Pushing back on Recent ‘Sea Level is Worse Now’ Claims: The Coordinate System That Ate the Coastline

Charles Rotter

In the annals of modern scientific discovery, a few moments stand out as turning points for human understanding. Galileo pointing a telescope at Jupiter. Newton contemplating a falling apple. The invention of the spreadsheet’s “auto-sum” function. And now we must add another milestone: the discovery that sea level rises dramatically whenever someone chooses a different vertical reference datum.

A recent paper in Nature, Sea level much higher than assumed in most coastal hazard assessments, announces with grave concern that sea level is “much higher than assumed in most coastal hazard assessments.” The oceans, it seems, have quietly climbed upward while nobody was looking. Not by the slow millimetres per year that have been the traditional talking point, but by an impressive leap achieved through the formidable power of coordinate system management.

Abstract

The impacts of sea-level rise and other hazards on the coasts of the world are determined by coastal sea-level height and land elevation¹. Correct integration of both aspects is fundamental for reliable sea-level rise and coastal hazard impact assessments^2,3, but is often not carefully considered or properly performed. Here we show that more than 99% of the evaluated impact assessments handled sea-level and land elevation data inadequately, thereby misjudging sea level relative to coastal elevation. Based on our literature evaluation, 90% of the hazard assessments assume coastal sea levels based on geoid models, rather than using actual sea-level measurements. Our meta-analyses on global scale show that measured coastal sea level is higher than assumed in most hazard assessments (mean offsets [standard deviation] of 0.27 m [0.76 m] and 0.24 m [0.52 m] for two commonly-used geoids). Regionally, predominantly in the Global South, measured mean sea level can be more than 1 m above global geoids, with the largest differences in the Indo-Pacific. Compared with geoid-based assumptions of coastal sea level, the measured values suggest that with a hypothetical 1 m of relative sea-level rise, 31–37% more land and 48–68% more people (increasing estimates to 77–132 million) would fall below sea level. Our results highlight the need for re-evaluation of existing coastal impact assessments and improvement of research community standards, with possible implications for policymakers, climate finance and coastal adaptation.

https://www.nature.com/articles/s41586-026-10196-1

To appreciate the breakthrough, one must first understand the central claim. According to the paper, many coastal hazard studies referenced land elevation to a gravity-based surface known as the geoid. Meanwhile, the actual sea surface does not perfectly align with that theoretical surface. When researchers instead align their calculations to measured sea-surface height, the baseline shifts upward by about 24–27 centimetres on average.

The ocean did not move. The zero line moved.

This subtle distinction is the sort of detail that tends to get lost between the Methods section and the press release. The paper describes the discrepancy as a “global average underrepresentation of coastal sea-level height” of about 0.27 metres.

Translated into everyday language, the scientific community has discovered that if one changes the definition of zero, the number above zero changes.

In fairness, this is not entirely trivial. Vertical datums matter in geodesy. Earth is lumpy, gravity varies slightly across the globe, and the sea surface is influenced by currents, winds, and temperature differences. The result is that mean sea level is not a perfectly smooth surface.

But the paper then delivers its pièce de résistance. The authors reviewed 385 coastal hazard studies and concluded that more than 99 percent handled the vertical reference problem incorrectly.

Pause for a moment and savour that claim.

For decades, an entire scientific literature on coastal flooding has been built on calculations that, according to this new analysis, may have used the wrong vertical reference frame.

One might imagine the reaction if a similar finding appeared in another field. Suppose a medical study announced that 99 percent of previous blood pressure measurements were taken with the cuff on the wrong arm. Or if astronomers reported that almost every galaxy distance estimate had used the wrong unit conversion.

Yet in climate-related research this sort of revelation arrives with remarkable composure. The paper calmly refers to a “community-wide blind spot.”

Indeed.

The most entertaining part of the exercise comes when the authors quantify the consequences. If the sea-level reference is corrected, the estimated number of people below sea level after a one-metre rise increases dramatically—from roughly 34 to 49 million people to somewhere between 77 and 132 million.

That is a very large jump in population exposure.

The fascinating part is that the jump occurs without the ocean moving a single millimetre.

Instead, the coordinate system moved.

To be clear, the authors are not claiming that sea level suddenly rose 27 centimetres overnight. They are pointing out that previous studies sometimes used a simplified reference level when comparing land elevation with ocean height. When that reference is adjusted, the relative positions of land and sea shift on paper.

This is an entirely legitimate technical point.

But it also produces a scenario that borders on the comic.

Imagine telling a coastal resident: “Good news and bad news. The ocean hasn’t risen. But we’ve moved the zero line upward by 27 centimetres, so you now live below sea level.”

One can almost hear the puzzled response: “Does the water know that?”

The issue becomes even more entertaining when considering the magnitude of uncertainties in coastal elevation data. The paper acknowledges that satellite-derived digital elevation models can contain vertical errors of several metres.

This is not a typo.

Several metres.

Meanwhile, the long-term sea-level rise signal that drives policy discussions is typically measured in millimetres per year.

The result is a curious mismatch of scales. Scientists are attempting to detect millimetre-level trends using terrain data that can sometimes be wrong by orders of magnitude more. It is a bit like measuring the thickness of a sheet of paper using a yardstick that may or may not be bent.

This does not make the exercise useless. It simply means the uncertainties deserve far more attention than they usually receive in popular summaries.

Another amusing subplot in the paper concerns geography. The authors note that the discrepancies between geoid models and actual sea surface height are largest in parts of the Global South, where gravity data are sparse.

In some locations the mismatch can reach several metres.

This leads to a delightful possibility: depending on the model chosen, entire coastlines can appear to jump up or down relative to sea level.

One begins to see how the headline “Sea level is rising everywhere faster than everywhere else” might emerge from such a landscape of shifting reference frames.

If one datum says the ocean is here, another says it is there, and a third introduces a correction of several metres, the humble shoreline becomes a surprisingly flexible concept.

The authors are understandably concerned about the implications for coastal hazard assessments. If elevation data are misaligned with sea-level references, risk estimates could be biased.

That is a reasonable observation.

Yet the broader lesson might be even more interesting. The complexity of these calculations highlights how sensitive many climate-impact projections are to seemingly mundane technical choices: which dataset, which model, which coordinate system, which correction.

Change any of those inputs and the resulting maps can look dramatically different.

The public discussion of climate risk rarely reflects this level of technical nuance. Instead, the narrative tends to present projections as if they were straightforward measurements of the physical world.

The reality is closer to an elaborate modelling exercise in which multiple layers of assumptions interact with imperfect data.

None of this means sea level is not rising. Tide gauges and satellite altimetry clearly show a gradual upward trend over the past century. The point is that estimating who will be flooded, when, and by how much involves a complicated chain of geospatial processing steps.

And occasionally someone discovers that one of those steps was handled differently than assumed.

At which point the ocean appears to jump upward—at least on paper.

The moral of the story may be that Earth’s oceans are less volatile than our coordinate systems. While the water creeps upward at a stately pace measured in millimetres per year, the reference frames used to describe it can shift by centimetres or metres depending on the choice of model.

From a comedic standpoint, this opens a delightful new frontier in coastal science. Future research may reveal that sea level is even higher once someone remembers to update the projection file path or recalibrate the GIS plugin.

Until then, one can admire the remarkable versatility of the ocean. It manages to rise, fall, and remain exactly where it is—all depending on which line in the spreadsheet happens to be labelled zero.