What is the situation regarding the rising sea level?

According to the climate scenarios worked out by the Royal Netherlands Meteorological Institute KNMI in 2014, the sea level rise may increase to 100 cm by 2100. Up until now, the Intergovernmental Panel on Climate Change (IPCC) has been reckoning with a sea level rise of 80 cm by 2100. However, recent studies (conducted by KNMI and Utrecht University, among other bodies) indicate that the processes affecting the melting of the ice caps in Greenland and Antarctica are developing at a much faster rate than was previously assumed. According to the Delta Programme Signal Group, this is why we cannot preclude an even more rapid rise in sea level.

In the Delta Programme, we anticipate future climate change; we focus on 2050, looking ahead to 2100, and base our plans on the KNMI climate scenarios. Adaptive strategies and flexible measures enable us to factor in new knowledge and insights. In anticipation of the first interim evaluation of the Delta Programme in 2020, in September 2018 the Delta Programme mapped out the first insights into the potential impact of a more rapidly rising sea level for the Netherlands.

The issues Deltares studied, under the authority of the Delta Programme Commissioner, and the main conclusions of the reports are outlined below.

What was the reason behind the study?

Recent insights pointed to a possible acceleration in the rate at which land and sea ice in Antarctica is melting and fracturing. On 30 March 2016, the Nature journal published an article by De Conto and Pollard that contained new scientific insights into the fracturing process, and the accelerated rise in sea level that may result. KNMI translated the data into projections for the Dutch coast. Due to the potentially high impact predicted, in anticipation of a scientifically supported decision (in 2019) from the IPCC in this regard, new KNMI scenarios in 2021 and the first interim evaluation of the Delta Programme in 2020, the Delta Programme Commissioner commissioned a first, exploratory study to provide an overview of the potential consequences for the Netherlands of an accelerated rise in sea level.

What did Deltares investigate?

The aim of the study was to explore the potential consequences of an accelerated rise in sea level in respect of the Preferential Strategies of the Delta Programme. These Preferential Strategies consist of a set of measures that can be implemented as conditions change over time. In the study, Deltares set down how an accelerated rise in sea level could cause intervention points to arise regarding the coastal base, flood risk management and the freshwater supply. An intervention point is the time at which new or supplementary measures might be considered. The study also describes when and to what degree or at what rate of sea-level rise an intervention point may arise, and what effect this might have on the implementation of the measures set out in the Preferential Strategies.

What are the main conclusions of the study?

There is still a lot of uncertainty as to whether the rise in sea level will accelerate, and if so, to what degree. KNMI has stated that any acceleration in sea-level rise will only be seen in 2050, at the earliest. Based on Deltares’ first exploratory study, the set of measures currently set down in the Delta Programme (the Preferential Strategies) should be sufficient to keep our delta liveable and habitable up until at least 2050. The degree to which sea-level rise accelerates after that time is largely dependent on the worldwide reduction of CO2 emissions, and the impact such reduction will have on global warming and the melting of Antarctica’s ice sheets. In addition, there is a significant lack of certainty regarding exactly how an accelerated sea-level rise will affect sand transport along the Dutch coast, the pressure on storm surge barriers, and the salinisation of groundwater and surface water.

Monitoring in Antarctica and further research into the long-term processes that affect the Dutch coastal zone need to be intensified to reduce these uncertainties. Moreover, we should examine how large-scale/capital-intensive measures with a long life cycle can be implemented in a more adaptive manner in order to respond more effectively to uncertain developments.

The study showed that compliance with the Paris Agreement is of great importance to limiting an acceleration in sea-level rise and containing the related consequences for our delta. If the rise in global temperature is limited to no more than 2 degrees Celsius, the target set down in the Paris Agreement, the sea level in the Netherlands could rise by 1 to 2 metres by 2100.

Coastal base:

  • If the global temperatures rises by 2 degrees Celsius, in 2100 we will need 20 times more replenishment sand to allow the coastal base to keep pace with the rise in sea level. (Currently, approximately 12 million cubic metres of sand is needed to compensate for erosion and to allow the coastal base to grow accordingly.)
  • If a sea-level rise of 6 mm a year occurs (the current rise is 2 mm a year), the Western Wadden Sea will not be able to cope with the increase. For the Eastern Wadden Sea, the limit is a rise of 10 mm a year. A rise of 10 mm a year could take place in as early as 2050. Also, in the southwestern delta additional sediment will be needed in the long term to allow areas outside the dykes to keep pace with the rise in sea level.

Flood risk management:

  • If the sea-level rise accelerates, it may mean that from 2050 large hydraulic structures such as storm surge barriers and floodgates will have to be replaced 10 to 20 years earlier (10 years earlier at +2 degrees and 20 years earlier at +4 degrees.)
  • If the sea level rises by 0.65 metres, gravity drainage from the IJsselmeer into the Wadden Sea will be virtually impossible because the water level of the lake will be below the level of the Wadden Sea, even at low tide. If the sea level rises by 1.75 metres, depending on the prevailing flood safety requirements, the pumping capacity at the Afsluitdijk will have to be between 1000 and 3200 cubic metres a second to cope with excess rainfall and all the water discharged by the IJssel.

Freshwater supply:

  • From 2050, an accelerated sea-level rise could result in a significant increase in salt intrusion via rivers so that inflow points in the lower reaches areas will have to be closed more frequently and for longer periods, and the need for a more permanent and structural eastern supply will become more urgent. This may also result in increased demand for fresh water from the IJsselmeer.

What are the next steps?

The findings of this study will serve as input for the six-year review of the Delta Decisions and Preferential Strategies of the Delta Programme. This review, coordinated by the Delta Programme Commissioner, focuses on carefully checking if new insights and developments mean that the current Delta Decisions, Preferential Strategies, and Delta Plans should be amended. This review will be completed in 2020 and will be included in the Delta Programme 2021.

In response to the Delta Programme 2019 and the Deltares study into the possible consequences of an accelerated sea-level rise for the Delta Programme, at the State Opening of Parliament, the cabinet announced that it will support setting up a new research programme into the rise in sea level, aimed at reducing uncertainties and investigating the causes of sea-level rise and the impact it will have in the Netherlands. The programme will include focus areas put forward by the Delta Programme Commissioner: the relationship between global warming, the melting of ice sheets in Antarctica, and sea-level rise, and the potential consequences for the Netherlands.

Background

Sea level rise is caused by a combination of ocean warming (warmer water expands and takes up more room) and melting land ice. The accelerated rise over the past few decades has mainly been triggered by expanding water, the more rapidly melting land ice (on Antarctica and Greenland), and melting mountain glaciers. (It is not affected by the melting North Pole ice shelf: being sea ice, this does not add water.) At the regional level, wind patterns and the gravitational force of large ice masses on water (“gravitation effect”) could also play a role.

Gravity discharge of excess water into the sea is increasingly hampered by the rising sea level and continued soil subsidence, necessitating the use of pumps. In addition, the sea is encroaching more and more on the coast, through salt intrusion in ground and surface water, but also through increasing coastal erosion and rising storm surge levels. Furthermore, the sand shoals, mud flats, and salt marshes in, e.g., the Wadden Sea, Westerschelde, and Oosterschelde are at risk of “drowning” because they cannot keep up with the rising sea level. Sand replenishment will enable us to keep our coastline in place for years on end, even if the sea level continues to rise. In the future, we may have to look for other measures.

The effect of sea level rise may even be enhanced (or mitigated) by the subsidence (or rise) of the adjoining land. Geological movements and compaction of clay and peat cause the Dutch soil in the North-western part of the country to subside by an average of 10 cm per century. At some locations, however, the soil may subside by up to 2 cm per annum.