A new study published in the journal Nature has found that global assessments of coastal flood risk may have significantly underestimated the threat from sea-level rise because of incorrect assumptions about the actual height of coastal waters.
The research suggests that seas along many coastlines are effectively higher than previously assumed, meaning tens of millions more people could face flooding risks this century as climate change drives ocean levels upward.
The study, led by Katharina Seeger, researcher at the University of Padua, Italy, and Philip Minderhoud, hydrogeology professor at Wageningen University and Research in the Netherlands, analysed hundreds of scientific studies and coastal hazard assessments worldwide.
According to the study, around 90 per cent of the studies reviewed underestimated baseline coastal water heights by an average of about 30 cm (one foot). The researchers say the discrepancy arises from a mismatch between the way land elevation and sea levels are measured.
The authors describe the issue as a ‘methodological blind spot’ in coastal impact assessments. While land elevation models and sea-level measurements are individually accurate, they are often not aligned properly at the crucial boundary where land meets the ocean.
Many coastal impact studies assume that “zero metres” in land elevation datasets corresponds to the actual level of the sea, the researchers explain. However, this assumption does not always hold true, leading to systematic underestimation of coastal water levels.
In some parts of the Indo-Pacific region, the difference between the assumed and actual coastal water level can reach as much as one metre, the researchers note.
The problem also arises because many models assume simplified ocean conditions without waves, tides, currents or climatic influences such as El Niño. In reality, coastal waters are constantly influenced by these processes, making the effective water level along shorelines higher than the baseline used in many impact assessments.
Correcting this baseline significantly alters projections of coastal risk. The study estimates that if global sea levels rise by just over one metre by the end of the century, flooding could affect up to 37% more land area than previously estimated.
This would expose an additional 77 million to 132 million people globally to coastal inundation and extreme flooding.
The findings are particularly significant for the Global South, including parts of Asia and the Pacific, where the study found the largest discrepancies between assumed and actual coastal water levels.
Researchers say these regions already have large populations living in low-lying coastal zones and therefore face heightened vulnerability to sea-level rise and storm surges.
For India, the findings carry important implications. The country has a coastline of more than 7,500 km and several major cities, including Mumbai, Chennai, Kolkata and Visakhapatnam, located along low-lying coastal plains. Large stretches of India’s eastern coastline, including parts of Tamil Nadu, Andhra Pradesh and Odisha, are frequently exposed to cyclones and storm surges originating in the Bay of Bengal.
River delta systems such as the Ganga-Brahmaputra and Mahanadi deltas are particularly vulnerable because they are both low-lying and densely populated. Even small errors in elevation data can significantly alter projections of flood exposure in such areas.