Tens of millions more people are under threat from rising sea levels than was previously estimated, according to a study published March 4 in the journal Nature.

According to AP News, the alarming conclusion is that scientists and government planners have relied on mistaken research assumptions on how high coastal waters already are, according to researchers who analyzed hundreds of scientific studies and hazard assessments for the study. They calculating that about 90% of those reports underestimated baseline coastal water heights by an average of 1 foot. 

The problem is far more frequent in the Global South, the Pacific and Southeast Asia, and less so in Europe and along Atlantic coasts, the study found.

AP says the cause is a mismatch between the way sea and land altitudes are measured, according to 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.

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.