New Study Reveals Coastal Waters Higher Than Previously Measured, Threatening Millions More

Rising seas may threaten tens of millions more people than scientists and government planners originally estimated due to widespread measurement errors in coastal water heights, according to a new study published Wednesday in the journal Nature.

Researchers analyzed hundreds of scientific studies and hazard assessments, finding that approximately 90% underestimated baseline coastal water heights by an average of 1 foot, according to the study. The problem occurs more frequently in the Global South, the Pacific and Southeast Asia, and less often in Europe and along Atlantic coasts.

The issue stems from a mismatch between how sea and land altitudes are measured, said study co-author Philip Minderhoud, a hydrogeology professor at Wageningen University & Research in the Netherlands. He attributed the discrepancy to a “methodological blind spot” between the different measurement approaches.

Studies that calculate sea level rise impact usually “do not look at the actual measured sea level so they used this zero-meter” 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 reaches close to 3 feet, according to Minderhoud.

The measurement gap occurs because many studies assume sea levels without waves or currents, when the reality at the water’s edge involves oceans constantly affected by wind, tides, currents, changing temperatures and phenomena like El Niño, according to Minderhoud and Seeger.

Using more accurate coastal height baselines means that if seas rise by slightly 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 additional people, the study found.

The findings could 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. Southeast Asia, where the study finds the biggest discrepancy, already has the most people threatened by sea level rise, according to Levermann.

The real-world impact of these measurement discrepancies affects island nations particularly severely, according to Minderhoud. For 17-year-old climate activist Vepaiamele Trief, the projections represent immediate reality rather than abstract numbers. On her island home in the South Pacific archipelago of Vanuatu, the shoreline has visibly retreated during her lifetime, with beaches eroded, coastal trees uprooted and some homes now barely 3 feet from the sea at high tide.

On her grandmother’s island of Ambae, a coastal road from the airport to her village has been rerouted inland because of encroaching water, according to Trief. Graves have been submerged and entire ways of life feel under threat.

“These studies, they aren’t just words on a paper. They aren’t just numbers. They’re people’s actual livelihoods,” she 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.”

The study focuses on measurement accuracy at the critical intersection of water and land. Calculations that may be correct for seas overall or for land aren’t quite right at that key junction point, according to Seeger and Minderhoud, particularly in the Pacific region.

“To understand how much higher a piece of land is than the water, you need to know the land elevation and the water elevation. 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,” according to the study’s findings.

The research highlights the need for more precise baseline measurements to accurately assess coastal flood risks and plan appropriate responses to rising sea levels.