New research shows: unprecedented flood risk can be partly prevented

From once every century to once every year. New projections worked out by an international team that includes a TU Delft researcher, show that extreme sea level events will happen annually by the end of the century if no measures are taken to slow down global warming.

Extreme sea level (ESL) occurs when high tides are combined with storm surges and high wind waves. New research shows that by the end of this century, the frequency of ESLs that now happen once in 100 years will increase to once a year on most coastlines around the world.

These predictions were worked out by an international team of researchers from Italy, Greece, the UK and the Netherlands, and were recently published in the journal Nature Communications.

The reason for the increment of ESL is global warming and the progressive rise in mean sea level, most of which is driven by the thermal expansion of the ocean and ice melt from glaciers and ice-sheets in Greenland and Antarctica. 

‘What matters for the design of dykes is the water level during storms’

It is the first time that projections at a large scale consider all the factors that affect high water levels. These include mean sea level, tides, wind waves and storm surges. “We tried to predict water levels during storms because this is what actually matters in the design of dykes,” explains TU Delft researcher and co-author of the paper, Martin Verlaan. 

If no measures are taken to slow down global warming, projections show that in ESL conditions, the sea level will increase by between 58 cm and 172 cm, leaving most coastlines at unprecedented flood risk. Demographic estimates state that by 2100, coastal areas up to five metres above sea level will be home to 500 million people. To protect this population, coastal defences will have to be raised by at least 50 cm on average, and up to two metres locally. 

‘If we reduce CO2 emissions, ESL will be lower than if we continue in the existing path’

However, predictions also state that if CO2 emissions are mitigated, the impact of ESL will be significantly reduced. In this scenario, the sea level will rise between 34 cm and 76 cm during ESL. “The two scenarios illustrate that if we are able to reduce CO2 emissions, the impact of ESL will be significantly lower than if we continue along the current path,” clarifies Verlaan.

Sea level rise is not equal around the world, and the variations are due to at least three elements. First, the place where the ice is melting. “Counter intuitively, in Europe we are more affected by ice melting in Antarctica than in the Arctic. When the ice disappears, the gravity pull there decreases and the sea level drops, which means that on the other side of the world it rises.” The second element is the magnitude of tides. In a place where tidal amplitude is already 20 metres, a one metre sea level rise will have a relatively small effect. Finally, the effect of storms is modest on deep water compared to that on shallow seas. 

The Netherlands is a special case, according to Verlaan. On the one hand, tides are ‘decently large’, the coast is shallow and there is a big low lying area. But on the other hand, the country is well protected with sea defences that are already high, although they will have to be adapted to cope with changes in future weather. 

The team is now working on improving the models used to calculate the projections. It will include additional aspects such as floating ice, which if melts should not contribute to rise the sea level, but which could change the dynamics of tides and storms. 

Maria Rubal / science editor

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