Sea Level Rise May Threaten Millions More Than Previously Thought, Study Finds

A groundbreaking study published Wednesday in the journal Nature reveals that climate change’s rising seas could threaten tens of millions more people than scientists and government planners originally anticipated, due to fundamental errors in how coastal water heights are measured.

Researchers examined hundreds of scientific studies and hazard assessments, discovering that approximately 90% underestimated baseline coastal water heights by an average of one foot (30 centimeters). The problem is particularly pronounced in the Global South, the Pacific, and Southeast Asia, while occurring less frequently along European and Atlantic coastlines.

“It’s essentially a methodological blind spot,” explained Philip Minderhoud, a hydrogeology professor at Wageningen University & Research in the Netherlands and co-author of the study. The issue stems from a mismatch between the measurement systems for sea and land altitudes that creates significant errors where these systems meet.

Lead author Katharina Seeger of the University of Padua in Italy noted that many studies calculating sea level rise impacts “do not look at the actual measured sea level, so they used this zero-meter figure as a starting point.” In reality, baseline sea levels in some Indo-Pacific regions are already close to 3 feet (1 meter) higher than assumed in models.

The discrepancy arises because most studies incorrectly assume that coastal waters are calm, static surfaces when making measurements. In reality, the ocean’s edge is constantly affected by waves, tides, currents, temperature changes, and climate phenomena like El Niño—all factors often unaccounted for in satellite and land-based models.

“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 Ben Strauss, CEO of Climate Central and a sea level rise expert whose 2019 study was one of the few that got it right. “What this paper says is the vast majority of studies have just assumed that zero in your land elevation dataset is the level of the water. When in fact, it’s not.”

The consequences of these measurement errors are potentially devastating. When adjusting for a more accurate coastal height baseline, researchers determined that a sea level rise of just over 3 feet (1 meter)—projected by some studies to occur by the end of this century—could inundate up to 37% more land than previously estimated and threaten an additional 77 million to 132 million people worldwide.

“You have a lot of people here for whom the risk of extreme flooding is much higher than people thought,” warned Anders Levermann, a climate scientist at the Potsdam Institute for Climate Impacts Research in Germany, who wasn’t involved in the study.

For 17-year-old climate activist Vepaiamele Trief from Vanuatu in the South Pacific, these projections represent an existential reality. On her home island, the shoreline has visibly retreated during her lifetime. Beaches have eroded, coastal trees have been uprooted, and some homes now stand barely 3 feet from the sea at high tide.

“These studies, they aren’t just words on a paper. They aren’t just numbers. They’re people’s actual livelihoods,” Trief said. “Put yourself in the shoes of our coastal communities—their lives are going to be completely overturned because of sea level rise and climate change.”

In Vanuatu, the impacts are already evident. Roads have been rerouted inland, gravesites have been submerged, and traditional ways of life are increasingly threatened by rising waters.

Some scientists, however, believe the study may be overstating the implications. Gonéri Le Cozannet of the French geological survey suggested that “the problem is actually well understood, albeit addressed in a way that could probably be improved.” Rutgers University sea level expert Robert Kopp noted that most local planners are aware of their coastal issues and plan accordingly.

Minderhoud acknowledged that in high-impact areas like Vietnam, local authorities do have an accurate understanding of elevation levels. However, the widespread nature of the measurement discrepancies raises concerns about global preparedness for sea level rise.

This study emerges as a new UNESCO report warns of major gaps in understanding how much carbon the ocean absorbs, with models differing by 10% to 20% in estimating the size of that carbon sink. Together, these findings suggest that governments may be planning for coastal and climate risks with an incomplete picture of how oceans are changing.

“When the ocean comes closer, it takes away more than just the land we used to enjoy,” said Thompson Natuoivi, a climate advocate for Save the Children Vanuatu. “Sea level rise is not just changing our coastline, it’s changing our lives. We are not talking about the future—we’re talking about the right now.”