What is the situation regarding the rising sea level?
Approx. 10,000 to 20,000 years ago, the last ice age ended, the temperature rose, and the large ice caps of North America, Europe and Antarctica started to melt. As a result, the sea level rose rapidly, by approx. 120 m, to more or less its current level some 7000 years ago. The temperature then remained more stable and the rise in sea level declined. Since 1900, the sea level along the Dutch coast has risen at an average rate of 18 cm per century. Since the 1990s, the rise in sea level has started to increase slightly again around the globe. The average sea level along the Dutch coast differs greatly from year to year, due to temperature fluctuations, tidal movements, and wind climate. As a result, the recent (global) acceleration from 2 to 3 mm per annum is not clearly manifest here.
A rise in sea level is caused by a combination of ocean warming (warmer water expands and takes up more space) and melting ice caps. The accelerated rise over recent decades can mainly be attributed to water expansion, the accelerated melting of ice caps (in Antarctica and Greenland), and the melting of mountain glaciers. (The melting North Pole ice sheet does not affect the sea level: as this involves sea ice, it does not add water). At the regional level, wind patterns and the force of attraction between large ice masses and water (gravitation effect) may also play a part.
The rising sea level and continued land subsidence hamper the gravity-driven drainage of excess water into the sea; consequently, pumps need to be used. In addition, the sea is increasingly affecting the coastal strip, not only through salt intrusion in the soil and surface water, but also by increasing coastal erosion and rising storm surge levels. The shoals, mud flats and salt marshes of, for example, the Wadden Sea, the Westerschelde and the Oosterschelde are at risk of “drowning”, because they cannot keep pace with the rise in sea level.
Subsidence (or elevation) of the adjoining land may further add to (or weaken) the effect of a rise in sea level. Geological movements and compaction of clay and peat cause the soil in the north western part of the Netherlands to subside by an average of 10 cm per century, whereas the disappearing ice caps of the past have caused Scandinavia to elevate.
The Intergovernmental Panel on Climate Change (IPCC) expects global warming to result in an accelerated rise in sea level. According to the Royal Netherlands Meteorological Institute KNMI (climate scenarios for the Netherlands, 2014), by 2050 the Dutch sea level may be up by 15 to 40 cm compared to the period 1981-2010. By 2100, the difference may have increased to 100 cm. The agreements set down in Paris in December 2015, aimed at reducing global warming, have little bearing on this trend, as ocean water is very slow to respond to changes in air temperature. On the other hand, the processes affecting the melting of the ice caps in Greenland and Antarctica appear to be developing at a much faster rate than was previously assumed. Consequently, we cannot preclude an even more rapid rise in sea level. Geological observations have shown a rise of 1.5 m per century 100,000 years ago.
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, viz. a maximum rise in sea level of 100 cm by 2100. Adaptive strategies and flexible measures enable us to factor in new knowledge and insights, for example, with respect to accelerated sea level rise. Sand replenishments keep the coastline in place and preserve shoals. Once every six years we assess whether we need to adjust our course and/or the implementation of measures. For example, we can increase the frequency or volume of the sand replenishments if the rising sea level so requires.