Birgit Hünicke

Trends in the amplitude of Baltic Sea level annual cycle

Baltic Sea tide gauge data and climatic data sets are statistically analysed to investigate the centennial trends in the amplitude of the annual cycle of Baltic sea level. In almost all gauge stations analysed, an increase of the amplitude (winter-spring sea level) is detected. These trends are not large compared to the decadal variations of the annual cycle, but they are statistically significant. The magnitude of the trends is almost spatially uniform, with exception of the Skagerrak area. Since interannual and decadal variability of sea level displays a clear spatial pattern, the mechanism responsible for the trends in the annual cycle seem to be not regional, but affect the Baltic Sea basin as a whole. Several hypotheses are proposed to explain these centennial trends on the winter-minus-spring sea level: wind (through the SLP field), the barometric effect, temperature and precipitation. By elimination of three of the working hypothesis, seasonal Baltic precipitation remains a plausible candidate. For the other three, either the sign or magnitude of the trend makes them unlikely the sole explanation.

Regional differences in winter sea level variations in the Baltic Sea for the past 200 yr

Decadal sea level variations in selected stations located in the southwestern, central and eastern Baltic Sea are found to be less coherent in the 19th century than in the 20th century. The effect of the North Atlantic sea level-pressure (SLP), precipitation and air-temperature in the 19th and 20th centuries from gridded climate reconstructions, and their relationship to Baltic Sea level, are statistically analysed to explain this difference. The influence of these factors on sea level varies geographically. In the central and eastern Baltic, Sea level variations are well described by SLP alone, whereas in the southern Baltic Sea area-averaged precipitation better explains the decadal sea level variations. The evolution of precipitation in the 19th century could explain the different behaviour of the southern Baltic stations; however, the physical mechanism for this relationship remains unclear. The effect of temperature variations is either already contained in the SLP field or is less important for decadal sea level variations than the other two factors.

Recent Change—Sea Level and Wind Waves

This chapter describes observed changes in sea level and wind waves in the Baltic Sea basin over the past 200 years and the main climate drivers of this change. The datasets available for studying these are described in detail. Recent climate change and land uplift are causing changes in sea level. Relative sea level is falling by 8.2 mm year−1 in the Gulf of Bothnia and slightly rising in parts of the southern Baltic Sea. Absolute sea level (ASL) is rising by 1.3–1.8 mm year−1, which is within the range of recent global estimates. The 30-year trends of Baltic Sea tide gauge records tend to increase, but similar or even slightly higher rates were observed around 1900 and 1950. Sea level in the Baltic Sea shows higher values during winter and lower values during spring and this seasonal amplitude increased between 1800 and 2000. The intensity of storm surges (extreme sea levels) may have increased in recent decades in some parts of the Baltic Sea. This may be linked to a long-term shift in storm tracks.

Statistical Analysis of the Acceleration of Baltic Mean Sea-Level Rise, 1900–2012

We analyse annual mean sea-level records from tide-gauges located in the Baltic and parts of the North Sea with the aim of detecting an acceleration of sea-level rise over the 20\textsuperscript{th} and 21\textsuperscript{st} centuries. The acceleration is estimated as a (1) fit to a polynomial of order two in time, (2) a long-term linear increase in the rates computed over gliding overlapping decadal time segments, and (3) a long-term increase of the annual increments of sea level. The estimation methods (1) and (2) prove to be more powerful in detecting acceleration when tested with sea-level records produced in global climate model simulations. These methods applied to the Baltic-Sea tide-gauges are, however, not powerful enough to detect a significant acceleration in most of individual records, although most estimated accelerations are positive. This lack of detection of statistically significant acceleration at the individual tide-gauge level can be due to the high-level of local noise and not necessarily to the absence of acceleration. The estimated accelerations tend to be stronger in the north and east of the Baltic Sea. Two hypothesis to explain this spatial pattern have been explored. One is that this pattern reflects the slow-down of the Glacial Isostatic Adjustment. However, a simple estimation of this effect suggests that this slow-down cannot explain the estimated acceleration. The second hypothesis is related to the diminishing sea-ice cover over the 20\textsuperscript{th} century. The melting o of less saline and colder sea-ice can lead to changes in sea-level. Also, the melting of sea-ice can reduce the number of missing values in the tide-gauge records in winter, potentially influencing the estimated trends and acceleration of seasonal mean sea-level This hypothesis cannot be ascertained either since the spatial pattern of acceleration computed for winter and summer separately are very similar. The all-station-average-record displays an almost statistically significant acceleration. The very recent decadal rates of sea-level rise are high in the context of the 20\textsuperscript{th} and 21\textsuperscript{st} centuries, but they are not the highest rates observed over this period.

What Determines the Change of Coastlines in the Baltic Sea

The change of coastline positions of the Baltic Sea is mainly determined by both the eustatic sea-level change and the glacio-isostatic adjustment (GIA). For changes on the Holocene time scale, the relative sea-level change can be reconstructed from paleo-coastline positions and correspondingly dated sediments and organic remains. On the decadal scale, tide gauge data are available. Both data sets display the relative value of sea-level change resulting from the superposition of climatically and meteorologically induced factors, vertical crustal displacement, and related gravitational forces. The isolation of the GIA signal from the compound relative sea-level change data plays a critical role for future projections of coastline changes within the frame of coastal zone management. To separate different components of sea-level data sets, statistical methods for the exploration of empirical water level, meteorological, and GPS data are combined with analytical methods to solve the sea-level equation. In the result, the pattern of vertical crustal movement can be displayed as maps covering the uplifting Fennoscandian Shield and its subsiding belt. Whereas along the uplifting coasts morphodynamic processes play a subordinated role, in the subsiding Southeast and South, Quaternary sediments are permanently exposed to coastal erosion, sediment transport, and re-deposition. This mainly wave-driven sediment dynamics together with aeolian processes depend on meteorological forcing of the in general west-east directed air-flow from the northern Atlantic Ocean to Eurasia. Regional coastal morphogenesis can generally be described by alongshore sediment transport pattern deduced from the integration of subregional to local models of transport capacities. For future projection, coastlines and the morphology of the adjacent zones have to be regarded a function of its position related to the vertical displacement of the Earths crust, the regional climatic and meteorological conditions, and the geological setting. Results of climate modelling, the Earth’s visco-elastic response to the deglaciation, geological data and regional sediment transport capacities have to be interpreted comprehensively.

Quantifying and Predicting the Contribution of Sea-Level Rise to Shoreline Change in Ghana: Information for Coastal Adaptation Strategies

ABSTRACT Evadzi, P.I.K.; Zorita, E., and Hünicke, B., 2017. Quantifying and predicting the contribution of sea-level rise to shoreline change in Ghana: Information for coastal adaptation strategies. The purpose of this study is to estimate the contribution of sea-level rise (SLR) in Ghana over the last decades and provide an estimation of shoreline retreat due to the projections of regional SLR. This study first analyzes historical shoreline change in Ghana from 1974 to 2015 using satellite images and orthophotos. Second, this study quantifies the SLR contribution to historical shoreline change using sea-level trend estimates from satellite observations, results from digital elevation model analysis, and shoreline change rates. This study finally makes predictions of shoreline in Ghana on the basis of modified Intergovernmental Panel on Climate Change Fifth Assessment Report representative concentration pathways (RCPs) scenarios for Ghana. On average, sea level has risen by about 5.3 cm over the last 21 years and accounts for only 31% of the observed annual coastal erosion rate (about 2 m/y) in Ghana. On the basis of the projected model ensemble-mean rise in sea-level (2.6, 4.5, and 8.5 RCPs) scenarios and assuming that SLR will also contribute to 31% of shoreline retreat in the future, by the year 2025, about 6.6, 4.7, and 5.8 m of coastland in Ghana with lowest slope range (0–0.4%) are projected to be inundated respectively. These projected changes increase to 19.8, 20.7, and 24.3 m by 2050 and further to 36.6, 51.6, and 83.9 m by 2100 for the 2.6, 4.5, and 8.5 RCPs respectively. The analysis that separates sea-level contribution to coastal change from other contributing factors could provide useful information about climate impact for coastal adaptation strategies. This study, however, recommends further research into the anthropogenic and other factors that contribute about 69% of the annual erosion rate in Ghana to help improve adaptation efforts.

Modelling of Medium-Term (Decadal) Coastal Foredune Morphodynamics- Historical Hindcast and Future Scenarios of the Świna Gate Barrier Coast (Southern Baltic Sea)

Coastal foredunes are developed as a result of interplay among multi-scale land-sea processes. Natural foredune ridges along the Świna Gate barrier coast (southern Baltic Sea) developed since 6000 cal. year BP provide an excellent laboratory to study the land-sea interaction under a medium- to long-term climatic control. In this paper we investigate several basic driving mechanisms of coastal foredune morphodynamics as well as natural environmental factors involved in shaping the foredune geometry by a numerical model. The model couples a process-based module for subaqueous sediment transport and a probabilistic-type module for subaerial aeolian sand transport and vegetation growth. After an evaluation of the model performance for a 61-year (1951–2012 AD) historical hindcast of the foredune development along a 1 km-long section of the Świna Gate barrier coast, the model is applied for a future projection of the same area to 2050 AD based on three different climate change scenarios. The climate change scenarios represent three different impact levels with regard to their capacity to shape the coastal morphology. Simulation results demonstrate a remarkable variability in foredune development even along a small (1 km) coast section, implying that the medium-term land-sea interaction and foredune morphodynamics is quite sensitive to boundary conditions and various processes acting on multi-temporal and spatial scales. Foredune morphodynamics such as migration, bifurcation, destruction and separation are determined by different combinations of storm frequency, onshore sediment supply rate and relative sea-level change. In contrast to a low rate of relative sea-level rise during the last few decades, an accelerated sea level-rise over the twenty-first century predicted by existing literature, would result in a dramatic and non-linear response from the foredune development according to our simulations.

The Challenge of Baltic Sea Level Change

Baltic Sea level variability is caused by different climatic and geological factors that render their understanding more difficult than for other areas of the Earth. Yet this understanding is crucial to predict with reliability the sea-level rise in the Baltic Sea that will be brought about by anthropogenic climate change. We illustrate this complexity by a few, in our opinion, important questions that ultimately are related to the estimation of long-term trends in the presence of land crust movements, to the heterogeneity of the Baltic sea-level response to atmospheric forcing, and the difficulty of identifying a sea-level rise acceleration in the observed records.