Probability of flooding increasing due to rising sea level

Climate change is causing sea levels to rise. This makes it harder and harder to keep protecting the Netherlands against flooding. If we want to keep living in our low-lying country, we must keep working on flood protection and stay willing to invest in it. On this page, you can read how sea levels are rising, how fast this is happening and what it means for the risk of flooding.

What causes the sea level to rise?

The sea level is rising because Earth is warming. This warming is caused by the large volumes of CO2 being emitted. Via a range of processes, global warming is contributing to the rise in sea level:

• The warmer air warms the seawater. As water warms, it expands. Ergo, warmer seawater takes up more room and thus, the sea level will rise.
• Global warming causes Greenland and Antarctic glaciers and ice sheets to melt. This adds more seawater to the oceans, which causes the sea level to rise. According to the KNMI'23 climate scenarios, the ice sheets are melting at an accelerated pace.
• Global warming causes more changes, most of which have a regional impact, such as changes in ocean currents. Furthermore, melting ice sheets and sea ice reduce the salinity of seawater. The density of seawater is proportional to its salinity; less saline seawater will expand in volume.

What does sea level rise mean for the risk of flooding?

If we take no extra action, the risk of flooding will increase. That is why we must keep a close eye on the sea level and prepare measures in time. Sea level rise leads to higher water levels and higher waves. Dykes and other flood defences are not automatically designed for this. According to the Sea Level Rise Knowledge Programme, the Netherlands can keep protecting itself well for the time being. To do this, we must keep raising and strengthening dykes, add more and more sand along the coast and adapt or replace storm surge barriers in time. We will also need more and more pumps to carry water out to sea. How soon these measures are needed depends on how fast the sea level rises. Land subsidence can also make flood protection more challenging. That is why the Sea Level Rise Knowledge Programme also takes future land subsidence into account.

By how much has the sea level already risen?

According to the KNMI'23 climate scenarios the worldwide sea level has risen since 1900 by an average of 20 cm, i.e., an average rise of 1.7 mm annually. However, towards the end of the 1960s, a considerable acceleration set in all across the globe: between 2006 and 2018, the annual rise worldwide averaged 3.7 mm. Along the Dutch coast too, the sea level is rising faster and faster. Until 1993, the sea level rose by an average of about 1,8 millimetres per year. Since 1993, the average has been about 3,1 millimetres per year. This is shown by the latest measurements from the Deltares Sea Level Monitor.

By how much is the sea level still going to rise this century?

Will we continue to emit large volumes of CO2 in the years ahead? Then up to 2100, the sea level off the Dutch coast may rise by between 59 cm to 124 cm vis-à-vis the beginning of this century. If we reduce our CO2 emissions, the sea level will rise by between 26 cm to 73 cm. More information on this topic can be found in the brochure of the KNMI'23 climate scenarios. In addition to CO2 emissions, other – uncertain – processes may cause the sea level to rise even more. For example, if parts of the Antarctic ice sheet become unstable, and if the ice sheet starts melting at an accelerated pace, this may even cause the sea level to rise by 2.5 metres by 2100.

The Dutch Caribbean Dutch islands of Bonaire, Saba and Sint Eustatius also face sharply rising sea levels. The KNMI'23 climate scenarios show that sea level at Bonaire 2100 will rise between 31 cm and 78 cm under a low greenhouse gas emission scenario and between 55 cm and 127 cm under a high emission scenario. The upper limit could rise to 3.4 metres if the Antarctic ice sheet becomes unstable. Slightly less sea level rise is expected for Saba and Sint Eustatius.

How fast will the sea level rise after 2100?

In the long term, the sea level will continue to rise, even if we achieve a considerable reduction in CO2 emissions and manage to limit global warming to a maximum of 2⁰C, as per the international agreements. This is because the oceans and ice sheets are slow to respond to global warming. Such processes cannot be stopped just like that. The KNMI'23 climate scenarios show the following potential scenarios for 2300:

• If CO2 emissions are significantly reduced, the worldwide sea level will have risen by an average of 0.3 to 3 metres by 2300;
• Under a high emissions scenario, sea level rise around 2300 is 2 to 6 metres;
• If uncertain ice sheet processes in Antarctica are also included, the worldwide sea level rise may increase to 17 metres.

Expectations are that the sea level will not be in balance with the climate until 10,000 years from now, and will continue to rise until such time, even if we cease our greenhouse gas emissions now. If we manage to keep global warming to 2⁰C, the worldwide sea level will have risen by 6 m to 7 m by that time. At 3⁰C global warming, it will have risen by 10 m to 24 m, and at 5⁰C global warming by 28 m to 37 m.