For decades, California has been bracing for the sea. Communities from Pacifica to Imperial Beach have fought over seawalls, buyout programs to move people away from shorelines, and the loaded politics of “managed retreat.” Now, a new study suggests researchers globally may have built these plans on a shaky foundation.
Published Wednesday in Nature, the research from Katharina Seeger and Philip Minderhoud at Wageningen University in the Netherlands uncovered discrepancies in what most scientists thought they knew about how high the sea already sits at coastlines around the world.
The researchers analyzed 385 peer-reviewed studies on coastal flooding and sea-level rise published between 2009 and 2025 — including dozens cited in the United Nations’ most recent major climate assessments.
They found that more than 90% of those studies made the same fundamental mistake when measuring how high the sea sits relative to the land: instead of using actual, measured ocean heights, they relied on mathematical models that estimate where the ocean surface should sit based on Earth’s gravity and rotation.
These models, called geoids, essentially sketch a modeled ocean — one that exists in the absence of tides, currents, winds and temperature differences. The real ocean, of course, ignores those assumptions entirely.
In some regions, that means the actual sea level could be several meters higher, meaning millions more people are at risk of inundation than previously thought.
The researchers subsequently ran the corrected numbers, revealing that under a scenario in which sea levels rise by roughly 3 feet, up to 37% more land could fall below sea level than current assessments predict.
Between 77 million and 132 million more people worldwide could be at risk of inundation than previously thought.
Globally, real coastal sea levels are on average about a foot higher than previous models suggested.
The discrepancies tended to be largest in lower-income regions, including parts of Africa, Southeast Asia and the Pacific, where there are typically fewer local tide gauges and direct ocean measurements; these places are more reliant on models that perform poorly there.
For example, in some parts of Southeast Asia and the Pacific, the difference exceeds 3 feet. These regions are also the places already most exposed to rising seas and the risks that accompany them.
Although discrepancies are smaller in data-rich regions such as Western Europe and the eastern United States, where robust local monitoring has kept projections more closely calibrated, the study identifies the West Coast of North America as a region where sea levels have also been underestimated.
For California and the Pacific Coast, discrepancies hover around 0.25 to 2 meters.
These findings underscore the need for updated assessments to ensure coastal cities are prepared for the true scale of sea-level rise, emphasizing the importance of proactive planning.
Current hazard assessments show tidal flooding in some California coastal cities has already increased by more than 550% since 2000. Using those assessments, the state has recommended planning for between 1 and 6.6 feet of sea-level rise by 2100, with some scenarios projecting as much as 10 feet under the worst-case emissions conditions.
Current assessments also predict that a rise of just 20 inches could put $17.9 billion worth of buildings statewide at risk, and that two-thirds of Southern California’s beaches could vanish entirely by the century’s end.
However, these projections were shaped by the same body of hazard research that this study calls into question, and it is unclear how these estimates will change in light of potential discrepancies.
L.A. Times reporter Rosanna Xia has previously documented how the politics of coastal planning are contentious, with more than 30 municipalities wrestling with how best to approach it. All of this political friction is now in addition to this study’s findings, which suggest cities and residents may have consistently undershot the problem.
Ultimately, what this study’s findings highlight is that planning for a slow-moving, politically contentious issue like sea-level rise, one that doesn’t have the immediacy of a wildfire or an earthquake, is complex and constantly evolving. The question now is whether California’s planning and hazard assessments will catch up.