The fact that sea levels are rising due to climate change is nothing new. But where the current levels are has apparently been underestimated in many regions, as a meta-analysis reveals. Accordingly, more than 90 percent of previous studies on climate-related threats to coastal regions have used incorrect methodology and incorrect reference data. As a result, the current global average sea level was estimated to be 20 to 30 centimeters too low. In some regions, such as Southeast Asia, the discrepancy is more than one meter, as the researchers have determined. This also means: Because the forecasts for future increases refer to current values, more people and areas are affected by the threat of flooding and land loss.
Sea levels are also rising due to climate change. The thermal expansion of seawater and the influx of meltwater from the thawing ice masses in the polar regions are driving the levels upwards. This is leading to increasingly frequent floods, but also to gradual loss of land on many islands and coastal areas. In many areas, ground subsidence further increases the risk. To make matters worse, sea level rise is accelerating: from 2006 to 2015 the global average was around 3.6 millimeters per year, but in 2019 values of 6.1 millimeters were already reached. The increase is currently an average of around four millimeters per year. What doesn’t sound like much adds up: According to the latest world climate report (AR6), global sea levels could be 28 to 101 centimeters higher by 2100 than the average from 1995 to 2014. Depending on the region, however, the levels can reach significantly higher values, because ocean currents, water temperatures and winds also influence local sea levels.
The geoid as a reference is not enough
It is therefore essential for coastal regions and islands worldwide to know as accurately as possible where the sea level currently lies and what consequences a rise could have. But it is precisely on these points that many of the previous studies could be flawed, as Katharina Seeger and Philip Minderhoud from Wageningen University in the Netherlands have now discovered. “Our calculations show that the measured sea level in many coastal regions of the world is higher than assumed in most studies,” reports Seeger. For their study, they evaluated the methodology and results of 385 current publications on sea levels, coastal protection and risks to coastal areas. They came across a crucial “blind spot” in the methodology. On the one hand, this occurs when satellite data is not compared with local conditions and level measurements.
(Video: Wageningen University & Research)
The core of the problem is the use of the so-called geoid as a reference. This model shows the earth’s gravity field, which is shaped by the earth’s gravity and rotation. Earth’s geoid looks more like a dented potato than a sphere. Because the water in the world’s oceans follows the earth’s gravity, the shape and approximate height of the ocean surfaces can also be derived from the geoid. However, there is a catch: “In reality, sea level is influenced by additional factors such as wind, ocean currents and the temperature and salinity of the seawater,” explains Minderhoud. If this data is not included and the models are calibrated using local level measurements, incorrect assessments will occur. The second source of error is the use of different reference systems for sea level height and the height of the coast. “The heights of land and sea are measured by different satellites and often do not refer to the same geoid,” explains Seeger. If you want to correctly determine the sea level in relation to the respective coast, these different references must be taken into account and adjusted mathematically.
Up to one meter higher than expected
However, the evaluations by Seeger and Minderhoud showed that more than 90 percent of the studies they examined did not take these two factors into account or did not take them sufficiently into account. A further nine percent of the studies combined land elevation data and water level measurements, but had deficiencies in the mathematical comparison of these data. As a result, most studies determine an incorrect value for the current sea level: “Our calculations show that the measured sea levels on many coasts are higher than those determined in the studies,” reports Seeger. On average, previous risk analyzes have underestimated sea level by 20 to 30 centimeters. In some regions of Southeast Asia, such as the Mekong Delta, levels are even one meter higher than previously determined. “Other areas with large discrepancies are in Latin America, the West Coast of North America, the Caribbean, Africa, the Indo-Pacific and the Middle East,” the team said. However, the smallest deviations from the real levels occur on most European coasts. The previous data is also correct for the German coasts, because here the values are based on a dense water level measurement network and an elevation model that is already based on sea level.
The underestimated sea levels are particularly problematic when it comes to the risk of flooding and land loss. Because the forecasts for the rise in water levels caused by climate change – for example in the world climate report – are relative values: They indicate how much the sea levels will change compared to their current state. However, if the current levels are estimated too low, the future danger to coastal residents will also be underestimated. “Simply put: If the sea level on the coast of my island or port city is higher than previously thought, then the consequences of sea level rise will become apparent faster than predicted,” explains Minderhoud. The number of people affected by floods and land loss is also higher than expected. “Our corrected calculations show that with an increase of one meter compared to current levels, around 37 percent more land areas will fall below sea level and 68 percent more people will be affected,” the researcher continued.
In concrete terms, this means that between 55 and 101 million people already live in areas below sea level – previously it was assumed that only 10 to 15 million were affected. The researchers have determined that if sea levels rise by one meter, up to 132 million people could be affected. “Our findings make it necessary to reevaluate and, in most cases, update the underlying methodology of all existing coastal hazard studies,” says Minderhoud. Researcher Gabriel David from the TU Braunschweig, who was not involved in the study, comments: “The fact that this is now being revealed is very positive and shows that science should work exactly like this: every study makes us a little smarter – very good studies even a little more – new data and technologies enable new perspectives with which we can check and correct previous assumptions.”
Source: Katharina Seeger and Philip Minderhoud (Wageningen University & Research, Netherlands), Nature, doi: 10.1038/s41586-026-10196-1