Baltic Earth Working Group on Sea Level and Coastal Change in the Baltic Sea

Sea level varies at a range of scales, from a few hours to millennia. When considered in combination with weather-related drivers (winds, waves, river runoff, sea-ice cover) and on a backdrop of diverse coastal geology and geomorphology, coastal change varies spatially, often over short distances, and over time, sometimes in dramatic and unexpected ways. The Baltic Sea has some of the longest and best sea level records in the world. Together with satellite data and numerical modelling, this allows for high-resolution analysis of absolute (geocentric) sea level evolution in the context of global sea level change. At the shoreline, where change such as coastal erosion is experienced, relative sea level is influenced by land uplift or subsidence, so that much of the rocky northern coast is experiencing sea level fall while the southern sand-dominated coast is experiencing quite rapid sea level rise in line with global trends, leading to distinct coastal change that evolves over time.

Coastal change and (compound) coastal flooding are most obvious during storms when the sea level is elevated, the combined result of higher water levels that are driven by the dynamics of the North Sea, and processes driven by regional and local weather conditions against the backdrop of mean sea level rise. Because waves experienced at the shoreline are driven by local winds rather than by oceanic swell, wind direction is of the utmost importance, and climate change related long-term changes to wind, or winds from an unusual direction during a particular storm, can have very significant impacts. Communities along the coast have an urgent need for improved short-term prediction and climate projections of extreme sea levels and of waves at fine spatial scales applicable to small coastal segments and compartments, that are sensitive to shifts in wind directions and intensities. These analyses should also include the impact of seiches, meteotsunamis, the risk of compound flooding from rivers and sea, and changes to the duration and extent of sea ice cover. Less frequently considered, high waves coinciding with low water levels may impact the stability of the nearshore and navigation safety.

Coastal change is the result of waves and other coastal drivers mobilizing and transporting sediments. In situ data, essential for the estimation of actual sediment fluxes, is required for incorporation in improved sediment transport and coastal morphological models. Major progress towards the prediction of future coastal changes requires the integration of modelling, measurements and monitoring.

Because drastic coastal change and coastal flooding almost always occur during periods when elevated sea levels coincide with storms, the prediction of the nature and extent of sea level extremes is essential. At longer time scales this requires researchers to refine sea level predictions at a basin scale including improving our understanding of links between the Baltic Sea and the North Sea.  The input of climatologists, particularly concerning changing wind patterns (especially direction) is essential, for coastal scientists to be able to accurately predict extreme (both high and low) sea levels and wave conditions for small sedimentary segments and compartments. The Baltic Earth working group will have an important role in integrating the work of modellers, statisticians, climatologists and field scientists, utilising new data sources such as the Surface Water and Ocean Topography satellite and methods such as AI and machine learning, as well as bringing together diverse data sets held in different jurisdictions. The working group will provide robust advice to hazard researchers, coastal managers, engineers and the wider community, applicable at the local community as well as at regional and basin-wide scales.