A new scientific study has revealed that sea levels along coastlines worldwide may be significantly higher than previously estimated, raising concerns that millions more people could be at risk from coastal flooding.
The research, conducted by scientists at Wageningen University and published in the journal Nature, highlights critical errors in how sea level heights have been calculated in many climate impact studies.
According to the researchers, the discrepancies between actual sea levels and earlier estimates can reach up to 30 centimetres in some regions. In parts of Southeast Asia, however, the difference may be as much as 1.5 metres.
These findings suggest that tens of millions of people living in coastal zones may be far more vulnerable to rising seas and flooding than policymakers currently believe.
The study argues that the problem stems from the way scientists have historically combined coastal sea level data with land elevation measurements.
Researchers found that many studies relied on geoid models—mathematical representations of the Earth’s gravity field—rather than direct measurements of local sea levels.
This approach led to what the scientists describe as a “misjudgment of sea level relative to coastal elevation,” ultimately producing inaccurate flood risk assessments. The researchers reviewed around 400 recent scientific studies examining sea level rise and coastal hazards.
Their analysis revealed that approximately 99% of these assessments contained the same fundamental error in their calculations. Only one of the reviewed studies had correctly integrated sea level measurements with land elevation data, according to the researchers.
The findings are particularly concerning because many of these studies have informed international climate reports and national policy planning.
Several of the flawed studies have been cited in reports by the Intergovernmental Panel on Climate Change (IPCC), which governments worldwide rely on when shaping climate adaptation strategies.
Philip Minderhoud, a researcher specializing in land elevation and sea level rise, first noticed the problem more than a decade ago during fieldwork in Vietnam. While visiting the Mekong Delta roughly ten years ago, he observed that the sea level appeared much higher than what official coastal maps indicated.
The discrepancy prompted Minderhoud to investigate further, leading him to discover similar inconsistencies in coastal regions across Southeast Asia. Over time, he worked with fellow researcher Katharina Seeger to identify the root cause of the error.
Their work eventually showed that incorrect integration of elevation data and sea level measurements had been widely replicated in scientific models. As a result, many coastal hazard assessments may significantly underestimate the true extent of flood-prone areas.
In densely populated coastal regions, even small differences in elevation can have major consequences for flood risk. When the sea level baseline is miscalculated, maps used to identify vulnerable areas may incorrectly classify communities as safe when they are not.
The study’s authors warn that this could have serious implications for disaster preparedness and coastal planning. Governments rely heavily on scientific assessments to determine where to build flood defences, seawalls and protective infrastructure.
If sea levels are higher than previously believed, some countries may have less time than expected to strengthen coastal protection systems. The issue is particularly urgent for low-lying regions such as river deltas and island nations, where rising seas already threaten communities and ecosystems.
In areas like Southeast Asia, where many major cities sit near sea level, updated measurements could significantly expand the number of residents considered at risk. However, the researchers emphasized that not all coastal regions are affected equally by the miscalculation.
In some locations, the actual sea level may be slightly lower than previously assumed. The Dutch coastline, for example, is not impacted by the issue identified in the study.
Sea level measurements in the Netherlands are based on extensive local monitoring systems rather than global models, providing more precise data. The researchers hope their findings will encourage scientists and policymakers to reassess existing flood risk models and improve the accuracy of future studies.
Better measurement techniques and more localized data could help governments make more informed decisions about climate adaptation. As global sea levels continue to rise due to climate change, accurate data will be essential for protecting vulnerable coastal populations.
Without reliable measurements, experts warn, the world may be underestimating the scale and urgency of the threat facing millions of people living along the planet’s coastlines.