surrounding the city, massive fissures—some several kilometers long and meters wide—are opening up.

  • Agricultural Mismanagement: Rapid population growth and a lack of modern irrigation technology have led to thousands of illegal wells.
  • Structural Threats: These fissures now threaten to swallow power lines, rail tracks, and small villages, creating a “disappearing earth” scenario that is almost impossible to reverse.

The Geophysical Ripple Effect: Beyond Sinking

The crisis of Groundwater Depletion Sinking Cities extends beyond the elevation of our streets. When we remove billions of tons of water from the crust, we are literally altering the physics of the planet.

As detailed in recent geophysical studies by organizations like the American Geophysical Union, the massive redistribution of mass from underground aquifers to the oceans is contributing to polar drift. This is a profound example of how localized water management decisions can scale up to global geophysical consequences.

One of the most alarming aspects of land subsidence is its potential to trigger seismic activity. When the land settles unevenly, it changes the stress loads on local fault lines. The removal of water weight—and the subsequent compaction of the soil—can “unclamp” faults or increase the pressure in ways that lead to man-made earthquakes.

This connection is explored comprehensively in our foundational report, “The Unseen Link” (The Unseen Link: A Comprehensive Report on Climate Change, Groundwater, and Induced Seismicity). Understanding that our water consumption can move the very tectonic plates beneath us is essential for future urban planning.

land subsidence, groundwater depletion, urban planning, climate change, water management, geological hazards, Jakarta, Mexico City, Groundwater Depletion Sinking Cities,

Understanding the mechanics of pore pressure is key to solving the global subsidence crisis.

Moving Toward Hydrological Equilibrium

To solve the crisis of Groundwater Depletion Sinking Cities, we must shift from a “mining” mindset to a “management” mindset. This requires a multi-pronged approach:

  1. Managed Aquifer Recharge (MAR): Actively pumping treated wastewater or storm runoff back into the ground to maintain pressure.
  2. Circular Water Economies: Implementing “sponge city” concepts where every drop of rain is captured and utilized, reducing the need for deep-well pumping.
  3. Strict Regulatory Oversight: Ending the era of “wild west” groundwater pumping where whoever has the deepest well wins.

The science is clear: the ground is not as stable as we once thought. However, by acknowledging the systemic link between our water use and geological stability, we can begin to rebuild our cities on a foundation that will actually hold. For further reading on the global implications of these shifts, refer to the United Nations World Water Development Report.

As we have seen, the redistribution of water weight is even “Tilting Earth’s Axis“, proving that our footprint on this planet is deeper—and more fragile—than we ever imagined.