The Development of Climate Science of the Baltic Sea Region
Summary and Keywords
Dramatic climate changes have occurred in the Baltic Sea region caused by changes in orbital movement in the earth–sun system and the melting of the Fennoscandian Ice Sheet. Added to these longer-term changes, changes have occurred at all timescales, caused mainly by variations in large-scale atmospheric pressure systems due to competition between the meandering midlatitude low-pressure systems and high-pressure systems. Here we follow the development of climate science of the Baltic Sea from when observations began in the 18th century to the early 21st century. The question of why the water level is sinking around the Baltic Sea coasts could not be answered until the ideas of postglacial uplift and the thermal history of the earth were better understood in the 19th century and periodic behavior in climate related time series attracted scientific interest. Herring and sardine fishing successes and failures have led to investigations of fishery and climate change and to the realization that fisheries themselves have strongly negative effects on the marine environment, calling for international assessment efforts. Scientists later introduced the concept of regime shifts when interpreting their data, attributing these to various causes. The increasing amount of anoxic deep water in the Baltic Sea and eutrophication have prompted debate about what is natural and what is anthropogenic, and the scientific outcome of these debates now forms the basis of international management efforts to reduce nutrient leakage from land. The observed increase in atmospheric CO2 and its effects on global warming have focused the climate debate on trends and generated a series of international and regional assessments and research programs that have greatly improved our understanding of climate and environmental changes, bolstering the efforts of earth system science, in which both climate and environmental factors are analyzed together.
Major achievements of past centuries have included developing and organizing regular observation and monitoring programs. The free availability of data sets has supported the development of more accurate forcing functions for Baltic Sea models and made it possible to better understand and model the Baltic Sea–North Sea system, including the development of coupled land–sea–atmosphere models. Most indirect and direct observations of the climate find great variability and stochastic behavior, so conclusions based on short time series are problematic, leading to qualifications about periodicity, trends, and regime shifts. Starting in the 1980s, systematic research into climate change has considerably improved our understanding of regional warming and multiple threats to the Baltic Sea. Several aspects of regional climate and environmental changes and how they interact are, however, unknown and merit future research.
Climate variations have been of concern for most generations of people living in the Baltic Sea region and in most of the world (Lamb, 1995). These variations have been regarded as unreliable, with rapid shifts and fluctuations causing living conditions to become difficult, including severe winter conditions, crop failure, frost, flood, drought, and storm damage on land and on sea. Attributing the causes of climate variability and change has often been hampered by the spatial and temporal limitations of available observations and by an incomplete understanding of the driving mechanisms that leaves room for speculation as to both the reasons for changes and the role of the climate in them. This is also the case for the Baltic Sea region, known as an area with rather good data coverage, which can be illustrated by tracking certain ideas closely related to the climate from the time when observations started in the 18th century to the early 21st century. The development of climate science cannot be tracked without considering that general scientific discussions often arise in response to pressing environmental and social problems. Problems such as sinking water levels, fishery failures, eutrophication, crop failure, and unusually cold or mild winters often lead to polarized scientific discussions with only one problem in mind. Climate change, either natural or anthropogenic, has often been used as a wild card when attributing changes instead of critical evaluation of observations or understanding the reasons behind them. Since the 1980s, increasing concern about anthropogenic climate change due to increased greenhouse gas (GHG) emissions has generated multidisciplinary international research programs and assessments that have improved both climate science and communication with society. The marine environment has been regarded as a free resource for increasing exploration and is under multiple stresses from many drivers; anthropogenic climate change due to increased GHG emissions is one factor that interacts with others. There is growing concern that marine system resilience will decrease in the future if management plans do not succeed in restoring a healthy environment that includes reductions in fishing pressure, reduction in nutrient loads to the sea, and reduction in atmospheric CO2.
Developing Understanding of Regional Climate Change
Sinking Water or Land Uplift?
In the past, most people believed that the land was stable. However, early observations around the shores of the Baltic Sea indicated that sea levels were sinking. Stones carved with runic texts were found quite far from the shore where they were believed to be carved and shallow harbors were gradually abandoned as the water level apparently declined. Several ideas were proposed to explain these sinking waters and here the historical presentation follows that of Ekman (2009). In the 18th century, Celsius (1743) studied what are known as “seal rocks” (Figure 1), which were economically important places for seal hunting and therefore described in written records.
He noted that the seal rocks needed to be close to the mean sea level for the seals to climb onto them. Based on four seal rocks, Celsius concluded that the sea level was falling and, using a crude method, estimated the first rate of sea level decline. This result aroused considerable interest, prompting new observations and discussion of the reasons. In the mid-18th century, Runeberg (1765) and Ferner (1765) argued that the water decrease was instead a matter of land uplift. This uplift differed between regions, the largest values being found in the Gulf of Bothnia, the northern extension of the Baltic Sea. It was a long time, however, before the pattern of land uplift was mapped, because the pattern was hidden by large amounts of unreliable data (Ekman, 2009). Early ideas about the Ice Age come from observation that mountain glaciers could transport large blocks and form long moraines. These blocks and moraines could be found not only close to the glaciers but over much larger areas indicating that the land had earlier been covered by ice. The idea of postglacial uplift was proposed in the mid-19th century by Jamieson (1865) and then later by others, though the causes of the uplift were strongly debated. A major shift in ideas was impossible until new knowledge of the thermal history of the earth was available. Ideas about changes in sun–earth orbital motions as a reason for climate change had already been proposed in the 19th century. However, after Milanković (1920) made his mathematical contribution by explaining the earth’s movement around the sun, new studies could better relate climate variations to orbital periods and ice periods. From that point, many scientists connected climate change to oscillations from warm to cold periods at different timescales. For example, Kotov and Harff (2006) studied Greenland ice and Baltic Sea sediment records, identifying periodicities of 900, 500, and 400 years in the Baltic Sea.
Detecting Climate Change Through Direct Observations?
Temperature records in Europe are available starting from the mid-18th century; for example, Uppsala and Stockholm temperature observations started in 1722 and 1756, respectively. Observed air temperatures (Figure 2) form the basis of understanding of the climate and have been studied from various regions and periods often based on land observations.
However, these data need to be homogenized (e.g., Camuffo & Jones, 2002). Homogenization of temperature records, like those from Stockholm (Figure 2), includes corrections for observation hours, missing data, thermometer positions, and protection from direct sunlight and urban warming (Moberg, Bergström, Ruiz Krigsman, & Svanered, 2002). Homogenizing air temperature data is therefore important as the corrections needed are large, and the meteorology community has been leading this work. Similar efforts in other scientific communities are limited, implying that most available marine and terrestrial observations must be handled with care.
Climate statistics can be studied from observations such as temperature, sea level, and ice coverage, and often indicate quite large seasonal, interannual, decadal, and longer variations. Much climate variability is natural and part of the dynamic sun–atmosphere–ocean–land system but human-induced effects are of increasing concern. Detecting climate changes and attributing their causes have been major challenges in past decades. When anthropogenic climate change due to GHG emissions is being studied, one often looks for increasing and decreasing trends in climate parameters parallel to the observed increased carbon dioxide concentrations in the atmosphere. In the 1980s scientists became more interested in trends than oscillations, and it became obvious that trends depend strongly on the time period studied. In the 1990s, concern about global warming generated a number of studies of horizontally averaged global and regional temperature datasets (e.g., Luterbacher et al., 2004; Mann et al., 1998). Available long-term datasets were blended and various statistical methods were used, including the creation of proxy temperature data from, for example, tree rings. Observations from the ocean were largely missing, however. The resulting global gridded datasets illustrated global warming in the 20th century, providing strong indications of climate change due to increased atmospheric GHG levels. They also prompted widespread scientific debate on whether global and regional average values were good measures of climate change. For example, coarse data or grids do not always resolve the underlying physics and, with such complex geometry as the Baltic Sea, many processes such as wind mixing, upwelling, and strait flows are neglected. Several studies of the Baltic Sea have been using gridded data with a horizontal resolution of 1×1 degrees or coarser resolution, with most observations taken from land. A resolution of 1×1 degrees is too coarse for resolving marine conditions over the Baltic Sea (e.g., see Omstedt, Chen, & Wesslander, 2005) and corrections for wind, air temperature, and precipitation are therefore introduced based on statistical relations for present climate conditions.
Are Marine Resources Unlimited?
The seas have been widely explored since the mid-2nd millennium inspired by the notion of infinite marine resources and many discoveries have been made. Fishing boats and ships transitioned from being sailing ships and steamers (powered by coal and wood) to ships powered by petroleum (i.e., bunker oil and other petroleum products) with increasing capacities to fish, travel globally, and stay longer at sea. Marine environmental considerations also led to research vessel expeditions. The Challenger expedition (1872–1876), under the command of Charles Wyville Thomson and supported by the British Royal Navy, marked the start of modern oceanography by systematically collecting data from around the world. This and other efforts also inspired expeditions in coastal seas such as the Baltic Sea. In the summer of 1877, Fredrik Laurentz Ekman made temperature and salinity measurements all around Sweden, from the Skagerrak to the Bothnian Bay (Ekman & Pettersson, 1893). Similar expeditions were performed by other countries (reported by e.g., Matthäus, 2006), improving our understanding of the temperature and salinity conditions of the Skagerrak–Baltic Sea system. Documentations of historical fisheries also illustrate that the Baltic Sea fish populations have fluctuated at decadal and even longer timescales. However, the reasons for the fluctuations are only partly known (MacKenzie et al., 2002). Another example of pioneering marine research is the Piscatorial Atlas by Olsen (1883), illustrating healthy benthic ecosystems with oyster banks in the Kattegat and the North Sea at the end of the 19th century. Today these early expedition data are important sources of information on sea conditions and marine ecosystems before modern industrialization. Comparing this early data with modern information, it is clear that marine ecosystems have undergone large changes, with declines of large fish species, disappearing fish species, and cascading effects that have influenced the marine environment. The reasons for these changes are believed to be high fishing pressure and damaging fishing methods, but eutrophication and spreading of anoxic bottom water have also influenced the ecosystem. The idea that marine resources were limited and sensitive to fishing pressure gained acceptance over the 20th century, but another concern was whether changes in fishing success observed over the past millennium were also related to climate.
Why Is Fishing Success Changing?
Dramatic changes in fishing success have been reported throughout history (e.g., Lamb, 1995) and several long-term datasets are available. For example, information about the herring fishery a millennium back is available for the Swedish coast of the Skagerrak, and nine periods, each several decades long, of large catches have been identified, the latest in 1877–1906. These periods are defined as times when large numbers of herring approach the shore and are thus easy to catch, which strongly influence the economic conditions and society development along the coasts (Figure 3).
The reasons for these periods of plentiful herring have been debated, and studies have found correlations with various climate factors. Alheit and Hagen (1996) demonstrated that at decadal time scales, the periods of abundant herring along the Swedish west coast coincided with cold winters with strong ice cover off Iceland, and they concluded that climate variations govern the alternating periods of herring abundance. The concept of regime shift, or a change with no return, become later more popular in time series interpretation, particularly in the biological science communities. For example, Möllmann and Diekmann (2012) reviewed regime shifts in northern hemisphere marine ecosystems, finding rapid shifts in the late 1980s and early 1990s. One hundred years before, in the late 19th century, concern arose about the well-being of fish stocks, and scientists from various countries realized that international cooperation was needed, leading to the formation of the International Council for the Exploration of the Sea (ICES) in 1902. The international monitoring of oceanographic parameters as well as fish stock distributions and movements was then implemented, giving advice on fishing reductions to balance the increased fishing pressure.
What Can Observation of Winter Conditions Teach Us?
Important information about climate dynamics is obtained from observations of ice and air temperatures. Historical ice observations in the Baltic Sea region are available from a number of sources, such as journals, diaries, customs information, military records, and church notes. Based on these and temperature observations, in the 1940s, Professor Jurva from Helsinki was able to reconstruct the maximum ice extent in the Baltic Sea back to 1720 (Jurva, 1952). The time series of maximum ice extent (MIB) illustrates how winter can vary (Figure 4): in some years, almost the whole Baltic Sea, including the Kattegat, was ice-covered, while in other winters, only about 10% was ice covered In 1877, the time series illustrates a regime shift with the Little Ice Age ending in the Baltic Sea and start of a new and warmer climate period (Omstedt & Chen, 2001).
Similar reconstructions are available for annual ice breakup in Tallinn (Tarand & Nordli, 2001) and ice conditions along the German coast (Koslowski & Glaser, 1999). The last two time series cover periods extending back to 1500. These datasets reveal large temporal variation in which 90% of the variance in the time series is at time scales of less than 15 years (Omstedt et al., 2004). Many aspects of winter climate dynamics have been analyzed, uncovering oscillation patterns, trends, and regime shifts. By putting available long-term datasets for the Baltic Sea region together, Eriksson et al. (2007) demonstrated that for century-scale variability, the records did not suggest strong periodicity; instead, an event-type conceptual model appears adequate to characterize Baltic Sea climate variability. Over the last 500 years, one can identify a number of cold and warm periods, with the 19th-century Little Ice Age, ending late in the 19th century, being the coldest. These time series illustrate the stochastic behavior of climate variability in the region and the importance of taking a long-term perspective when interpreting climate dynamics.
Why Does the Winter Climate Change?
The close relationship between winter conditions in the Baltic Sea region and large-scale atmospheric circulation or the extent of Arctic sea-ice cover has been discussed for many years. For example, the single number characterizing the large-scale atmospheric circulation, the winter North Atlantic Oscillation (NAO) index, is closely correlated with winter temperatures and maximum ice extent in the Baltic Sea (MIB). However, Omstedt and Chen (2001) demonstrated that the correlation between winter NAO and MIB is not stable and instead has changed considerably over time and therefore does not necessary hold in changing climate conditions. Characterizing the complex atmosphere using just a single “golden” parameter and relating this parameter to other single observations, such as ice or herring abundance periods, seems questionable. Climate and biogeochemical data often display stochastic behaviors, and with too few observations, these behaviors can easily be misinterpreted as oscillations, trends, or regime shifts. Simplified statements based on too few observations are misleading and common in science, reflecting that scientists are also influenced by ideas discussed in the society and dependent on available research programs.
The coupled atmosphere–ocean–land–biosphere system is much more complex than shown in simplified statistical relations, as indicated by the fact that statistical correlation may work for some periods but not others. In the late 1990s, coupled atmosphere–ocean–land models started to be developed in the Baltic Sea area (Rummukainen et al., 2001) following developments within global climate modeling but with higher numerical resolution and forced by global climate models outside the regional model domains. In the early developmental stage, only physical processes were considered in these models. More processes, such as biogeochemical and land surface processes, were later incorporated, increasing the need for interdisciplinary research. In model development, it was recognized that different climate models, particularly global models, were giving quite different results. To better interpret the models, outcome ensemble means based on many model storylines were introduced to capture model uncertainty. The importance of learning more about the whole system and not just individual processes was starting to be realized at both the global and regional levels. International networks such as the Intergovernmental Panel on Climate Change (IPCC) were at the forefront of planning how new processes could be added to the coupled models, following a pragmatic approach taking advantage of rapidly increasing computer-processing capability. The close connection between science and societal considerations generated a need for large-scale assessments of both climate science and communication, which were initiated by IPCC in 1990 (IPCC, 1990) and by BALTEX for the Baltic Sea in 2008 (BACC Author Team, 2008).
Is Anoxic Bottom Water Natural or Anthropogenic?
Throughout history, the sea has been regarded as infinite, calling for no limitations of human activity. This idea also dominated in coastal seas, illustrated by the wide range of waste often just dumped into the sea. For example, during the 1960s, the total loads of nutrients from atmospheric and land depositions increased rapidly (Gustafsson et al., 2011; Gustafsson et al., 2012) in the Baltic Sea region because agriculture used high amounts of fertilizers and wastewater treatment from point sources was not yet in place. Eutrophication has become a widespread problem in many coastal areas since the mid-20th century, producing large areas of anoxic waters. As the Baltic Sea is a semi-enclosed brackish water body with strong salinity stratification and limited exchange with the North Sea, stagnation periods are common, resulting in anoxic bottom water. Observations of marine sediments have revealed a shift from laminated to mixed sediments occurring long ago, and anoxic bottom water seems to be a natural part of semi-enclosed coastal water bodies. Systematic investigation of major inflows and stagnant deep waters was started in the 1960s by Fonselius (1962, 1967, 1969). Matthäus (2006) developed the major Baltic inflow (MBI) index by analyzing many MBIs since 1880 and relating them to different driving forces, such as atmospheric circulation and river inflows, improving the understanding of inflow dynamics. The first MBI index was presented by Matthäus and Franck (1992). However, observations indicate that oxygen conditions in the central Baltic Sea have deteriorated since the 1950s (e.g., Hansson and Gustafsson, 2011; Savchuk, 2010), attributable to an increased anthropogenic supply of plant nutrients. Scenario calculation has also indicated that climate warming will have several effects on the primary production and oxygen balance in the Baltic Sea (Meier et al., 2011), increasing the spreading of anoxic waters if successful managements are not in place.
How Will Increasing Atmospheric Carbon Dioxide Concentrations Influence Climate?
In the late 19th century, Arrhenius (1896) demonstrated how carbon dioxide, as one of the GHGs, could influence the air temperature. Variation in atmospheric carbon dioxide was also discussed as a possible explanation for large climate changes, such as ice ages. The topic was then forgotten, and it was mainly believed that human influence was insignificant compared with natural forces, such as solar activity and ocean circulation (Maslin, 2004). Direct measurements of atmospheric carbon dioxide were started by Keeling in the late 1950s, and soon these measurements constituted major evidence of human influence and indications that the ocean was unable to take up all the increasing atmospheric CO2. It was not until the 1980s that new estimates of global mean temperatures indicated global warming, rather than the earlier expected global cooling. The global mean temperature calculations of Mann et al. (1998), followed by several others, provided strong support for the global warming concept at the same time as these calculations were being strongly questioned due to lack of data and the statistical methods used. From long-term climate datasets for the Baltic Sea region, several temperature datasets were corrected and analyzed. Using gridded land station data, it was demonstrated that regional warming had been occurring since the late 19th century (BACC I Author Team, 2008; BACC II Author Team, 2015), with the strongest warming found in the spring.
How Will Increasing Atmospheric Carbon Dioxide Concentrations Influence the Baltic Sea?
With observations of growing carbon dioxide concentrations in the atmosphere, the uptake of carbon dioxide became a major research topic. It was estimated that of anthropogenic CO2 emissions over the 2000–2008 period, 29% were absorbed by land and 26% by oceans, with the remaining 45% staying in the atmosphere (Gattuso & Hansson, 2011). Direct pH observations also indicated decreasing ocean pH (IPCC, 2013), a new and unexpected threat, as the oceans have a high buffering capacity and were believed to be insensitive to acidification. Marine acidification may have several impacts on the marine life as, for example, the way in which it influences the calcium carbonate saturation by reducing the concentrations of carbonate ions and therefore possibly affecting the survival of calcifying organisms (e.g., Riebesell & Tortell, 2011; Andersson et al., 2011; Tyrrell et al., 2008).
Studies of the carbon system in the Baltic Sea have been conducted since the early 20th century. One parameter that has been measured is total alkalinity (Hjalmarsson et al., 2008), which is strongly related to salinity but also depends on the mineralogy of the drainage basins. Although considerable research effort has gone into examining the marine inorganic carbon system of the Baltic Sea, some aspects of the carbon system are missing. Salinity varies from normal ocean values to almost zero and the dissociation constants applied in the carbon system calculations are still problematic, as are the acid–base aspects of organic alkalinity. As eutrophication, acidification, and climate change are closely connected through the primary production and mineralization of organic matter and through atmospheric and river loads, the effects of carbon dioxide concentrations on the Baltic Sea are still only partly understood.
Current State of Regional Climate Science
Coastal Seas in Modern Time Are Under Multiple Anthropogenic Threats
The climate is defined by the statistical description in terms of means and variability of relevant quantities over a period of decades. The statistics are strongly influenced by the latitude and the atmosphere-land surface properties which may vary considerable on regional scale. Some climate aspects at the Baltic Sea regional scale are shown in Figure 5, which uses satellite data based on a 34-year period. The data consist of cloud information from the Advanced Very High Resolution Radiometer (AVHRR) sensor carried by polar-orbiting, operational meteorological satellites (Karlsson et al., 2016).
The regional cloud climate varies greatly over the Baltic Sea and over land during spring and summer, with up to 50% less cloudiness over the sea with a number of consequences for the environment. The figure shows that the underlying land–sea interface strongly influences not only weather time scale but also climate time scale, meaning there is a need to consider the land–sea–atmosphere coupling also on regional scale.
Past centuries have seen the development and organization of regular observation and monitoring programs—major achievements because our basic understanding of the climate depends on available observations. The resulting databases are now available to the research community, partly as station data, but with a growing number of freely available gridded datasets at decadal and centennial time scales. This is a major advance that would be impossible without international cooperation, the rapid development of computer science and the Internet. In the early days, data were available mainly through written reports and from individual scientists or national institutions and were difficult for scientists to access. The free availability of datasets also supports the development of more accurate forcing functions for Baltic Sea models, making it possible to improve the modeling of the Baltic Sea–North Sea system, including the development of coupled land–sea–atmosphere models.
Despite great improvements in observations, models, and international assessment efforts, Baltic Sea ecosystems still face pressure from multiple anthropogenic threats having many unknown effects. Excessive nutrient leakage into the seas, which creates eutrophication and anoxic bottom water with phosphorous-leaking sediments and dead ecosystems, calls for management action. The marine ecosystem is also being degraded by overfishing and threatened by anthropogenic factors, such as heavy metal, organic, and plastic pollution. On top of this, anthropogenic climate change due to increased greenhouse gas (GHG) levels is believed likely to cause new problems, such as warmer water, reduced sea ice, changed salinity, increased areas of anoxic waters, marine acidification, coastal erosion due to increased sea levels and changed winds and waves, and changed marine ecosystems (e.g., Jutterström et al., 2014). Strong societal support is therefore needed for developing successful programs related to sustainable fishery, nutrient loads reduction, and atmospheric CO2 reduction and a healthy Baltic Sea.
Data and Visibility
Reliable long-term datasets freely available to the scientific community and society are the basis for management efforts to reduce the environment’s vulnerability and to enhance the resilience to observed and future changes. Since the 1990s the availability of data in the form of direct observations and from satellites, gridded datasets, proxy data, and model data has grown immensely. Global reanalysis datasets are generally coarse in horizontal resolution, so to resolve the geometry of the Baltic Sea, efforts have begun to generate high-resolution gridded data but these data sets are still not yet evaluated. Oceanographic data have also become much more accessible and new important measurement platforms, such as FerryBoxes and satellites, have provided observations with better temporal and spatial resolution. However, hydrological data, such as river runoff and nutrient and carbon load data, are still difficult to obtain, access relies on contacts with experts, and only data that are several years out of date are available. Marine data are available from ICES, HELCOM, and national institutions, but accompanied by inadequate overview information and supported by weak efforts to provide homogeneous and coherent long-term datasets which allow determination of trends and other climate statistics. Improved expert-designed Web information on complex marine phenomena is needed as a basis for communicating complex information with the public and stakeholders.
Climate Warming and Marine Acidification
Global and regional climate assessments (IPCC, 2013; BACC II Author Team, 2015) have found increasing CO2 concentrations in the atmosphere and warming. The warming in the Baltic Sea region is expected to continue throughout the 21st century. The climate signal is limited to temperature, ice extent, and sea level rise; later climate change is expected to affect the water cycle, including river runoff and nutrient and carbon loads, as well. Regional climate warming is associated with increased atmospheric GHG levels and circulation changes, such as more frequent anticyclonic atmosphere circulation and increased westerlies (BACC II Author Team, 2015). It is plausible that this warming is related to increased atmospheric GHG levels, though formal regional attribution studies are lacking and the relative importance of different drivers has not yet been evaluated.
Since preindustrial times, the atmospheric carbon dioxide level has increased from 280 to 400 ppm with a corresponding increase in the surface partial pressure of carbon dioxide. In marine waters, this implies a decrease in pH of 0.15 units. However, the recent alkalinity increase in the central Baltic Sea has diminished this by roughly 0.03 pH units (BACC II Author Team, 2015). Alkalinity strongly interacts with pH and increased weathering on land may partly counteract the higher atmospheric carbon dioxide concentrations (Figure 6).
On the other hand, a decrease in alkalinity from, for example, acidic precipitation may partly increase the acidification (Müller et al., 2016). The cumulative effects of atmospheric sulphur and nitrogen loads on the Baltic Sea pH balance (Omstedt et al., 2015) indicate that, from the preindustrial state at about 1750 to conditions as of 2010, these effects are one order magnitude less than that of atmospheric carbon dioxide. Climate change impacts on the basin-scale freshwater biogeochemistry are still unknown (BACC II Author Team, 2015) but changes in terrestrial ecosystems—for example, a northward shift in the boreal forest—may significantly influence the biogeochemistry of the region’s rivers. The Baltic Sea is also influenced by river loads of organic material, but organic carbons and organic alkalinity are not yet included in most Baltic Sea models. However, despite the uncertainties, Baltic Sea future acidification is expected to be closely related to increased atmospheric carbon dioxide concentrations, which, together with the pH level, will depend on society’s ability to restrict atmospheric emissions of carbon dioxide.
Changes in Sea Level and Land Rise
Sea level changes in the Baltic Sea are influenced by various factors, including meteorological factors (e.g., wind, air pressure, temperature, net precipitation, and sea ice); astronomical factors (i.e., the sun and moon); hydrological factors (e.g., river runoff); and oceanographic factors (e.g., sea levels in the North Sea-Skagerrak region, water temperature, and salinity), all of which vary over time scales ranging from minutes to millennia and longer. The short- and long-term sea level variations of the Baltic Sea behave differently. For short-term sea level variation over less than one month, the Baltic Sea behaves like a closed basin with variations caused by rapid wind setups and seiche waves. For long-term sea level variations over more than a month, the Baltic Sea behaves like a fjord with the maximum amplitude in the north and the minimum in the south (Samuelsson & Stigebrandt, 1996; Stigebrandt, 1980). The Baltic Sea is one of the best- and longest-sampled sea areas in the world, with tidal gauge datasets covering more than two centuries, the longest of which covers three centuries (e.g., Hünicke et al., 2015). Tidal gauge observations indicate quite different trends and need to be related to a common mean sea level and corrected for the geographical distribution of land uplift. The global sea level rise during the 20th century has been dominated by the contributions of ocean thermal expansion and glacier melting (IPCC, 2013). Ekman (2003) divided the Stockholm sea level record into two periods, 1774–1864 and 1865–2008, and calculated the trends for the two periods to −4.75 mm/year and −0.374 mm/year respectively (Figure 7).
Assuming that the first period reflects conditions before present climate period and giving a land uplift of 4.75 mm/year that continues through the whole record, he could estimate an absolute sea level rise of about 1mm/year since 1865. Later satellite data with measuring lengths of about five to ten years have estimated land rise and showed quite large horizontal variations (Lidberg et al., 2010; Richter et al., 2012). Due to the uncertainty in satellite data in measuring land rise it is not yet possible from satellite to detect whether the absolute Baltic Sea level is rising or not.
Changes in Water Balance and Salinity
The water balance of the Baltic Sea controls the salinity and involves a number of processes, such as inflows and outflows, river runoff, and net precipitation (e.g., Leppäranta & Myrberg, 2009). Groundwater inflow occurs but is estimated to be small. Key processes determining Baltic Sea salinity are the freshwater input, sea-level-driven exchange through the Danish Straits, and turbulent mixing. The Baltic Sea mean salinity displays large-scale variations of typically ~1‰ on a time scale of several decades (Meier & Kauker, 2003a; Winsor et al., 2001, 2003), reflecting the cumulative changes of river runoff. Meier and Kauker (2003b) identified two major causes of mean salinity variability—namely the cumulative changes of river runoff and the low-frequency variations in the wind field over Scandinavia. The salinity stratification is dependent on entrainment from inflowing saline waters and on a complex interaction of turbulent mixing in shallow and deep areas. When sea levels are higher in the Kattegat than the southern Baltic Sea, the Kattegat surface water and more saline water from Skagerrak flows across shallow sills along the bottom. This inflow is the source of the inflowing deep water in the Baltic Sea, which enters and mixes in a number of sub-basins before interleaving in the central Baltic Sea, giving rise to the salinity stratification. When the sea level is higher in the Baltic Sea than in the Kattegat, an outflowing brackish surface water layer mixes with deeper and saltier layers to form the Baltic current along the western coast of Sweden and Norway. The possibility that sea level rise may cause increased salinity inflows is under discussion (e.g., Arneborg, 2016) and a better understanding of changed strait geometry and mixing conditions is needed to be able to estimate the effects of sea level rise due to climate change. The impact of a global sea level rise on hypoxia was examined by Meier et al., (2016) illustrating that the applied model showed rather small effects with a sea level rise of 0.5 meters. However for larger sea level rises this may lead to increased salt water inflows causing higher salinity and increased vertical stratification and expanded dead bottom areas.
Using observed and gridded meteorological and hydrological data, Baltic Sea models can quite realistically simulate the salinity stratification. Climate model scenarios indicate a tendency towards reduced salinities (Meier et al., 2012). However, both global and regional climate models have severe biases in their water and heat balances, so it is still uncertain how salinity will change (e.g., BACC II Author Team, 2015). Baltic Sea calculations based on forcings from climate models must therefore rely on corrections using different kinds of statistical methods (Omstedt et al., 2012). Climate models are often too wet in the Baltic region, calculating unrealistically high river discharges. Donnelly et al. (2014) analyzed river discharge to the Baltic Sea based on climate projections and hydrological modeling. They found uncertainties in atmospheric projections and hydrological modeling and demonstrated that the statistical corrections need to be investigated. Statistical corrections are problematic as they violate all conservation principles and neglect change with time. Predictions of future climate changes in the Baltic Sea salinity are therefore still highly uncertain.
Changes in Oxygen and Nutrient Loads
The oxygen concentration in surface water is related to the air–sea exchange, water temperature, and primary production. Warmer water can dissolve less oxygen than can colder water, implying that climate warming may reduce the surface oxygen concentration and therefore also the oxygen concentration in the inflowing deep water. In the deeper layers, the oxygen concentration depends on the oxygen concentration in the inflowing water and on the oxygen consumption due to the mineralization of biological matter. When oxygen concentrations reach zero, the water is anoxic, causing large changes in chemical reactions and severe damage to marine ecosystems. The threshold for biological life is sometimes roughly estimated to be 2 mg O2 L–1 and water having oxygen concentrations less than 2 mg O2 L–1 is considered hypoxic. Stagnation periods (i.e., periods lacking inflowing dense water) occur naturally in the Baltic Sea due to the restricted exchange in the entrance area. Direct measurements at the sediment–water interface during an inflowing oxygen-rich period illustrate rapid change and improvements, with a changed bacterial community and increased phosphorous retention (Rosenberg et al., 2016). Experiments in pumping oxygen-rich surface water in the By Fjord on the Swedish West Coast (Stigebrandt et al., 2015a) indicate similar dynamics at the sediments-water interface. It has been suggested that to improve ecosystem function, the feasibility of pumping oxygen-rich surface water into deeper Baltic Sea layers should be investigated (Stigebrandt & Gustafsson, 2007; Stigebrandt et al., 2015b).
Since the second half of the 20th century, amount of anoxic and hypoxic waters have increased due to increased anthropogenic nutrient input, anoxic bottom water with phosphorous-leaking sediments, and the Baltic Sea’s natural susceptibility to nutrient enrichment (Andersen et al., 2017). Attribution studies of the causes of Baltic Sea oxygen conditions related to climate warming, change in inflow dynamics, and rivers are needed. The large and saline inflow in 1951, the long stagnation period during the 1980s, and the increased nutrient loads since the 1960s may all have contributed to creating the early-21st-century state of the Baltic Sea. Whether nutrient load reductions on land will help, or whether climate change and phosphorous-leaking anoxic sediments will counteract these efforts, are being debated. Monitoring, research, and systematic assessments are therefore crucial in guiding management efforts to improve the ecological status of the Baltic Sea.
Ecosystems and Climate Change
Salinity, with its strong horizontal and vertical gradients, is no mere oceanographic parameter but, together with poor oxygen conditions, strongly influences the marine ecosystem (Snoeijs-Leijonmalm et al., 2017). Key pelagic species in the Baltic Sea are few and cod, herring, and sprat are dominant. Salinity changes due to climate change may influence both pelagic and benthic ecosystems (BACC II Author Team, 2015) with unpredictable consequences. Even early-21st-century climate conditions may already have had severe consequences for the interaction between different trophic levels. One example of how humans may influence the ecosystem and environmental conditions in the Baltic Sea is through cod overfishing. Strong reduction in cod populations may lead to increased sprat populations, decreasing the amount of zooplankton and possibly allowing more phytoplankton growth, in turn causing further depletion of oxygen. In addition, various fishing optimization measures may have detrimental effects on fish productivity (Svedäng & Hornborg, 2014). Anthropogenic factors such as climate change and high fishing pressure may influence the trophic levels in many ways, possibly eroding ecosystem resilience and influencing environmental conditions, such as algal blooms and oxygen concentrations. The relative importance of various drivers, not only climate change, therefore needs to be evaluated.
Dramatic climate changes have occurred in the Baltic Sea region at many different time scales and with large influence for the humans living in the region. The knowledge of climate has been hampered by the temporal and spatial limitations of observations and by an incomplete understanding of the driving mechanisms, leaving room for speculation as to both the reasons for changes and the role of climate. Past centuries have witnessed major achievements, namely, the development and organization of regular observations; monitoring programs; international assessments addressing fishery, pollution loads and climate change; and atmospheric and hydrologic modeling as well as Baltic Sea modeling. This article has traced the development of climate science of the Baltic Sea region since observations began in the 18th century. The scientific literature is full of crucial knowledge but also misleading errors that are communicated to society. Freely available observations, data products, and models as well as regular assessments are fundamental to science. It must be realized that science is not just a quest to discern the truth; science is also a social process in which researchers are strongly influenced by currently discussed ideas and available research programs.
Why the water level was sinking around the Baltic Sea could not be determined until the idea of postglacial uplift and the thermal history of the Earth were better understood in the 19th century and periodic behavior in climate related time series attracted more scientific interest. Herring fishing successes and failures led to investigations of fishery and climate change and to the realization that fishing itself has strongly negative effects on the marine environment. International assessment efforts were initiated in response to this realization. Scientists later introduced the concept of regime shifts when interpreting abrupt changes in fish stock data. The increasing amount of anoxic deep water in the Baltic Sea and anthropogenic eutrophication has prompted debates about what is natural and what is anthropogenic. The scientific outcome of these debates forms the basis of international management efforts to reduce nutrient leakage from land. However, climate warming may reduce the surface water uptake of oxygen from the atmosphere, and phosphorous-leaking anoxic sediments may counteract management efforts to reduce nutrient fluxes from land.
Observed increases in atmospheric CO2 levels and their effects on global warming have focused the climate debate on trends and generated a series of international and regional assessments and research programs. These have considerably improved our understanding of climate and environmental changes, giving more impetus to earth system science in which both climate and environmental factors are analyzed together.
The Baltic Sea is facing serious climate and environmental threats, and strong international management plans are needed based on the best available knowledge. The multiple observable threats cannot be understood and combated singly. Instead, scientists need to interact in earth system science communities, using detection, attribution, and scenarios as tools to demonstrate to society the main reasons for changes and possible future storylines.
The author wishes to thank Marcus Reckermann, Lennart Bornmalm, Bertil Quirin (who provided Figure 3), Anders Moberg (who provided Figure 2), Martin Ekman (who provided Figure 1), Karl-Göran Karlsson, Abhay Devasthale (who provided Figure 5), Eduardo Zorita for fruitful discussions, and valuable comments from an anonymous reviewer. Also thanks to the Ekman Foundation at University of Gothenburg for economical support and Stephen Sanborn at Proper English for improving the text. This work forms part of the Oxford Research Encyclopedias and the Baltic Earth program.
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