Sea levels are higher than thought and millions more may be at risk

A study in Nature found about 90% of reviewed assessments underestimated coastal water baselines by roughly 1 foot on average, which could increase land inundation and place tens of millions more people at risk if seas rise a little more than 3 feet.

Researchers published a study in Nature reporting that many prior assessments have underestimated how high coastal waters already are. They reviewed hundreds of scientific studies and hazard assessments and found about 90% underestimated baseline coastal water heights by an average of about 1 foot. The authors say the discrepancy arises from a mismatch in how sea and land elevations are measured where water meets land. The study notes the problem is more common in the Global South, especially the Indo-Pacific and Southeast Asia, and less frequent in parts of Europe and Atlantic coasts. Key findings: - About 90% of reviewed studies and assessments used baseline values that, on average, underestimated coastal water heights by roughly 1 foot. - The core issue is a methodological mismatch between sea-level measurements and land elevation datasets, often assuming a "zero" land elevation equals still-water level at the shoreline. - Using corrected baselines, the authors estimate that if seas rise a little more than 3 feet, inundation could affect up to 37% more land and place about 77 million to 132 million additional people at increased risk. - The largest discrepancies were reported in the Indo-Pacific region, including Southeast Asia and small island nations; discrepancies were smaller along many European and Atlantic coasts. Summary: The study indicates that commonly used baselines may understate current coastal water levels, which would raise estimates of future exposure to sea level rise in many regions. The findings imply more land and more people could face increased flood exposure if higher sea levels materialize. Undetermined at this time.