Researchers examined hundreds of scientific studies and hazard assessments, calculating that about 90 per cent of them underestimated baseline coastal water heights by an average of 30 centimetres, according to the new 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.
Studies that calculate sea level rise impact usually do not look at the actual measured sea level and use a "zero-metre" figure as a starting point, said lead author Katharina Seeger of the University of Padua in Italy.
In some places in the Indo-Pacific, the difference is close to one metre, Minderhoud said.
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 one metre - as some studies suggest will happen by the end of the century - waters could inundate up to 37 per cent 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.
"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.
Minderhoud pointed to island nations in that region as an area where the reality of discrepancy hits home.
Calculations that may be correct for the seas overall or for the land aren't quite right at that key intersection point of water and land, Seeger and Minderhoud said.
"To understand how much higher a piece of land is than the water, you need to know the land elevation and the water elevation," said sea level rise expert Ben Strauss, chief executive of Climate Central.
"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," added Strauss, who wasn't part of the research.
