The present-day sea-level rise is a major indicator of climate change. The sea level in European seas has risen at a rate of 2.5 to 4 millimeters per year (von Schuckmann et al., 2018). The aim of this paper is to present the sea level variability in the Baltic Sea, as based on satellite altimetry. For this purpose, the paper presents a methodology of the investigation of the Baltic Sea level changes based on the Optimum Dataset (OptD) method. The OptD method was used to identify characteristic points from the analyzed data set. For detailed theoretical and empirical tests, the sea level anomaly was used. The time series were created from the data set after introducing the OptD method, in the period from January 1993 to December 2017. The time series are then used to characterize sea level trends, and inter-annual and semi-annual variability in the Baltic Sea region. The results prove that the linear change is higher at points which are located in the northern part of the Baltic Sea, while it is lower at points located in the western part of the Baltic Sea. The average trend is 4.1±0.2 mm/yr. However, the annual cycles in the sea level variations measured by altimetry reach maximum values in approximately the same months (November/December) in the whole Baltic Sea area. We find that there occur substantial regional deviations in sea level depending on the latitude and longitude. Our results confirm the need for research into the sea level variability in the Baltic Sea region.
In this study, the vertical crustal movements of the southern Baltic coast were determined based on two independent methods: tide gauge and GNSS observations. The mean sea level change trends were determined from five tide gauge observations and satellite altimetry data along the Polish coastal zone. The vertical crustal movements at nearby GNSS stations were also determined. We used the tide gauge water level data from the Permanent Service for Mean Sea Level (PSMSL) and from the Institute of Meteorology and Water Management National Research Institute, Poland (1951-2017 and 1993-2017) as well as sea level anomalies obtained from the Copernicus Marine and Environment Monitoring Service (CMEMS) (1993-2017). The time series for GNSS stations eveloped with the PPP technique obtained from Nevada Geodetic Laboratory (NGL) and last - squares approach were used for the analysis. The results provide a view of the absolute vertical crustal movements of the Polish coast. The absolute vertical crustal movements, calculated from tide gauge data and satellite altimetry for the time period between 1951-2017 ¬– from +2.20 mm/yr ±0.42 mm/yr to +2.68 mm/yr ±0.31 mm/yr. A comparison was made, and it showed that the absolute movements determined from two data sets were different. This may be a consequence of several factors: short and incomplete time series, other unidentified movements of a GNSS station, a tide gauge of own movements, human activity, geological and hydrological factors, the method of the time series elaboration and of the data used to work out the vertical crustal movements. It is thought that the results are very significant despite the existing differences in the absolute crustal movements, because the sea level is a unique index in studies of climate impact on all changes on the Earth and crustal movements are closely linked to it.