A study published Wednesday in the journal Nature found that roughly 90% of scientific studies and hazard assessments have underestimated baseline coastal water heights by an average of 1 foot (30 centimeters), raising the prospect that climate change’s rising seas may threaten tens of millions more people than scientists and government planners previously calculated. The error stems from a mismatch between the ways sea and land altitudes are measured — a gap researchers called a “methodological blind spot.”
If seas rise by just over 3 feet (1 meter) by century’s end — a threshold some projections support — the study estimates that waters could inundate up to 37% more land and threaten between 77 million and 132 million more people than earlier assessments indicated, with the greatest discrepancies concentrated in the Global South, the Pacific, and Southeast Asia.
What the studies got wrong
The problem, according to study co-author Philip Minderhoud, a hydrogeology professor at Wageningen University & Research in the Netherlands, is that hazard models measuring sea level rise impact typically measure sea and land elevations in separate coordinate systems that each work correctly on their own — but diverge at the shoreline where the two meet.
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. The practical effect, Minderhoud said, is that many studies assume sea levels without accounting for the constant influence of waves, tides, currents, changing temperatures, and phenomena such as El Niño that routinely push water higher at the coast.
In parts of the Indo-Pacific, that gap between assumed and actual baseline coastal water heights can be close to 3 feet (1 meter), Minderhoud said.
Ben Strauss, CEO of Climate Central, who was not part of the research, said the error is fundamental. “To understand how much higher a piece of land is than the water, you need to know the land elevation and the water elevation. And what this paper says the vast majority of studies have done is to just assume that zero in your land elevation dataset is the level of the water. When in fact, it’s not,” he said. Strauss’s own 2019 study was among the few the new paper identified as having gotten the baseline right.
“It’s just the baseline that you start from that people are getting wrong,” he said.
Where the risk is greatest
The error is far more pronounced in the Global South, the Pacific, and Southeast Asia than in Europe or along Atlantic coasts, according to the study. Anders Levermann, a climate scientist at the Potsdam Institute for Climate Impacts Research in Germany who was not part of the research, said the implications for those regions are significant.
“You have a lot of people here for whom the risk of extreme flooding is much higher than people thought,” Levermann said. Southeast Asia, he said, already has the most people threatened by sea level rise — and the study finds the biggest discrepancies there.
On her island in the South Pacific archipelago of Vanuatu, 17-year-old climate activist Vepaiamele Trief said the shoreline has visibly retreated within her lifetime. Beaches have eroded, coastal trees have been uprooted, and some homes now stand barely 3 feet (about 1 meter) from the sea at high tide. On her grandmother’s island of Ambae, a coastal road from the airport to her village has been rerouted inland because of encroaching water. Graves have been submerged.
“These studies, they aren’t just words on a paper. They aren’t just numbers. They’re people’s actual livelihoods,” Trief said. “Put yourself in the shoes of our coastal communities — their lives are going to be completely overturned because of sea level rise and climate change.”
Thompson Natuoivi, a climate advocate for Save the Children Vanuatu, said the consequences are already unfolding. “Sea level rise is not just changing our coastline, it’s changing our lives. We are not talking about the future — we’re talking about the right now.”
Some scientists see less cause for alarm
Not all outside researchers agreed that the study’s implications are as sweeping as the authors suggested.
“I think they’re exaggerating the implications for impact studies a bit — the problem is actually well understood, albeit addressed in a way that could probably be improved,” said Gonéri Le Cozannet, a scientist at the French geological survey.
Robert Kopp, a sea level expert at Rutgers University, said most local planners already know their coastal conditions and plan accordingly. Minderhoud acknowledged that Vietnam, one of the highest-impact areas in the study, has an accurate sense of its own elevation.
A concurrent gap in ocean carbon data
The findings appeared alongside a new UNESCO report warning of major gaps in scientific understanding of how much carbon the ocean absorbs. That report said models differ by 10% to 20% in estimating the size of the ocean’s carbon sink, raising questions about the accuracy of global climate projections that depend on those estimates.
Together, the two studies suggest governments may be planning for coastal and climate risks with an incomplete picture of how the ocean is changing.