Ole Bøssing Christensen and Erik Kjellström
The ecosystems and the societies of the Baltic Sea region are quite sensitive to fluctuations in climate, and therefore it is expected that anthropogenic climate change will affect the region considerably. With numerical climate models, a large amount of projections of meteorological variables affected by anthropogenic climate change have been performed in the Baltic Sea region for periods reaching the end of this century.
Existing global and regional climate model studies suggest that:
• The future Baltic climate will get warmer, mostly so in winter. Changes increase with time or increasing emissions of greenhouse gases. There is a large spread between different models, but they all project warming. In the northern part of the region, temperature change will be higher than the global average warming.
• Daily minimum temperatures will increase more than average temperature, particularly in winter.
• Future average precipitation amounts will be larger than today. The relative increase is largest in winter. In summer, increases in the far north and decreases in the south are seen in most simulations. In the intermediate region, the sign of change is uncertain.
• Precipitation extremes are expected to increase, though with a higher degree of uncertainty in magnitude compared to projected changes in temperature extremes.
• Future changes in wind speed are highly dependent on changes in the large-scale circulation simulated by global climate models (GCMs). The results do not all agree, and it is not possible to assess whether there will be a general increase or decrease in wind speed in the future.
• Only very small high-altitude mountain areas in a few simulations are projected to experience a reduction in winter snow amount of less than 50%. The southern half of the Baltic Sea region is projected to experience significant reductions in snow amount, with median reductions of around 75%.
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.
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.
The warming of the global climate is expected to continue in the 21st century, although the magnitude of change depends on future anthropogenic greenhouse gas emissions and the sensitivity of climate to them. The regional characteristics and impacts of future climate change in the Baltic Sea countries have been explored since at least the 1990s. Later research has supported many findings from the early studies, but advances in understanding and improved modeling tools have made the picture gradually more comprehensive and more detailed. Nevertheless, many uncertainties still remain.
In the Baltic Sea region, warming is likely to exceed its global average, particularly in winter and in the northern parts of the area. The warming will be accompanied by a general increase in winter precipitation, but in summer, precipitation may either increase or decrease, with a larger chance of drying in the southern than in the northern parts of the region. Despite the increase in winter precipitation, the amount of snow is generally expected to decrease, as a smaller fraction of the precipitation falls as snow and midwinter snowmelt episodes become more common. Changes in windiness are very uncertain, although most projections suggest a slight increase in average wind speed over the Baltic Sea. Climatic extremes are also projected to change, but some of the changes will differ from the corresponding change in mean climate. For example, the lowest winter temperatures are expected to warm even more than the winter mean temperature, and short-term summer precipitation extremes are likely to become more severe, even in the areas where the mean summer precipitation does not increase.
The projected atmospheric changes will be accompanied by an increase in Baltic Sea water temperature, reduced ice cover, and, according to most studies, reduced salinity due to increased precipitation and river runoff. The seasonal cycle of runoff will be modified by changes in precipitation and earlier snowmelt. Global-scale sea level rise also will affect the Baltic Sea, but will be counteracted by glacial isostatic adjustment. According to most projections, in the northern parts of the Baltic Sea, the latter will still dominate, leading to a continued, although decelerated, decrease in relative sea level. The changes in the physical environment and climate will have a number of environmental impacts on, for example, atmospheric chemistry, freshwater and marine biogeochemistry, ecosystems, and coastal erosion. However, future environmental change in the region will be affected by several interrelated factors. Climate change is only one of them, and in many cases its effects may be exceeded by other anthropogenic changes.
The growing concern about global warming has turned focus in Sweden and other Baltic countries toward the connection between history and climate. Important steps have been taken in the scientific reconstruction of climatic parables. Historic climate data have been published and analyzed, and various proxy data have been used to reconstruct historic climate curves. The results have revealed an ongoing regional warming from the late 17th to the early 21st century. The development was not continuous, however, but went on in a sequence of warmer and colder phases.
Within the fields of history and socially oriented climate research, the industrial revolution has often been seen as a watershed between an older and a younger climate regime. The breakthrough of the industrial society was a major social change with the power to influence climate. Before this turning point, man and society were climate dependent. Weather and short-term climate fluctuations had major impacts on agrarian culture. When the crops failed several years in sequence, starvation and excess mortality followed. As late as 1867–1869, northern Sweden and Finland were struck by starvation due to massive crop failures.
Although economic activities in the agricultural sector had climatic effects before the industrial society, when industrialization took off in Sweden in the 1880s it brought an end to the large-scale starvations, but also the start of an economic development that began to affect the atmosphere in a new and broader way. The industrial society, with its population growth and urbanization, created climate effects. Originally, however, the industrial outlets were not seen as problems. In the 18th century, it was thought that agricultural cultivation could improve the climate, and several decades after the industrial take-off there still was no environmental discourse in the Swedish debate. On the contrary, many leading debaters and politicians saw the tall chimneys, cars, and airplanes as hopeful signs in the sky. It was not until the late 1960s that the international environmental discourse reached Sweden. The modern climate debate started to make its imprints as late as the 1990s.
During the last two decades, the Swedish temperature curve has unambiguously turned upwards. Thus, parallel to the international debate, the climate issue has entered the political agenda in Sweden and the other Nordic countries. The latest development has created a broad political consensus in favor of ambitious climate goals, and the people have gradually started to adapt their consumption and lifestyles to the new prerequisites.Although historic climate research in Sweden has had a remarkable expansion in the last decades, it still leans too much on its climate change leg. The clear connection between the climate fluctuations during the last 300 years and the major social changes that took place in these centuries needs to be further studied.