The world average sea level has risen by more than 20 cm in the last hundred years, of which 9 cm in the last 25 years. The rate of increase over the last three decades has been 3.4 mm per year and is expected to increase further. And that is bad news for the Dutch BES islands of Bonaire, Saba and St. Eustatius. A new KNMI report indicates that the water around these islands will probably rise just a little faster than the world average sea level.

Caribbean Netherlands follows the global average increase

Measurements show that sea-level rise in the Caribbean Netherlands in the past was comparable to the world average rise (3.3 mm per year on Bonaire and 2.9 mm per year on St Eustatius and Saba; Figure 1). This is not self-evident, because the sea level can vary greatly regionally. This is largely due to the fact that the large ice sheets of Greenland and Antarctica exert a strong gravitational pull on the ocean water. The melting of an ice sheet reduces this attraction and leads to a redistribution of the water in the oceans.

The sea-level rise caused by the meltwater is therefore not the same everywhere: close to the ice caps, the sea level rise is less, far away more. For example, the sea level rise in the Netherlands is higher than the world average when Antarctic land ice melts, but lower when land ice melts off Greenland. However, the BES islands are exactly so far away from both ice sheets that the redistribution of ocean water has little impact on sea level. It is therefore expected that the sea level at the BES islands will continue to keep pace with the global average rise in the future. Only a substantial change in ocean currents can change this.

Sea level rise around the BES islands until and after 2100

The expected sea-level rise in 2100 compared to the period 1986-2005 is between 30 cm and 120 cm. The sea level at the BES islands will probably rise slightly faster (figure 2). The lower limit is the scenario in which the climate agreement of 1.5 degrees of warming is followed and the sensitivity of the land ice to warming is smaller than in the current models. The upper limit is the scenario in which greenhouse gas emissions continue to increase at the current rate and land ice is more sensitive than in the model simulations. This upper limit is 2 meters in the year 2150.

Beyond 2100, sea levels will continue to rise even if the Paris Agreement is followed. This means that if the temperature rise is limited to between 1.5 and 2 degrees, the sea level rise will eventually still be between 3 and 20 meters. However, this can take hundreds to thousands of years.

More frequent extreme water levels

The fate of the island state of Tuvalu in the Pacific Ocean, which is expected to disappear under water this century, the BES islands are spared for the time being. Nevertheless, a higher average sea level for the Caribbean will more regularly lead to extreme water levels, for example, if a storm or hurricane passes by. The BES islanders may then have to deal with flooding, saltwater ingress and coastal erosion further inland. Some parts then become uninhabitable.

Regional information on sea-level rise helps to be better prepared and to take effective adaptation measures in a timely manner. Computer models that can simulate the ocean vortices in the Caribbean could better estimate the future sea-level around the islands. Also, through more knowledge about the variability of the regional sea level, a warning may be issued earlier for extremely high water on the coast.

Volcano versus coral reef

Not every island has the same amount to fear. Saba and St. Eustatius are volcanic islands and are higher above sea level than Bonaire, which is built on a coral reef (figure 3). In any case, every island suffers from the consequences of sea-level rise. In addition to water safety due to the more frequent occurrence of extreme water levels, erosion of the coast also has a significant impact on tourism. Tourism is highly dependent on the beaches that gradually disappear into the sea. In any case, the next generation of islanders will end up in a completely different landscape.

KNMI climate report