New Delhi: Coastal sea levels are much higher than most flood-risk studies have assumed so far, new research has found. That means millions more people could be in danger than earlier assessments suggested.
Devastating flooding could hit coastal areas far earlier than governments have been planning for, suggests the study ‘Sea level much higher than assumed in most coastal hazard assessments’, published this week in Nature. The risk, it notes, is especially high in the “Global South”.
Reviewing hundreds of previous studies, researchers Katharina Seeger and Philip S J Minderhoud of Wageningen University in the Netherlands found that most rely on the wrong baseline to estimate sea levels.
“From all evaluated studies, more than 99% did not use sea-level information, omitted or made errors during sea-level datum conversion and missed crucial datum and processing documentation, rendering the studies irreproducible,” the Nature paper said.
Here’s a look at the study’s findings and what they mean.
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A dangerous ‘blind spot’
The study found that coastal sea levels are an average of 24 to 27 centimetres higher than previous estimates. In some regions of Southeast Asia and the Indo-Pacific, the gap exceeds 1 metre.
The discrepancy comes from how researchers measure the ocean. Most studies rely on geoid models—mathematical representations of Earth’s shape based on gravity—rather than actual measurements of local sea levels.
“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,” the study read.
Geoid models provide a smooth, theoretical ocean surface that doesn’t account for currents, winds, tides or water temperature, which are affected by factors such as climate and weather phenomena. The researchers called it a “community-wide blind spot”, also noting that errors are worst where data is scarcest.
“In data-rich countries in the Global North, the global geoids represent coastal sea level relatively well (for example, Eastern United States, Northern Europe and Western Europe), whereas in the more data-sparse Global South, regions such as Latin America, East Africa and the Indo-Pacific, with Southeast Asia and Oceania as global hotspots, the geoids substantially underrepresent actual sea-surface height, ranging from several decimetres up to several metres locally,” the study said.
Seeger and Minderhoud reviewed 385 studies published between 2009 and 2025. They recalculated flood exposure using four global elevation datasets properly aligned to actual sea-level measurements.
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How many are at risk?
Sea-level rise has long been on the threat radar for coastal communities, with the UN Intergovernmental Panel on Climate Change (IPCC) estimating levels could increase by 28-100 cm by 2100. The new Nature study suggests that the actual situation is even more alarming.
When actual sea-level measurements are used, the study found that a 1 metre rise in sea level would affect 31 per cent to 37 per cent more coastal land than earlier estimates suggested. The population under threat increases by 48 per cent to 68 per cent under the revised estimates, amounting to 77 million to 132 million people.
Southeast Asia confronts the greatest risk. In the region, land area below mean sea level could increase by up to 94 per cent, affecting 46.9 million people.
“If sea level is higher for your particular island or coastal city than was previously assumed, the impacts from sea level rise will happen sooner than projected before,” Minderhoud told The Guardian.
It’s not necessarily bad news for everyone. In some regions, including parts of the northern Mediterranean coast, Antarctica, and some Atlantic and Pacific islands, geoid models can actually overestimate sea-surface height, the paper notes.
“This study probably reveals only the tip of the iceberg, as the evaluated publications form only a representative selection of the full body of coastal hazard assessments,” the paper said.
(Edited by Asavari Singh)