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Sea levels around the world's coasts are higher than modelling often assumes, potentially putting millions more people at risk from rising oceans, scientists have said.
Researchers have warned of a "blind spot" in many studies into the impacts of rising seas and other hazards for people living on coastlines around the world, which have frequently underestimated current sea levels.
As a result, more land and people could be at risk from climate change-driven rising seas, and for impacts to hit sooner than previously thought, the scientists from Wageningen University and Research in the Netherlands warn.
In a study published in the journal Nature, they said that 90% of coastal hazard studies use land elevation measurements and then rely on "geoid" models that make estimates for sea levels based on gravity and the rotation of the Earth.
But actual sea height is also determined by ocean currents and large-scale circulation, winds, tides, seawater temperature and water saltiness, Dr Philip Minderhoud, one of the study authors said.
"Our results show how important it is to account for this difference by using actual sea-level measurements instead of the widely-used assumption that geoid height equals present mean sea level", he said.
The study calculated the difference between coastal land elevation and sea level around the world and compared it to 385 recently published studies.
The findings suggest most research has underestimated coastal sea-level height across the world by an average of 30cm (1ft), while in some areas of the world such as south-east Asia and the Indo Pacific sea levels are around one metre (3ft) higher on average than previously thought.
The findings have significant implications for coping with one of the major impacts of climate change – rising sea levels as the oceans warm and ice melts.
The most recent UN science review projects rises of between 0.28m and 1.01m by the end of this century, compared to 1995-2014 levels.
Study co-author Katharina Seeger said: "Our calculations show that measured coastal sea levels in many places on Earth are higher than is often assumed in coastal impact studies."
And she warned: "Studies that do not properly account for actual sea level, underestimate the amount and exposure of coastal area and population around the world."
And while satellites can measure both land elevation and, very accurately, sea level, the use of different satellites for each set of measurements, with often different baselines, makes it hard to combine the data, the researchers warn.
Calculations carried out by the pair suggest 37% more land than previously estimated will fall below sea level in the event of 1m of relative sea-level rise, with the number of people hit rising from up to 50 million to up to 132 million.
Dr Minderhoud added: "Simply put, if sea level in reality is higher for your particular island or coastal city than was previously assumed, the impacts from sea-level rise will happen sooner than projected before."
The first "light bulb" moment that led to the findings occurred in Vietnam's Mekong Delta a decade ago, Dr Minderhoud said, where international impact assessments assumed the land would become inundated with sea level rises of 1.5m to 2m. But he could see the surface water level, directly connected to sea levels, was already in many places within several tens of centimetres of the land surface.
Further research unveiled the same discrepancy for Myanmar's Ayeyarwady Delta, and then in other areas.
The researchers said there was a need to re-evaluate and update the methodology for existing coastal hazard studies, and urged governments to assess whether the issue affects their strategies for coastal adaptation and protection against rising seas.
Commenting on the research, Dr Matt Palmer from the Met Office Hadley Centre and University of Bristol said it was a "very important paper" whose findings mean that the impact of sea-level rise under climate change have been "systematically underestimated".
"Put another way, we could see devastating impacts from coastal flooding earlier than expected from climate projections – particularly in the Global South."
Dr Alex Arnall, at the University of Reading, added: "For poorer coastal communities, higher exposure doesn't just mean more land below sea level.
"It means greater strain on homes, incomes and local economies, and fewer options for responding to the changes ahead."
And the University of Bristol's Prof Daniela Schmidt warned that sea level rises will also alter coastlines, result in erosion and destroy wetlands, mudflats, mangroves and corals that protect the coasts, provide for wildlife and store carbon.
