Coasts are those regions of the world where the land has an impact on the state of the sea, and that part of the land is in turn affected by the sea. This land–sea interaction may take various forms—geophysical, biological, chemical, sociocultural, and economic. Coasts are conditioned by specific regional conditions. These unique characteristics result, in heavily fragmented regional and disciplinary research agendas, among them geographers, meteorologists, oceanographers, coastal engineers, and a variety of social and cultural sciences. Coasts are regions where the effects and risks of climate impact societal and ecological life. Such occurrences as coastal flooding, storms, saltwater intrusion, invasive species, declining fish stocks, and coastal retreat and morphological change are challenging natural resource managers and local governments to mitigate these impacts. Societies are confronted with the challenge of dealing with these changes and hazards by developing appropriate cultural practices and technical measures. Key aspects and concepts of these dimensions are presented here and will be examined in more detail in the future to expand on their characteristics and significance.
Hans von Storch, Katja Fennel, Jürgen Jensen, Kristy A. Lewis, Beate Ratter, Torsten Schlurmann, Thomas Wahl, and Wenyan Zhang
While not all projected climate change impacts are affecting especially and directly at all the cities of the Baltic Sea region (bsr), including its basin, those cities expect very different direct as well as indirect impacts of climate change. The impacts are also a matter of location, if the city with its built structures and concentration of population is located in the northern or southern part of this basin, or more inland or directly at the coast. As there are many different definitions in use trying to determine what a city is, also in the different national contexts of the bsr, here it is cities in the sense of being human-dominated densely populated areas, which are also characterized by higher concentrations of built-up areas, infrastructure, and soil-sealing as well as socioeconomic roles than rural settlements are. Those characteristics render cities also especially vulnerable to climate change impacts while there are some opportunities arising too. There are many studies on climate change impacts on the Baltic Sea itself as well as on the various ecosystems, but the studies on the observed as well as potential future impacts of climate change on cities are disperse, many are also of a national character or concentrating on a small number of cases, leaving some cities not well studied at all. This renders an all-encompassing picture on the cities within the bsr difficult and even more complicated as every city provides a mix of built-up and open structures, of socioeconomic structure and role in a region, nation-state, or even on an international level, and further characteristics. Their urban development is dependent on manifold various interdependencies as well as climatic and nonclimatic drivers, such as, to name just a few diverse examples, urban to international governance processes, or topography and location, or also different socioeconomic vulnerabilities within the Baltic Sea basin. Accordingly every urban society and structure provides specific exposure, vulnerabilities, and adaptive capacity. Generally, the cities of the bsr have to deal with the impacts of temperature rise, natural hazards, and extreme events, and, depending on location and topography, with sea-level rise. With reference to temperature rise and the increase of heat waves, it is important to consider that cities of a certain size within the Baltic Sea basin contribute to their own urban climatic conditions and provide already urban heat islands. Also, urban planning and building facilitated by local political decisions contribute to the extent of urban floods as well as their damage, as these are regulating, for example, the sealing of soils or new built-up areas in flood-prone zones.
The environmental history of Venice over the last millennium has been reconstructed from written, pictorial, and architectural documentary sources, used in a synergistic way. The method of transforming a document into an index and then into calibrated numerical values according to an international system of units has been applied in the case of Venice and its geographical and climate peculiarities. Because frost constituted a dramatic challenge for the city, a series of severe winters is well documented: The city was sieged by ice, meaning Venetians had to cross the ice transporting food, beverages, and wood for burning in carts, as recorded in written reports and visual representations. The sea level in the 18th century has been reconstructed based on paintings by Canaletto and Bellotto, who took advantage of a camera obscura to precisely draw the views of the city and its canals.. These paintings accurately represent the green algae belt that corresponds to the level of soaking created by marine waters at high tide. This has made it possible to measure how much the green algae (and therefore the seawater) has risen since the 18th century. Similarly, a painting by Veronese has enabled the reconstruction of sea level rise (SLR) since 1571. Another useful proxy is the water stairs of the Venetian palaces. These were originally built to access boats and are now (almost) totally submerged and covered with algae. As the sea level rose, these steps became submerged underwater. The depth of the lowest step is therefore representative of how much the sea level rose after the stair was built. This proxy has allowed the relative sea level since 1350 to be reconstructed, and an exponential trend in the rising of the sea level has been identified. Venice has at times been flooded by seawater, including tsunamis at the beginning of the second millennium. A long series of sea floods due to storm surges triggered by particular meteorological situations shows that the flooding frequency is related to the exponential SLR. In the 1960s, there was a sharp increase in frequency of flooding, which coincided with the digging of deep and wide canals, excavated to allow the passage of tankers. This increased the exchange of water between the sea and the lagoon. Proxies based on archaeological remains, as well as geological-biological cores extracted from the coastal area and dated with isotopic methods, cover long time periods; the longest record reaching 13 ka BP. However, the time resolution is reduced, thus providing good data for physical geography purposes.
Post-glacial aquatic ecosystems in Eurasia and North America, such as the Baltic Sea, evolved in the freshwater, brackish, and marine environments that fringed the melting glaciers. Warming of the climate initiated sea level and land rise and subsequent changes in aquatic ecosystems. Seminal ideas on ancient developing ecosystems were based on findings in Swedish large lakes of species that had arrived there from adjacent glacial freshwater or marine environments and established populations which have survived up to the present day. An ecosystem of the first freshwater stage, the Baltic Ice Lake initially consisted of ice-associated biota. Subsequent aquatic environments, the Yoldia Sea, the Ancylus Lake, the Litorina Sea, and the Mya Sea, are all named after mollusc trace fossils. These often convey information on the geologic period in question and indicate some physical and chemical characteristics of their environment. The ecosystems of various Baltic Sea stages are regulated primarily by temperature and freshwater runoff (which affects directly and indirectly both salinity and nutrient concentrations). Key ecological environmental factors, such as temperature, salinity, and nutrient levels, not only change seasonally but are also subject to long-term changes (due to astronomical factors) and shorter disturbances, for example, a warm period that essentially formed the Yoldia Sea, and more recently the “Little Ice Age” (which terminated the Viking settlement in Iceland). There is no direct way to study the post-Holocene Baltic Sea stages, but findings in geological samples of ecological keystone species (which may form a physical environment for other species to dwell in and/or largely determine the function of an ecosystem) can indicate ancient large-scale ecosystem features and changes. Such changes have included, for example, development of an initially turbid glacial meltwater to clearer water with increasing primary production (enhanced also by warmer temperatures), eventually leading to self-shading and other consequences of anthropogenic eutrophication (nutrient-rich conditions). Furthermore, the development in the last century from oligotrophic (nutrient-poor) to eutrophic conditions also included shifts between the grazing chain (which include large predators, e.g., piscivorous fish, mammals, and birds at the top of the food chain) and the microbial loop (filtering top predators such as jellyfish). Another large-scale change has been a succession from low (freshwater glacier lake) biodiversity to increased (brackish and marine) biodiversity. The present-day Baltic Sea ecosystem is a direct descendant of the more marine Litorina Sea, which marks the beginning of the transition from a primeval ecosystem to one regulated by humans. The recent Baltic Sea is characterized by high concentrations of pollutants and nutrients, a shift from perennial to annual macrophytes (and more rapid nutrient cycling), and an increasing rate of invasion by non-native species. Thus, an increasing pace of anthropogenic ecological change has been a prominent trend in the Baltic Sea ecosystem since the Ancylus Lake. Future development is in the first place dependent on regional factors, such as salinity, which is regulated by sea and land level changes and the climate, and runoff, which controls both salinity and the leaching of nutrients to the sea. However, uncertainties abound, for example the future development of the Gulf Stream and its associated westerly winds, which support the sub-boreal ecosystems, both terrestrial and aquatic, in the Baltic Sea area. Thus, extensive sophisticated, cross-disciplinary modeling is needed to foresee whether the Baltic Sea will develop toward a freshwater or marine ecosystem, set in a sub-boreal, boreal, or arctic climate.
Thomas Wahl and Sönke Dangendorf
Sea level rise leads to an increase in coastal flooding risk for coastal communities throughout the world. Changes in mean sea level are caused by a combination of human-induced global warming and natural variability and are not uniform throughout the world. The key processes leading to mean sea level rise and its variability in space and time are the melting of land-based ice and changes in the hydrological cycle; thermal expansion due to warming oceans; changes in winds, ocean currents, and atmospheric pressure; and, when focusing on the relative changes between the land and the ocean, any vertical motion of the land itself (subsidence or uplift). In addition to the change in mean sea level, which is the main climatic driver for changes in coastal flooding risk in most regions, additional changes in tides, storm surges, or waves can further exacerbate, or offset, the negative effects of mean sea level rise. Hence, it is important to analyze, understand, and ultimately project the changes in all of these sea level components individually and combined, including the complex interactions between them. Advances in sea level science in the 21st century along with new and extended observational records including in situ and remote sensing measurements have paved the path to being able to provide better and more localized information to stakeholders, particularly in the context of making decisions about coastal adaptation to protect the prosperity of coastal communities and ecosystems.