Correcting Coastal Baselines Could Put 77–132 Million More People at Risk From Sea Level Rise

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A sweeping review published in Nature this week warns that tens of millions more people may be at risk from sea level rise than previous studies have estimated because most assessments start from the wrong coastal height. Researchers found that about 90 percent of reviewed hazard studies and assessments underestimated baseline coastal water heights by an average of about 1 foot (30 centimetres), a discrepancy that — if not corrected — could mean substantially more land and people are exposed as seas rise.

The study’s lead author, Katharina Seeger of the University of Padua in Italy, and co-author Philip Minderhoud, a hydrogeology professor at Wageningen University & Research in the Netherlands, say the problem stems from a “methodological blind spot” at the interface where sea meets land. Satellite and land-elevation datasets each work well in their domains, they say, but when combined many assessments simply assume a zero-meter land elevation equals mean sea level. In reality the shoreline is influenced by waves, tides, currents, temperature-driven water expansion and climate patterns such as El Niño — factors not captured when a static zero baseline is applied.

Using corrected baselines, the authors calculate that if sea levels rise by a little more than 3 feet (about 1 metre) — a scenario some studies project by the end of this century — inundation could cover up to 37 percent more land than previously estimated and put an additional 77 million to 132 million people at risk. Minderhoud told reporters the discrepancy is most pronounced in the Global South, especially across the Indo-Pacific, Southeast Asia and many Pacific island coasts, while it is less common along European and Atlantic shorelines.

The study’s findings carry immediate implications for coastal planning, insurance, disaster response and international adaptation finance. “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 was not part of the study. The expanded exposure estimates could increase the scale and cost of relocation, sea defences and other adaptation measures, and reshape priorities for scarce funding in vulnerable countries.

Pacific island voices underscore the human stakes behind the numbers. Seventeen-year-old climate activist Vepaiamele Trief, from Vanuatu, described visible shoreline retreat on her home islands: eroded beaches, uprooted coastal trees, homes that are only about 3 feet from the sea at high tide, and a rerouted road on Ambae after encroaching water submerged the original route. “These studies, they aren’t just words on a paper. They aren’t just numbers. They’re people’s actual livelihoods,” Trief said, emphasising that adjusted assessments reflect lived realities in the region.

Some scientists welcomed the correction while urging nuance. Ben Strauss, CEO of Climate Central, noted his 2019 study was among the few to account for baseline elevation discrepancies and agreed the baseline assumption is often wrong. But other outside experts cautioned that the new paper may overstate broader implications for impact studies; they say the measurement issue is known within some technical circles, although approaches differ and could be improved. The Nature study does not negate prior regional work but highlights widespread inconsistency and the need for harmonised methods.

As policymakers and planners in Fiji and across the Pacific prepare climate adaptation strategies, the study represents the latest development in understanding coastal risk: an argument that more precise, locally calibrated measures of coastal water and land elevations are essential to correctly estimate exposure. Correcting the baseline could change which communities are prioritised for protection, relocation or other interventions as sea levels continue to rise.