A new peer reviewed study suggests that sea level rise risk has been systematically underestimated in much of the research used to evaluate coastal flooding and land loss. The paper, published March 4 in Nature, concludes that many coastal impact studies start from an incorrect baseline because the reference sea level they use is lower than actual coastal sea levels in many parts of the world.
That matters because sea level projections are often used to estimate when specific coastlines could be overtopped, how far storm surge could travel inland, and how many people and assets might be exposed. If the starting point is wrong, the impacts of future rise arrive sooner and hit harder than models suggest.
What The Researchers Found
The study reviewed the research landscape and found that more than 90% of existing sea level studies rely on a reference sea level that does not match measured coastal conditions. Lead author Katharina Seeger said the results make it necessary to re-evaluate and update the methodology underpinning many coastal hazard assessments.
The core issue is that a large share of studies either lack direct measurements of coastal sea level or combine ocean level and land elevation data in ways that can miss real world coastal dynamics. The researchers describe this as a gap between sea level science and land based hazard analysis.
Why Baselines Matter For Flood Risk
Global sea level rise is driven mainly by ocean warming and expansion and by melting land ice, including mountain glaciers and large ice sheets. But sea level does not rise evenly across the planet. Local conditions such as currents, tides, winds, temperature patterns and weather cycles can make coastal water levels higher or lower than a simple global average would imply.
The study argues that many assessments effectively treat sea level as a smoother surface than it is at the shoreline. In reality, the coast is shaped by constant motion from tides and currents and by variability tied to climate patterns, which can shift the practical water level that coastal communities experience.
How Much More Exposure Could There Be
Using the updated approach, the authors estimate that a hypothetical 1 meter rise, a little more than 3 feet, could place up to 37% more land below sea level than previously assumed. That would translate to an additional tens of millions of people exposed globally, with the study citing a range of 77 million to 132 million people potentially affected under that scenario.
The research does not change how much sea level might rise in the future. Instead, it suggests that the consequences of any given amount of rise may be larger and could arrive sooner in many places because the coastline starts from a higher effective water level than many models assume.
Regions Flagged As Higher Risk
The underestimation problem appears especially pronounced in parts of the global south. The study highlights Southeast Asia and the Pacific as regions where the gap between assumed and measured coastal sea levels is particularly notable. It also reports underestimates in Latin America, the west coast of North America, the Caribbean, Africa, the Middle East and the Indo-Pacific.
That geographic pattern is important because many of these regions already face significant exposure to storm surge, cyclones and rapid urbanization along low lying coasts. A higher baseline increases the likelihood that regular high tides and seasonal variability push water into areas previously thought to be safe until later in the century.
How Experts Are Reacting
External researchers have described the findings as meaningful. Climate scientist Michael Mann, who was not involved in the study, said the distinction the authors draw appears to have been overlooked widely and could mean that the community has been underestimating impacts in key regions. He also noted there remains uncertainty tied to changes in ocean circulation and other dynamics, but said the study looks significant.
The authors say they hope their method becomes a new standard for more accurate assessments of future coastal impacts, particularly as governments, insurers and planners rely on these models to guide infrastructure investment and adaptation decisions.