The sea is higher than we thought and millions more are at risk, study finds

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Climate change’s rising seas may threaten tens of millions more people than scientists and government planners originally thought because of mistaken research assumptions on how high coastal waters already are, a new study said.

Researchers studied hundreds of scientific studies and hazard assessments, calculating that about 90% of them underestimated baseline coastal water heights by an average of 1 foot (30 centimeters), according to the study in the journal Nature. It’s a far more frequent problem in the Global South, the Pacific, and Southeast Asia, and less so in Europe and along Atlantic coasts.

The cause is a mismatch between the way sea and land altitudes are measured, said study co-author Philip Minderhoud, a hydrogeology professor at Wageningen University & Research in the Netherlands. And he attributed that to a “methodological blind spot” between the different ways those two things are measured.

Each way measures its own areas properly, he said. But where sea meets land, there are a lot of factors that often don’t get accounted for when satellites and land-based models are used. Studies that calculate sea level rise impact usually “do not look at the actual measured sea level, so they used this zero-meter” figure as a starting point, said lead author Katharina Seeger of the University of Padua in Italy. In some places in the Indo-Pacific, it’s close to 3 feet (1 meter), Minderhoud said.

One simple way to understand that is that many studies assume sea levels without waves or currents, when the reality at the water’s edge is of oceans constantly roiled by wind, tides, currents, changing temperatures, and things like El Niño, said Minderhoud and Seeger.

Adjusting to a more accurate coastal height baseline means that if seas rise by a little more than 3 feet (1 meter)—as some studies suggest will happen by the end of the century—waters could inundate up to 37% more land and threaten 77 million to 132 million more people, the study said.

That would trigger problems in planning and paying for the impacts of a warming world.

This article and video were provided by The Associated Press.

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[Large waves hitting a concrete wall]

Katharina Seeger (interview): “In our study, we looked at the relation between actual measured sea level and what was commonly assumed to represent sea level along the world’s coastlines. So because for having reliable coastal hazard impact assessments, we need to correctly combine the measured land elevation with the sea level.”

[Resident standing by flowing floodwaters]

[Floodwaters]

Katharina Seeger (interview): “So we find that the measured global coastal sea level is an average 20 centimeters to 30 centimeters higher than the commonly assumed one. And this difference is irrespective of the age of the geoid model. And if we then, and we see that the offset is even larger in regions like Southeast Asia and the Indo-Pacific where we observe offsets of up to more than one meter.”

[Eroding coastline]

Katharina Seeger (interview): “If we take this hypothetical one meter relative sea level rise, we figure out that this underestimates global coastal population by up to 12%.”

[Sinking buildings due to sea level rise]

[Sainte Jeanne D’Arc school]

Philip Minderhoud (interview): “If you talk about small island development states in this region, that their actual situation on the ground where they see the sea level and they see that it’s going to impact them, let’s say rather soon, for example, is not reflected in this specific body of scientific literature. So if you look at the studies, they give you basically a future perspective that is not representative for what the people on the ground, on these islands, for example, are already experiencing on their daily lives.”

[Plenary session and U.N. climate talks]

[COP30 president André Corrêa do Lago hitting gavel]

Philip Minderhoud (interview): “Policymakers, governments that are trying to manage vulnerable coastlines around the world, especially in these hotspot areas where we see the biggest discrepancies, for them, our study is basically sort of call to action to make sure that Let’s say the plans that they’re currently having are based on reliable assessments.”

[Major flooding in the downtown area of Alcacer do Sal]

[People walking in flooded area]

Philip Minderhoud (interview): “And this will lead to more accurate insights, more accurate assessments, and therefore more, let’s say, reliable understanding of what’s happening in the Earth around the world.”

[Flooded streets]

[Car inundated by floodwaters]

This script was provided by The Associated Press.

The related lesson for this video will be published next Wednesday.