Sea level: Dutch coast and worldwide, 1890-2014

Sea levels along the Dutch coast show a steady, linear increase over the past 125 years

The Dutch Directorate for Public Works and Water management (Rijkswaterstaat) coordinates measurements of sea level along the Dutch North sea coast. There are six stations with unique lengths. They all start before the year 1890: Delfzijl, Harlingen, Den Helder, IJmuiden, Hoek van Holland and Vlissingen. The oldest registrations are for Vlissingen and date from 1862. However, measurements become more reliable from 1890 onwards. From that year all data were gathered in an automated way. Moreover, all measurements were fixed relative to the same point, the NAP. We have averaged the six stations and estimated a trend over the period 1890-2014. Trend model estimates with uncertainties come from the Integrated Random Walk (IRW) model (Visser et al., 2015). See figure.

The trend in the data appears to show a regular pattern for he complete series 1890-2014. In jargon this pattern is denoted by the term 'linear'. The annual increment accounts for
1,86 ± 0,15 mm per year (± 2-s limits). Seen over the complete period 1890-2014 sea level rose by 23 cm.
The Dutch Royal Meteorological Institute (KNMI) developed four climate scenarios for the Netherlands. As part of their scenarios sea level rise is given along the Dutch coast up to the year 2100. For details see KNMI (2014) and PBL/KNMI (2015).

No acceleration along the Dutch coast

Analysis of the sea level data averaged over six stations, do not show any acceleration. This is also true for recent years. This finding is based on the estimation of various mathematical trend models (Visser et al., 2015). With 'acceleration' we mean sea level data which show a trend pattern that deviates from linear. A positive deviation points to acceleration, a negative deviation to a deceleration. Our finding is consistent with that presented by Baart et al. (2012).

Wahl et al. (2013) analyse 30 stations along the European North sea coast. They present linear trends for all stations. Their estimates for stations along the Dutch coast correspond well with those presented here (figure 2 and table 1 in the article of Wahl et al.). Linear trends along the Belgium coast have been reported by Verwaest (2005). Wahl et al. (2011) found for stations along the German Bight that stations lying nearest to the Netherlands showed a linear pattern over time. However, stations lying more in the north, showed (small) accelerations in recent years.

Global sea level rise

The trend pattern in global sea level rise has been estimated various research groups. Church and White (2011) found over the period 1901-1990 an annual increment of 1,5 ± 0,2 mm per year; Hay et al. (2015) estimate annual increments of 1,2 ± 0,2 mm per year. Over the period 1993-2010 both research groups report a strong linear increment: 2,8 ± 0,5 mm per year and 3,0 ± 0,7 mm per year, see second graph. Furthermore, the graph shows a sea level series composed on the basis of satellite data, starting in 1993 (Watson et al., 2015). The satellite series shows a pattern that is approximately linear, accounting for 2,6 ± 0,4 mm per year. This value is somewhat lower than that found by Church and White and Hay et al.. However, the differences lie within the uncertainty limits presented.

We note that sea levels vary over different regions of the oceans. These variations come from differences in water temperatures, wind speeds and variations in the gravity field of the earth (even if tide flows are averaged over the globe, the gravity field shows variations). See
Church et al. (2010). Two-weekly maps from satellites give a clear picture of these height variations. See the following animation with two-weekly sea levels over the period 2003-2008: NOAA, Sea level animations.
The role of anthropogenic forcing has been revealed in a recent study of Slangen et al. (2016). Also see Dangendorf (2016).