As the climate heats up, sea levels are already rising around the planet. Scientific research shows that millions of people live in areas facing inundation, but now, a new study finds those numbers have been vastly underestimated.
As many as 132 million more people than previously thought may be in the path of rising seas, according to a study published in the journal Nature. That's if sea levels rise by three feet from where they were from 1995-2014, something that could happen by the middle of the next century, depending on how much humans are able to curtail the burning of fossil fuels that produce heat-trapping emissions.
The discrepancy comes from the starting point of current sea levels. The new study finds that most scientific research uses ocean heights that are about 10 inches lower than they actually are today.
That means the full impact of future sea level rise is underestimated because more land is facing inundation than previous studies have shown. The underestimate is greatest in Southeast Asia and the Indo-Pacific region, where sea levels are more than three feet higher today than most research assumes.
Ocean scientists say newer studies are taking the discrepancy into account. It also doesn't likely affect communities planning for sea level rise, because they use direct measurements to do more localized studies. Still, climate researchers say having an accurate idea of the overall global impact is important, especially for vulnerable countries urging the world to do more in international climate negotiations.
"People from an island that is very lowly elevated and is already feeling the effects of sea level rise, they know from their own experience standing at the coastline how high the water is," says Philip Minderhoud, associate professor at Wageningen University in the Netherlands and co-author of the new study.
Calculating the global oceans
Globally, sea levels have already risen 8 to 9 inches since 1880. As the planet's temperature warms, glaciers and polar caps are melting, releasing freshwater into the ocean. Ocean temperatures are also heating up, which causes the water to expand and take up more space. The effect is even greater in places where the land is sinking, like on the U.S. East Coast.
Coastal communities are already seeing the impacts during storms and hurricanes, where higher sea levels mean more water is surging onto land. Flooding is also happening on sunny days, because high tides are getting even higher.
Study author Minderhoud says he was working in Vietnam's Mekong Delta when he realized the water was much higher than global studies accounted for. He and colleagues looked at 385 sea level rise studies from 2009 to 2025 and found that 90% of them were starting with current sea levels that were too low.
This "methodological blind spot," as Minderhoud calls it, comes from the computer models that scientists use. To estimate current sea levels, they use a gravitational model of the planet, also known as a geoid model. It simulates how the oceans meet the land, taking into account gravity and the Earth's rotation. But it doesn't consider other factors that affect how high the ocean is, like tides, ocean currents and trade winds.
"In a way, a geoid gives you the surface of the ocean in a calm situation, so without disturbances," he says.
Using actual measurements of current sea level, Minderhoud and colleagues found that up to 37% more land area and 68% more people would be affected by the roughly three feet of sea level rise, compared to what most current studies show.
More recent sea level studies are starting to account for this discrepancy. "There is a growing awareness in the community that this data is valuable and will improve the assessments, so we do see that trend already," Minderhoud says.
These findings could help provide a more accurate global picture of climate change impacts, something important to low-lying islands and nations around the world. At international climate talks, they've pressed wealthier nations to cut their emissions, as well as reimburse poorer countries for the flooding impacts they're already experiencing.
Other scientists point out that this new study may not affect how local communities plan for sea level rise. To assess their vulnerabilities, planners typically use more detailed data about land and sea elevations in their own regions.
"If you're actually doing adaptation planning, presumably any responsible planner is going to know where the water actually is, and not just be using a global level screening analysis to figure out where the water is," says Bob Kopp, professor of Earth and planetary sciences at Rutgers University, who was not involved in the new study.
Assessing local risk means looking at where people live, the building and infrastructure and the dynamics of coastal conditions, including any protections that communities have built, Kopp says.
Analyzing those aspects ultimately will help determine how many people are affected by sea level rise. That's because it informs how local governments will handle it, like building sea walls and elevating infrastructure, restoring marshes and other natural coastal protections, or even moving people out of the way.
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