Accurately monitoring sea levels
Monitoring sea levels is important for ensuring water safety and coastal protection in the Netherlands, both now and in the future. The Deltares Sea Level Monitor shows trends based on measurements. This allows us to understand the past and look ahead to how sea level rise will develop over the next 15 years. Until 1993, the average annual sea level rise was 1.8 mm. After 1993, this rose to 3.1 mm per year.
Based on measurement data, Deltares determines the average sea level along the Dutch coast each year on behalf of Rijkswaterstaat. This is very important for understanding how sea levels are developing and what impact this has on water safety in our country. It helps water managers and policymakers make important decisions, such as reinforcing dykes and determining how much sand needs to be supplied each year to protect the coast.
From 262,000 measurements per year to an average rise
The Sea Level Monitor uses measurement data from six tide stations along the Dutch coast: Delfzijl, Harlingen, Den Helder, IJmuiden, Hoek van Holland and Vlissingen. Every year, we collect approximately 262,000 measurements, which we analyse in detail. We calculate the average value (the sea level value over a year), taking into account major changes caused by wind and tide. We also take into account land subsidence at the tide stations in order to make the data as accurate as possible. We compile the annual figures in a single graph, from around 1900 (when we first had access to sea level data) to the present day. The trends reflected in these data points show the rise in sea level over the years. This can be used to make predictions for the next 15 years.
From 1993 onwards: an annual average rise of 3.1 mm
In addition to publishing the annual sea level height, Deltares publishes a more comprehensive report every four years, showing sea level rise over the years. The reports from 2014 and 2018 establish that sea level has risen by an average of approximately 1.9 mm per year since 1900. This was the best estimate for the following years. The Sea Level Monitor 2022 provided new insights. The measurements from 2018 to 2021 were so much higher than the average of 1.9 mm per year that the straight line could no longer be extended to 2022. Looking back, the trend break occurred around 1993. The sea level data for 2022, 2023 and 2024 confirm this. Since 1993, the average sea level rise has been 3.1 mm per year. A higher trend in recent decades is in line with the global trend of accelerating sea level rise.
It is striking that sea levels have been higher than expected according to the trend in the last two years. These kinds of spikes of a few centimetres occur more often, but there is not always a clear explanation. Sometimes we can suspect a cause, such as the drop in sea level after the major eruption of the Philippine Pinatubo volcano in the early 1990s, which led to a cooling of the Earth. The last two years have been extreme climate years worldwide, including an El Niño event, which also affected sea level. See also the FAQ on sea level rise.
Sea level rise has many causes
Sea level rise has many causes. Firstly, ice caps and glaciers are melting, which leads to more water in the sea. In addition, global warming causes seawater to expand, causing sea levels to rise. Changes in groundwater, snow cover and water reservoirs also play a role. Regional differences in sea level rise are influenced by factors such as ocean currents, wind, atmospheric pressure and changes in the seabed.
