Scientists studying coastal flood risk have been working with sea-level estimates that are systematically too low, according to a study published in Nature on March 4, 2026. Researchers Christian Seeger and Philip Minderhoud found that the vast majority of global coastal hazard assessments conducted over the past 16 years contain a shared technical error that makes the ocean appear shallower than it actually is relative to the land. The consequences fall hardest on low-lying nations in Southeast Asia and the Pacific, where tens of millions of additional people now face greater exposure to flooding and erosion than planners assumed.
A 16-Year Blind Spot in Coastal Science
The core problem is deceptively simple. Coastal hazard studies need to know how high the sea sits relative to the ground. To do that, researchers rely on a mathematical surface called the geoid, a gravity-based model of Earth’s shape that serves as a proxy for mean sea level. But the geoid is not the same thing as actual local sea level, and the two can differ significantly depending on ocean currents, temperature, and regional gravity variations. According to the Nature analysis, roughly 90% of the 385 coastal hazard and exposure assessments published between 2009 and 2025 treated the geoid as if it were identical to local sea level. More than 99% of those assessments mishandled vertical datum conversion and sea-level referencing in some way.
That assumption introduced a consistent downward bias. The study estimates global mean offsets of about 0.24 to 0.27 meters depending on which geoid model was used. Separate reporting puts the average figure at roughly 30 centimetres higher than assumed. The discrepancy between those two numbers, 24-to-27 centimetres versus 30 centimetres, likely reflects differences in how global averages and regional peaks are reported, but both point in the same direction: the water is closer to people’s doorsteps than almost every recent assessment has shown.
Why the Error Hits Hardest in the Global South
The offset between the geoid and true local sea level is not uniform. It varies by region, and some of the largest gaps appear along the coasts of Southeast Asia and the Indo-Pacific. The Nature study’s abstract notes explicit regional differences in the Indo-Pacific, where warm ocean currents and complex gravity fields push actual sea levels well above what the geoid predicts. These are also regions where large populations live on low-lying river deltas, meaning even a small measurement error translates into a large increase in the number of people exposed to storm surges, tidal flooding, and saltwater intrusion.
The geographic pattern creates a troubling equity dimension. Wealthier nations in the North Atlantic and Northern Europe tend to have dense tide-gauge networks and local vertical reference systems that partially correct for geoid errors. Countries in the Global South often lack that infrastructure and depend more heavily on the satellite-derived global models that carry the bias. The result is that the places with the fewest resources to adapt are also the places where the error is largest and least likely to be caught. Associated Press coverage on the study frames the stakes bluntly: tens of millions more people are potentially at risk, with uneven impacts concentrated in the Global South, Pacific, and Southeast Asia.
How Better Terrain Models Exposed the Gap
Part of what made this study possible is a new generation of coastal terrain data. The researchers compared their findings against models including the DeltaDTM, a global terrain dataset constructed using space-based imagery and ICESat-2 laser altimetry. DeltaDTM and similar tools provide higher-resolution elevation data for coastlines than older satellite-based digital elevation models, which often overestimate ground height in flat, vegetated, or built-up coastal zones. By cross-referencing these improved terrain models with tide-gauge records, Seeger and Minderhoud were able to quantify how far off the standard geoid-based approach has been.
The distinction matters because coastal flood models are only as good as their inputs. If the baseline sea level is set too low and the ground elevation is set too high, the model will dramatically undercount the number of people and structures in the flood zone. Correcting both inputs simultaneously, as this study effectively does, compounds the revision. Communities that appeared to have a comfortable margin above high tide may turn out to have very little buffer at all.
What This Means for Climate Adaptation Planning
The practical fallout extends well beyond academic debate. National governments, international development banks, and insurance companies all rely on coastal hazard assessments to decide where to build sea walls, which communities to relocate, and how to price flood risk. If those assessments have been systematically optimistic for more than a decade, then billions of dollars in infrastructure investment and climate adaptation funding may have been allocated based on flawed baselines. A brief summary on the Nature homepage frames the finding plainly: coastal sea levels may be higher than previously thought.
The study also raises a pointed question about how climate adaptation aid is distributed. International climate finance already skews toward middle-income and upper-middle-income countries that can assemble bankable project proposals. If the technical models used to justify those proposals systematically understate risk in the poorest coastal nations, the mismatch between need and funding grows wider. Correcting the vertical datum error could shift the map of global climate vulnerability in ways that demand a reallocation of resources toward the Indo-Pacific and sub-Saharan African coastlines where the bias is most severe.
A Systemic Fix, Not a Quick Patch
One striking aspect of Seeger and Minderhoud’s work is how mundane the underlying mistake is. The authors do not accuse previous researchers of ignoring sea-level rise or misreading climate projections. Instead, they trace the problem to routine steps in data processing: choosing a vertical reference, converting between coordinate systems, and documenting those choices clearly. In many of the 385 studies they reviewed, the geoid-based reference surface was simply assumed to represent local mean sea level, and that assumption was carried through into flood maps and exposure estimates without further scrutiny.
Fixing the problem, therefore, will not be as simple as publishing a single correction factor. The offset between geoid height and true sea level depends on local ocean dynamics and gravity, and it must be calibrated using tide gauges or other in situ measurements wherever possible. In places without such measurements, scientists will need to rely on regional ocean models and satellite altimetry to estimate the gap. That means future coastal hazard assessments will have to budget more time and expertise for vertical datum work, and journals and funding agencies may need to insist on clearer documentation of how sea levels are referenced.
The authors also point out that the error is entangled with other known issues in coastal risk modelling, such as the use of low-resolution elevation data and simplified representations of tides and storm surges. Correcting the geoid and sea-level mismatch will sharpen the picture, but it will not eliminate uncertainty. Instead, it should be seen as one step in an ongoing effort to build more realistic, locally grounded assessments that can support difficult policy decisions about where to invest in protection and where to plan for retreat.
Implications for Coastal Communities
For residents of low-lying coasts, the technical debate over vertical datums translates into very concrete questions. A village that was previously mapped as sitting 40 centimetres above projected high tide might now be reassessed as only 10 or 15 centimetres above that level, or even slightly below it. That difference can determine whether a community qualifies for relocation assistance, whether a new housing development is approved, or whether an existing sea wall is deemed sufficient for the coming decades.
Local planners and engineers may also need to revisit design standards. If baseline sea levels are higher than assumed, then freeboard allowances (the extra height added to levees and seawalls above expected water levels) may no longer be adequate. Ports, wastewater treatment plants, and coastal roads built to outdated benchmarks could face more frequent overtopping and flood damage. Updating those standards will take time and money, but delaying the adjustment risks locking in infrastructure that is vulnerable from the day it is completed.
Community engagement will be essential as these reassessments unfold. Explaining to residents that past maps were based on a subtle but systematic error will not be easy, especially in places where trust in authorities and outside experts is already fragile. Yet transparent communication about the new findings, and about the limits of current knowledge, may help communities make informed choices about adaptation, from elevating homes to restoring protective wetlands.
Rebuilding Trust in Coastal Risk Science
The discovery of a long-running, widely shared error raises uncomfortable questions for the scientific community. Peer review did not catch the misuse of the geoid in hundreds of papers, and many influential assessments were built on top of those flawed foundations. Going forward, researchers and journals may need to treat vertical referencing and datum conversion as core methodological issues rather than technical footnotes.
At the same time, the episode illustrates how science corrects itself as new data and tools become available. The emergence of high-precision terrain models, improved tide-gauge records, and better documentation standards made it possible to identify and quantify the problem. For readers seeking deeper context on how such corrections shape public understanding of risk, outlets like Guardian subscriptions and other specialist reporting can provide ongoing coverage of how climate science feeds into policy.
Ultimately, the revised sea-level baselines underscore a sobering reality: even without any additional warming, many coastal regions are closer to the edge than previously recognised. As governments, insurers, and communities revisit their plans, they will be working from a new starting point—one in which the ocean, quite literally, is higher than the maps had shown. For those following the evolving science and its implications, staying engaged through outlets that offer detailed climate reporting and tools such as reader accounts can help keep the conversation grounded in the latest evidence as coastal communities confront difficult choices about their future.
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*This article was researched with the help of AI, with human editors creating the final content.
