How much higher are sea levels than assumed?

Coastal mean sea levels exceed common assumptions

Multiple recent analyses show that many coastal risk assessments have been working with sea-level estimates that are too low. Meta-analyses and a major peer-reviewed paper found that commonly used methods and datasets have, on average, underestimated current coastal mean sea level by several decimetres. One synthesis reported average underestimates in the range of roughly 20 to 30 centimetres; other work suggests that methodological gaps could have missed up to a century of change in some regions.

These underestimates matter because they directly affect flood maps, infrastructure planning, and the tally of people and assets at risk. When baseline sea level is higher than assumed, even modest increments of future rise translate into earlier and more frequent coastal flooding, higher storm surges, and greater land loss than projected under the older assumptions.

Key takeaways

• The mismatch stems from widely used calculation methods and data gaps that fail to capture long-term and recent accelerations in sea-level change.
• Tens of millions more people worldwide may be exposed to coastal hazards sooner than risk assessments predicted.
• Urgent updates to coastal planning, building codes, and flood insurance models will be needed to reflect the revised baseline.

Actionable responses include recalibrating local and national sea-level datasets, incorporating the revised baselines into hazard maps, and accelerating adaptation measures such as managed retreat, strengthened levees, and nature-based defenses. Policymakers and planners should treat the revised estimates as a prompt to reassess vulnerability and accelerate protective actions—because the tide lines used for decision-making are now understood to be higher than previously thought.