What’s happening with sea level rise?
The KNMI Climate Scenarios show that, by 2100, the sea level in the Netherlands could be more than a metre higher than it is now. In its report from August 2021, the Intergovernmental Panel on Climate Change (IPCC) assumes a maximum global rise of 1 metre by 2100. The IPCC does not rule out a global rise of 2 metres by 2100 and 5 metres by 2150 because of the considerable uncertainties about the effects of climate development on Antarctica, even though the probability of these scenarios actually occurring is low at present. In 2023, the KNMI will publish new scenarios in which these recent insights will be applied to the situation of the Netherlands.
How fast are sea levels rising now?
Global sea level rise has now increased from 2 mm to 4 mm a year. This is because sea level rise varies considerably around the world, in part because of the effects of wind. Studies by the Royal Netherlands Meteorological Institute (KNMI) and Utrecht University show that the land ice in Greenland and Antarctica is melting much faster than previously assumed. This may be causing the possible acceleration in sea level rise.
You can read here about the investigations conducted by Deltares on behalf of the Delta Commissioner and what the main conclusions are:
Why was the study initiated?
Recent insights indicate a possible acceleration in the rate at which the land ice in Antarctica is melting and fracturing. On 30 March 2016, Nature published a paper by De Conto and Pollard that described new scientific insights into this fracturing process, and the accelerated rise in sea level that could result. The KNMI applied the data to the Dutch coast. The Minister of Infrastructure and Water Management (I&W) and the Delta Commissioner launched the Sea Level Rise Knowledge Programme in 2019 because of the potentially major impact of this acceleration in sea level rise. The intermediate review of the Knowledge Programme was published in 2023 and the final results will follow in 2026.
What did Deltares investigate?
The aim of the study was to review the potential consequences of accelerated sea level rise for the preferred strategies in the Delta Programme. Those strategies consist of a combination of measures that can be used as the circumstances change. The study shows how the acceleration in sea level rise can cause tipping points – the points in time at which new or additional measures may be needed – affecting the coastal foundation, flood risk management and freshwater supplies. The study describes when and at what degree or rate of sea level rise a tipping point can occur, and the implications for the implementation of the measures in the preferred strategy.
What are the principal conclusions of the report?
There is still considerable uncertainty as to whether sea level rise will accelerate faster and, if so, to what extent. On the basis of the initial exploratory study by Deltares, the package of measures assumed by the Delta Programme at present (the preferred strategies) would seem to be adequate to keep our delta liveable and habitable until at least 2050. The rate at which sea level rise accelerates depends primarily on the reduction of the levels of carbon emissions worldwide, and how that will affect global warming and the melting of Antarctica. Furthermore, there is still considerable uncertainty about the precise effect of accelerated sea level rise on sand transport along the Dutch coast, the burden on the flood defences, and the salinisation of groundwater and surface water.
The monitoring of Antarctica and further research into long-term processes in the Dutch coastal zone need to be intensified to reduce these uncertainties. An assessment is also required of how large/capital-intensive measures with a long lifespan can be implemented more adaptively in order to respond better to uncertain developments.
The study shows that complying with the Paris Agreement is vitally important to limit the acceleration of sea level rise and the associated impacts on our delta. Global warming of no more than 2 degrees as stipulated by the Paris Agreement could lead to a rise in sea levels in the Netherlands of 1 to 2 metres by 2100.
- If the global temperatures rises by 2 degrees, we will need 20 times more sand in 2100 for nourishment operations to allow the coastal foundation to keep up with sea level rise. (At present, approximately 12 million m3 of sand is applied annually to compensate for erosion and to ensure that the coastal foundation rises in line with the sea level.)
- If sea level rise exceeds 6 mm a year (the current rise is 2 mm a year), the Western Wadden Sea will no longer be able to keep up. The limit for the Eastern Wadden Sea is a rise of 10 mm a year. A rise of 10 mm a year could be seen as early as 2050. Extra sediment will also be needed in time in the Southwest Delta to allow unprotected areas outside the dikes to keep pace with sea level rise.
Flood risk management:
- If sea level rise accelerates, the replacement of large engineering structures such as storm surge barriers and locks will become an issue after 2050 between ten (assuming warming of 2 degrees) and twenty years (assuming warming of 4 degrees) earlier.
- If the sea level rises by 0.65 m, gravity drainage from the IJsselmeer to the Wadden Sea will become virtually impossible because the level of the lake will be below the level of the Wadden Sea, even at low tide. A sea level rise of 1.75 metres will require pumping capacity on the Afsluitdijk barrier dam of between 1000 and a maximum of 3200 m3/s to drain all the IJssel discharge and precipitation surplus, depending on the requirements for flood risk management.
From 2050 onwards, an accelerated sea level rise could lead to a significant increase in salt intrusion through the rivers, forcing inlet points in the Rhine-Meuse estuary area to close more often and for longer, and increasing the need for a more permanent and structural influx from the east. This could also result in an increase in demand for water from the IJsselmeer.
What are the next steps?
The findings of the Deltares study were included in the six-year periodical evaluation of the Delta Decisions and preferred strategies of the Delta Programme. This evaluation, which is coordinated by the Delta Commissioner, involves carefully checking whether new insights and developments mean that changes have to be made to the current Delta Decisions, preferred strategies and Delta Plans. The results of the periodical assessment can be found in the 2022 Delta Programme.
The Sea Level Rise Knowledge Programme was launched in 2019. In the years ahead, we want to establish a clearer picture of the uncertainties relating to sea level rise for the Netherlands. In addition, we want to have a clear understanding of the extent to which current strategies are tenable or require modification: which supplementary measures may be needed? We are also exploring the action perspectives for the distant future (in other words, after 2100). This process is required in order to be as prepared as possible – taking into account the ongoing uncertainties – for a range of sea level scenarios. We can then make the right decisions when they are needed. In this way, we can keep the Netherlands safe and liveable in the future as well. You can read more about the Sea Level Rise Knowledge Programme in this brochure.
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 was primarily triggered by expanding water, the faster melting of land ice (in Antarctica and Greenland), and melting glaciers in the mountains. At the regional level, wind patterns and the gravitational force exerted by large ice masses on water could also play a role.
The gravity discharge of excess water into the sea is increasingly hampered as sea levels rise and land continues to subside, and it becomes necessary to resort to pumping. In addition, the sea is having an increasing impact on the coast: there is salt intrusion in groundwater and surface water as well as increasing coastal erosion and higher storm surge levels. Furthermore, there is a risk that the sandbanks, mud flats, and salt marshes in areas such as the Wadden Sea, Western Scheldt and Eastern Scheldt may be permanently submerged. With sand nourishment operations, we can maintain the position of the coastline for years to come, even as sea levels rise. Other measures may have to be found in the future.
The effect of sea level rise can be further amplified (or mitigated) by land subsidence (or rising land levels) in the adjoining land. Geological movements and the compaction of clay and peat are leading to land subsidence in the north-west of the country at an average rate of 10 cm a century. At some locations, however, the land can subside by up to 2 cm a year.