Sea levels along coasts around the world are much higher than assumed because of errors in the way they have been calculated, according to a study by Wageningen University and published in scientific magazine Nature.

In some places, the real sea level is 30 centimeters higher than assumed and in others, including coastal zones in Southeast Asia, the discrepancy can be as big as a metre and a half, the researchers say.

This, they argue, means that tens of millions more people are living in flood-prone areas and that countries may be strapped for time to strengthen coastal defences as sea levels continue to rise.

The mistake, which the researchers found in 99% of the impact assessment studies they evaluated, lies in the incorrect integration of coastal sea level height and land elevation, resulting in the “misjudgment of sea level relative to coastal elevation”.

The majority of hazard assessments are “based on geoid models rather than actual sea level measurements”, they say, which has led to the potentially disastrous miscalculations.

Researcher Philip Minderhoud spotted the problem when on a field trip to the Mekong Delta in Vietnam some 10 years ago. The sea level was much higher than indicated on the coastal maps, and Minderhoud noticed the same was true in the coastal areas of other countries in Southeast Asia.

Minderhoud, who is an expert in land elevation and rising sea levels, eventually pinpointed the mistake.

He and fellow researcher Katharina Seeger evaluated some 400 recent studies and concluded there was just one that had got it right.

Several of these studies have been used in the reports by the UN climate panel IPCC and are used by countries to determine climate policy.

In some cases, the sea level is lower than assumed but that does not include the Dutch coast, which has not been affected by the report. Dutch sea level rise is based on local measurements.