Rising sea levels are already affecting coastal communities, exacerbating high tide events like this “king tide” in California’s Mill Valley. A new study shows researchers may be underestimating how many people globally will be affected.
Josh Edelson/AFP via Getty Images
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Josh Edelson/AFP via Getty Images
As the climate warms, sea levels are already rising around the planet. Scientific research shows that millions of people live in flood-prone areas, but now, a new study finds that those numbers are vastly underestimated.
According to a study published in the journal, 132 million more people than previously thought may be in the path of rising seas the nature. If sea levels rise three feet from where they were from 1995-2014, that could happen by the middle of the next century, depending on how much humans are able to reduce the burning of fossil fuels that produce heat-trapping emissions.
The difference comes from the initial level of current sea levels. A new study finds that most scientific research uses sea levels that are 10 inches lower than they 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 underestimation is highest in Southeast Asia and the Indo-Pacific region, where sea levels are more than three feet higher today than most research predicts.
Marine scientists say new studies are taking the difference into account. This does not affect communities planning for sea level rise, as they use direct measurements to make more localized studies. Still, it’s important for climate researchers to have an accurate idea of the overall global impact, especially for vulnerable countries, forcing the world to do more in international climate negotiations.
“Island people, who are at very low elevations and are already experiencing the effects of sea level rise, know how high the water is from their own experience standing on the coast,” says Philip Minderhoud, assistant professor at Wageningen University in the Netherlands and co-author of the new study.
Calculation of global oceans
Globally, sea levels have already risen 8 to 9 inches since 1880. As the planet’s temperature warms, glaciers and polar caps melt, releasing freshwater into the ocean. Ocean temperatures are also warming, which causes water to expand and take up more space. The impact is even greater in places where land sinks, such as the US East Coast.
Coastal communities are already seeing the effects during storms and hurricanes, where higher sea levels mean more water is rising inland. Flooding also occurs on sunny days, as high tides are still rising.
Study author Minderhoud was working in Vietnam’s Mekong Delta when he realized the water was much higher than global studies had calculated. He and colleagues looked at 385 studies of sea level rise from 2009 to 2025 and found that 90% of them show that current sea levels are too low.
This “methodological blind spot,” as Minderhoud calls it, comes from the computer models scientists use. To estimate current sea levels, they use a model of the planet’s gravity, also known as a geoid model. It simulates how the oceans meet the Earth, taking into account gravity and Earth’s rotation. But it doesn’t account for other factors that affect how high the sea rises, such as tides, ocean currents and trade winds.
“In a way, the geoid gives you the ocean surface in calm conditions, so without disturbances,” he says.
Using actual measurements of current sea level, Minderhoud and colleagues found that 37% more land area and 68% more people would be affected by about three feet of sea level rise than most current studies show.
More recent sea level studies are accounting for this difference. “There’s a growing awareness in the community that this data is valuable and can improve assessments, so we already see that trend,” says Minderhoud.
These findings help provide a more accurate global picture of the impacts of climate change, which is important for low-lying islands and nations around the world. In international climate talks, they have urged rich countries to cut their emissions, as well as reimburse poor countries for the effects of floods they are already suffering.
Other scientists suggest that this new study will not affect how local communities plan for sea level rise. To assess their vulnerabilities, planners often use more detailed data about land and sea levels in their own regions.
“If you’re really doing adaptation planning, presumably any responsible planner would know where the water is and not use a global-scale screening analysis to find out where the water is,” says Bob Kopp, a professor of earth and planetary sciences at Rutgers University who was not involved in the new study.
Assessing local risk means looking at the dynamics of coastal conditions, including where people live, building and infrastructure, and any defenses communities have built, Kopp says.
Analyzing those factors will ultimately help determine how many people are affected by sea level rise. Because it informs how local governments manage it, whether by building seawalls and upgrading infrastructure, restoring marshes and other natural coastal defenses, or diverting people.
