Sea levels are much higher than often assumed. How is that possible?

[OKIsItJustMe]

https://www.wur.nl/en/longread/sea-levels-much-higher-than-assumed-how-possible

When Philip Minderhoud travelled through Vietnam in 2015, he sensed that something was not quite right. Minderhoud was working on his PhD research on land subsidence in the Mekong Delta, one of the largest deltas in the world. He had brought along a series of maps indicating how high the land was supposed to be elevated: one and a half metres above sea level, and in some places even two metres, according to the maps.

Minderhoud looks around, across the vast delta. The landscape is as flat as a pancake. And almost everywhere he looks, he sees the same thing: the water level is far higher than the maps suggest. In many parts of the delta, the surface water level — directly connected to the sea — is not one and a half metres but only a few decimetres below the land surface. The maps simply do not match reality. And that is not because of the weather or the tides.

The maps Minderhoud consults in Vietnam come from reports by international organisations. How can it be, he wonders, that these reports present such a distorted picture? And how widespread are these inaccuracies? In the years that follow, he delves deeper into the methods and calculations underpinning the relative height of sea level and land elevation.

How sea levels are measured
In the Netherlands, sea level elevation is no mystery. Since the end of the 19th century, the Netherlands has used Normal Amsterdam Level (NAP) as a reference frame for measuring the country's elevation. Nieuwerkerk aan de IJssel is 6.78 metres below NAP, while the Vaalserberg is 322 metres above it. Many people intuitively think that 0 metres NAP equals sea level, but sea level has now risen to about 10 centimetres above NAP. A house 1.50 metres above NAP is therefore actually about 1.40 metres above the current average sea level for the Dutch coast.

…

Seeger, K., Minderhoud, P.S.J. Sea level much higher than assumed in most coastal hazard assessments. Nature (2026). https://doi.org/10.1038/s41586-026-10196-1

[Lovie777]

1. Melting ice....

[OKIsItJustMe]

2. The inaccurate estimates are based on "melting ice" and other factors

How is it possible that so many scientific studies continued to use assumed sea level based on geoid models, when more accurate, global sea level measurements had been available for at least twenty years? This may have been due to the fact that most prominent studies were conducted by Western scientists, who were accustomed to the reliable results of geoid models in their own regions. As geoid assumptions worked relatively well, they may not have realized the need to use direct sea level measurements in their analyses. This assumption has led to an underestimation of coastal exposure in these studies, especially at the coasts of South-East Asia and the Indo-Pacific.

[mwmisses4289]

3. Doesn't topography change?

What was accurate 30 years ago may not be accurate now because of erosion due to weather, watercourses changing, flooding etc.

[OKIsItJustMe]

5. Topography changes, perhaps most notably due to subsidence

Take India for example:
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL120539

Vertical Land Motion and Human Exposure Across India's Coastal Regions
Quantao Zhu, Pei-Chin Wu, Meng (Matt) Wei, Estelle Chaussard
First published: 14 February 2026 https://doi.org/10.1029/2025GL120539
Quantao Zhu and Pei-Chin Wu are contributed equally to this work.

Abstract
In India, over 200 million people live within 100 km of the coastline, and many reside in low-lying areas exposed to increasing flood risks associated with sea-level rise. However, the role of vertical land motion (VLM)—particularly land subsidence—in shaping this coastal exposure remains poorly quantified. Here we present the first assessment of VLM across India's coastal zone, using 8 years of Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) time series data (2016–2024). This comprehensive data set reveals widespread subsidence with several notable hotspots: Ahmedabad, Chennai, Amaravathi, Kochi, Kakinada, and Kolkata. The five major deltas along the east coast display extensive subsidence up to 20 mm/yr. Over 8.5 million residents live in coastal areas experiencing subsidence greater than 5 mm/yr. Our findings suggest that coastal subsidence in India is more extensive than previously recognized and poses a significant challenge for long-term land-use planning in coastal zones.

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[eppur_se_muova]

4. Another lesson on the importance of standards, and the need for vigilant maintenance. nt