Widely used method for calculating sea level rise may be a century behind in change
Hundreds of global and regional studies of sea level rise and coastal flooding may have underestimated sea levels by an average of 20 to 30 centimeters.
Of 385 peer-reviewed studies published from 2009 to 2025, about 99 percent of the ocean’s height is incorrectly estimatedleading to sea level approximations that were at least a century off from predicted sea level rise, researchers report March 4 in Nature. These included 45 studies referenced by the United Nations Intergovernmental Panel on Climate Change in its report. Sixth evaluation report.
The results suggest that the consequences of future sea level rise are even more severe than expected. One meter rising sea levels – which could happen within a century – would overwhelm areas inhabited by up to 132 million people, the researchers said, an increase of up to 68 percent more than previously suggested.
Sea level rise is slow but dangerous if you ignore itsays climatologist Anders Levermann of the Potsdam Institute for Climate Research in Germany. “That’s basically what we did without knowing it,” he said. “These estimates now tell us that we are much further into the future than we thought. »
Physical geographers Katharina Seeger and Philip Minderhoud of Wageningen University in the Netherlands discovered this discrepancy after evaluating hundreds of global and regional studies on sea level rise, storm surges, tsunamis and coastal hazards in general. More than half were published in the last five years. Their analysis revealed a common error in 90 percent of the research evaluated regarding the type of data used.
Typically, scientists and engineers assessing an area’s vulnerability to coastal hazards compare land elevation with sea level. Ideally, land elevation data includes real-world measurements, such as those collected by satellite. Likewise, sea level data should include measurements collected by tide gauges, ocean buoys, satellites, or other monitoring instruments.
But Seeger and Minderhoud found that most of the studies they evaluated neglected to include direct measurements of sea level, relying instead on wonky numerical shapes called geoids.
A geoid can be imagined as an irregular, wavy blue ball representing the global ocean based on data about Earth’s gravity and rotation. But using geoids to estimate sea level poses two major problems. First, they can be offset by several meters in areas lacking gravitational data. Second, geoids do not take into account ocean circulation, currents, winds, tides, water temperature, and other factors influencing sea level.
The new work shows that most research has not corrected for these geoid flaws with actual measurements when estimating sea level, Minderhoud said during a March 3 press briefing. Such corrections are common practice in oceanography but have not yet been widely adopted by coastal hazard researchers, he said.
In addition to the 90% of studies that relied on the geoid hypothesis, an additional 9% had misaligned measurements of sea level and land elevation, the researchers found. Less than 1 percent of the studies evaluated aligned the data correctly, Seeger said.
Using a global dataset of sea level, based largely on satellite measurements of the ocean surface, Seeger and Minderhoud estimated how far off the studies evaluated were. They found that the studies underestimated coastal sea level heights by an average of 24 to 27 centimeters (about 10 inches), depending on the specific geoid used.
In some places the gap was much larger due to lack of data. For example, in parts of Southeast Asia and the Indo-Pacific, sea levels are more than a meter higher than studies had estimated. In a small number of places, studies have overestimated sea levels. These regions include the northern Mediterranean, Antarctica, and some islands in the Atlantic and Pacific. The smallest differences were found in eastern North America and northern and western Europe.
However, given the overall underestimation, ocean advance “is even worse than has been reported,” says coastal geologist Patrick Barnard of the University of California, Santa Cruz. This new work, he says, highlights how important it is for planners to avoid using the results of aggregate studies in local adaptation plans without additional verification.
To facilitate future studies, Seeger and Minderhoud have produced publicly available coastal sea level data that incorporates the most recent measurements. “We hope that as a scientific community we can … move forward together,” Seeger said.
