The Earth’s climate is strongly affected by the partitioning of carbon between its mobile reservoirs, primarily between the atmosphere and the ocean. The distribution between the reservoirs is being massively perturbed by human activities, primarily due to fossil fuel emissions, with a range of consequences, including ocean warming and acidification, sea-level rise and coastal erosion, and changes in ocean productivity. These changes directly impact valuable habitats in many coastal regions and threaten the important services the habitats provide to mankind. Among the most productive and diverse systems are coral reefs and vegetated habitats, including saltmarshes, seagrass meadows, and mangroves. Coral reefs are particularly vulnerable to ocean warming and acidification. Vegetated habitats are receiving heightened attention for their ability to sequester carbon, but they are being impacted by land-use change, sea-level rise, and climate change. Overall, coasts play an important, but poorly quantified, role in the global cycling of carbon. Carbon reservoirs on land and in the ocean are connected through the so-called land–ocean aquatic continuum, which includes rivers, estuaries, and the coastal ocean. Terrestrial carbon from soils and rocks enters this continuum via inland water networks and is subject to transformations and exchanges with the atmosphere and sediments during its journey along the aquatic continuum. The expansive permafrost regions, comprised of ground on land and in the seabed that has been frozen for many years, are of increasing concern because they store vast amounts of carbon that is being mobilized due to warming. Quantitative estimates of these transformations and exchanges are relatively uncertain, in large part because the systems are diverse and the fluxes are highly variable in space and time, making observation at the necessary spatial and temporal coverage challenging. But despite their uncertainty, existing estimates point to an important role of these systems in global carbon cycling.
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Carbon, Coast, and the Climate
Katja Fennel, Tyler Cyronak, Michael DeGrandpre, David T. Ho, Goulven G. Laruelle, Damien Maher, and Julia Moriarty
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Climate and Coast: Overview and Introduction
Hans von Storch, Katja Fennel, Jürgen Jensen, Kristy A. Lewis, Beate Ratter, Torsten Schlurmann, Thomas Wahl, and Wenyan Zhang
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.
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Climate Change and Baltic Sea Genetic Diversity
Kerstin Johannesson
Up until the beginning of the previous millennium, threats to the environment were mostly local and hence less difficult to combat. In the early 21st century, a large human population and high and increasing demands of energy, food, and other resources have led to a global climate crisis affecting nearly all life on Earth, biodiversity, and human societies alike. How will such a dramatic environmental shift affect the ecosystems? The Baltic Sea ecosystem is an interesting example in this respect, as it includes a dramatic environmental shift, starting from a freshwater lake but 8,000 years ago turning into a marine (brackish) sea with initially somewhat higher salinity than in the early 21st century. Marine populations of species that colonized the brackish water basin had to tolerate or adapt to one third their original salinity, lower density of the water, no regular tides, winter ice, and generally colder waters. Overall, these were dramatic environmental changes of magnitudes and rates of change similar to what is predicted from contemporary global environmental changes, including climate change. Indeed, the Baltic Sea is a “Darwinian laboratory” that can be explored to understand the role of plasticity, genetic variation, genetic structure and architecture, and demography using the sharp footprints of both phenotypic and genetic changes of organisms that survived this rapid environmental shift. What is observed is firstly that many marine species outside did not make it into the Baltic Sea, but among those that were successful, almost all (among the >30 studied macrospecies) show large genetic changes due to genetic drift and directional selection. The latter show that adaptation to a strong environmental shift is possible over a relatively short period of time for some species. Among successful survivors were species with very large population sizes and thus large standing genetic variation. In some species, hybridization contributed to increased levels of genetic variation. Only very few macroscopic species show evidence of recent adaptation following new mutations, while this was common among microbes. Reduced rates of recombination among genes through chromosomal inversions enable local adaptation, and it certainly supported the establishment of at least a handful of Baltic populations of species. Cloning, is also overrepresented among Baltic populations, and there may be several reasons for this. Cloning conserves suitable genotypes in marginal environments, and it also supports efficient range expansion in species that under sexual reproduction would require two individuals (one female and one male) to arrive close together after migration into a new spot. Being in an enclosed sea, the Baltic populations of largely cold water–adapted species are now facing the next large challenge—a much more rapid warming than elsewhere in the oceans.
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The Development of Fish Stocks and Fisheries in the Baltic Sea Since the Last Glaciation
Henrik Svedäng
The fish fauna of the Baltic Sea reflects its 9 KY history of Arctic and temperate conditions and is a mixture of species that have invaded from the Atlantic and the continental watersheds. In spite of the challenging environmental conditions, such as low salinity in the entire Baltic Sea and varying temperature conditions, limiting the possibilities for successful reproduction, the number of species is comparably high.
Except for the Baltic Ice Lake and certain stages of the cold Yoldia Sea and freshwater Ancylus Lake, the fish fauna of the Baltic Sea, as recorded by archaeological and historical notes, has to a large extent remained unchanged. Some freshwater and cold-water species such as Arctic char may have disappeared while others, such as fourhorned sculpin and eelpout, have adapted and persist as “ice age relicts.” There are few viable introductions of novel species; the round goby may be the most conspicuous invasive species. The extinction rate is still low; the loss of sturgeon and the common skate within the HELCOM (the Helsinki Commission) area is due to fishing, and, for the riverine sturgeon, due to damming.
Since the formation of the Baltic Sea, fishing has played an essential role in supplying coastal settlements and their hinterlands and in trading. The archaeological and historical records have indicated fishing conducted with varying intensity using different methods.
Herring fishing has been a significant economic driver from the Middle Ages onward. Recent archaeological findings indicate that organized fishing was established at the outer archipelagos along the present Swedish east coast on predominately herring and cod archipelagos for self-sufficiency shortly before or during the Viking Age, and later to engage in barter.
The fact that the cod abundance has sometimes been sufficient for letting cod fishing be the most important fishery in the northern Baltic Proper alongside the fishery on herring may indicate that the eastern cod stock had relatively high productivity even when the Baltic Sea was significantly less eutrophic than it has been since the mid-20th century. This preindustrial variability in cod abundance suggests that climatic changes leading to changes in inflows of oceanic water may have affected salinity levels in the Baltic Sea.
Fisheries show substantial variability, especially over the last century. Fish production may have increased due to nutrient enrichment of the Baltic Sea. Higher yields have also been obtained due to higher fishing intensity and technological changes. Fishing has, therefore, become a major driver in shaping fish stocks.
The eutrophication of the Baltic Sea, leading to higher primary productivity and increasing water temperature and reductions in ice cover, may have led to changes in ecosystem structure and productivity. It should be underscored that such changes may also be amplified by the increasing fishing pressure on the cornerstone species such as herring, leading to significant disruptions in the food web.
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Effects of Climate Change and Fisheries on the Marine Ecosystem of the Baltic Sea
Christian Möllmann
Climate change and fisheries have significantly changed the Baltic Sea ecosystem, with the demise of Eastern Baltic cod (Gadus morhua callarias) being the signature development. Cod in the Central Baltic Sea collapsed in the late 1980s as a result of low reproductive success and overfishing. Low recruitment and hence small year-classes were not able to compensate for fishing pressures far above sustainable levels. Recruitment failure can be mainly related to the absence of North Sea water inflows to the Central Baltic deep basins. These major Baltic inflows (MBIs) occurred regularly until the 1980s, when their frequency decreased to a decadal pattern, a development attributed to changes in atmospheric circulation patterns. MBIs are needed for ventilation of otherwise stagnating Baltic deep waters, and their absence caused reduced oxygen and salinity levels in cod-spawning habitats, limiting egg and larval survival. Climate change, on the other hand, has promoted a warmer environment richer in zooplanktonic food for larval Baltic sprat (Sprattus sprattus). Resulting large year-classes and low predation by the collapsed cod stock caused an outburst of the sprat stock that cascaded down to the zoo- and phytoplankton trophic levels. Furthermore, a large sprat population controlled cod recruitment and hence hindered a recovery of the stock by predation on cod eggs, limiting cod larval food supply.
The change in ecosystem structure and function caused by the collapse of the cod stock was a major part and driver of an ecosystem regime shift in the Central Baltic Sea during the period 1988 to 1993. This reorganization of ecosystem structure involved all trophic levels from piscivorous and planktivorous fish to zoo- and phytoplankton. The observed large-scale ecosystem changes displayed the characteristics of a discontinuous regime shift, initiated by climate-induced changes in the abiotic environment and stabilized by feedback loops in the food web. Discontinuous changes such as regime shifts are characteristically difficult to reverse, and the Baltic ecosystem recently rather shows signs of increasing ecological novelty for which the failed recovery of the cod stock despite a reduction in fishing pressure is a clear symptom. Unusually widespread deficient oxygen conditions in major cod-spawning areas have altered the overall productivity of the population by negatively affecting growth and recruitment. Eutrophication as a consequence of intensive agriculture is the main driver for anoxia in the Baltic Sea amplified by the effects on continuing climate change and stabilized by self-enforcing feedbacks. Developing ecological novelty in the Baltic Sea hence requires true cross-sectoral ecosystem-based management approaches that truly integrate eutrophication combatment, species conservation, and living resources management.
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The Genesis and Evolution of European Union Framework Programmes on Climate Science
Elisabeth Lipiatou and Anastasios Kentarchos
Although the first European Union Framework Programme (FP) for research and technological development was created in 1984, it was the second FP (FP2) in 1987 that devoted resources to climatological research for the first time. The start of FP2 coincided with the establishment of the Intergovernmental Panel on Climate Change in 1988, aimed at providing a comprehensive assessment on the state of knowledge of the science of climate change.
FP-funded research was not an end in itself but a means for the European Union (EU) to achieve common objectives based on the principle of cross-border research cooperation and coordination to reduce fragmentation and effectively tackle common challenges.
Since 1987, climate science has been present in all nine FPs (as of 2023) following an evolutionary process as goals, priority areas, and financial and implementation instruments have constantly changed to adapt to new needs. A research- and technology-oriented Europe was gradually created including in the area of climate science.
There has historically been a strong intrinsic link between research and environmental and climate policies. Climate science under the FPs, focusing initially on oceans, the carbon cycle, and atmospheric processes, has increased tremendously both in scope and scale, encompassing a broad range of areas over time, such as climate modeling, polar research, ocean acidification, regional seas and oceans, impacts and adaptation, decarbonization pathways, socioeconomic analyses, sustainability, observations, and climate services.
The creation and evolution of the EU’s FPs has played a critical role in establishing Europe’s leading position on climate science by means of promoting excellence, increasing the relevance of climate research for policymaking, and building long-lasting communities and platforms across Europe and beyond as international cooperation has been a key feature of the FPs. No other group of countries collaborates on climate science at such scale. Due to their inherited long-term planning and cross-national nature, the FPs have provided a stable framework for advancing climate science by incentivizing scientists and institutions with diverse expertise to work together, creating the necessary critical mass to tackle the increasing complex and interdisciplinary nature of climate science, rationalizing resource allocation, and setting norms and standards for scientific collaboration. It is hard to imagine in retrospect how a similar level of impact could have been achieved solely at a national level.
Looking ahead and capitalizing on the rich experience and lessons learned since the 1980s, important challenges and opportunities need to be addressed. These include critical gaps in knowledge, even higher integration of disciplines, use of new technologies and artificial intelligence for state-of-the-art data analysis and modeling, capturing interlinkages with sustainable development goals, better coordination between national and EU agendas, higher mobility of researchers and ideas from across Europe and beyond, and stronger interactions between scientists and nonscientific entities (public authorities, the private sector, financial institutions, and civil society) in order to better communicate climate science and proactively translate new knowledge into actionable plans.
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Historical Documents as Proxy Data in Venice and Its Marine Environment
Dario Camuffo
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.
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Perspectives From Coastal Ecosystems Through the Lens of Climate Change
Kristy A. Lewis, Giovanna McClenachan, Kristin DeMarco, Jennifer Salerno, and Katherine Thompson
Climate change impacts coastal ecosystems and the humans who live and work in these regions. The coastal ecosystem, or the coastal zone, is defined as the margin between land, air, and ocean where complex interactions occur between living and nonliving parts of the system. This interconnectivity generally gives rise to areas of high productivity, that is, areas where life flourishes. These systems are also some of the most dynamic (but also fragile) regions on the planet, supporting tremendous ecological, biological, and human diversity. Climate change, however, can alter sea temperature, raise sea level, and modify weather patterns, ultimately impacting the ecosystem services provided by the system. All these changes can have varying (positive, negative, or neutral) outcomes depending on the type of coastal ecosystem experiencing the impacts.
Some coastal ecosystems, like rocky coasts, will experience tropicalization leading to many species shifting further toward the poles. This process may lead to the expansion of oyster reefs and mangroves into salt marshes. Species distributions will also be affected by marine heat waves, which are periods of high temperature anomalies in the ocean. Prolonged periods of elevated sea surface temperatures can cause coral reefs and kelp forests to collapse. Ocean acidification, caused by increased CO2 uptake by the ocean, is likely to further limit coral reef and oyster larvae growth but may also lead to increases in some kelp species’ abundance and distribution.
Sea level rise will affect sediment dynamics and light availability for kelps, seagrasses, and submerged aquatic vegetation. This phenomenon will also pose a direct threat to coastal marshlands and vulnerable low-lying island nations. Other ecosystems, like oyster reefs and mangrove forests, may have some level of resistance to sea level rise, as their three-dimensional structure allows them to cope with rising water levels.
In conjunction with rising sea levels, modified weather patterns will greatly affect coastal ecosystems. Many coastlines, especially sandy beaches and barrier islands, will be impacted by more frequent and intense storms that disrupt the cycle of sediment erosion and accretion. These events may result in rapid changes to the shape and location of the coast or even complete loss of landmasses. Changes in storm intensities can also significantly damage coral reefs, cause kelp to be detached from the seafloor, erode marshland, and prevent seagrass establishment. However, storms may also potentially expand mangrove distributions, because the storm currents and waves may enable the transport of their seeds longer distances. Combined with increased air temperatures, mangroves will continue to expand into regions where they were previously not found.
Climate change impacts on coastal systems will be varied in degree and may even help some ecosystems flourish, rather than degrade. Given the many types of ecosystems that occur in the coastal zone, coupled with synergistic impacts of climate change and human disturbances, a collective effort will be needed to mitigate against the varied changes to these fragile ecosystems.
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Post-Glacial Baltic Sea Ecosystems
Ilppo Vuorinen
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.
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Projected Oceanographical Changes in the Baltic Sea until 2100
H.E. Markus Meier and Sofia Saraiva
In this article, the concepts and background of regional climate modeling of the future Baltic Sea are summarized and state-of-the-art projections, climate change impact studies, and challenges are discussed. The focus is on projected oceanographic changes in future climate. However, as these changes may have a significant impact on biogeochemical cycling, nutrient load scenario simulations in future climates are briefly discussed as well. The Baltic Sea is special compared to other coastal seas as it is a tideless, semi-enclosed sea with large freshwater and nutrient supply from a partly heavily populated catchment area and a long response time of about 30 years, and as it is, in the early 21st century, warming faster than any other coastal sea in the world. Hence, policymakers request the development of nutrient load abatement strategies in future climate. For this purpose, large ensembles of coupled climate–environmental scenario simulations based upon high-resolution circulation models were developed to estimate changes in water temperature, salinity, sea-ice cover, sea level, oxygen, nutrient, and phytoplankton concentrations, and water transparency, together with uncertainty ranges. Uncertainties in scenario simulations of the Baltic Sea are considerable. Sources of uncertainties are global and regional climate model biases, natural variability, and unknown greenhouse gas emission and nutrient load scenarios. Unknown early 21st-century and future bioavailable nutrient loads from land and atmosphere and the experimental setup of the dynamical downscaling technique are perhaps the largest sources of uncertainties for marine biogeochemistry projections. The high uncertainties might potentially be reducible through investments in new multi-model ensemble simulations that are built on better experimental setups, improved models, and more plausible nutrient loads. The development of community models for the Baltic Sea region with improved performance and common coordinated experiments of scenario simulations is recommended.
