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, according to Wednesday’s 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 their own areas properly, he said. But where sea meets land, there’s 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, 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 Nino, said Minderhoud and Seeger.

Adjusting to a more accurate coastal height baseline means that if seas rise by a little more than 3 feet — 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.

“You have a lot of people here for whom the risk of extreme flooding is much higher than people thought,” said Anders Levermann, a climate scientist at the Potsdam Institute for Climate Impacts Research in Germany, who wasn’t part of the study. And Southeast Asia, where the study finds the biggest discrepancy, has the most people already threatened by sea level rise, he said.

For 17-year-old climate activist Vepaiamele Trief, the projections aren’t abstract. On her island home in the South Pacific archipelago of Vanuatu, the shoreline has visibly retreated within her short lifetime, with beaches eroded, coastal trees uprooted and some homes now barely 3 feet 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 and entire ways of life feel under threat.