Baltic Earth is an independent research network of scientists from
all Baltic Sea countries that promotes regional Earth system research.
Within the framework of this network, the Baltic Earth Assessment Reports
(BEARs) were produced in the period 2019–2022. These are a collection of 10 review articles summarising current knowledge on the environmental and
climatic state of the Earth system in the Baltic Sea region and its changes
in the past (palaeoclimate), present (historical period with instrumental
observations) and prospective future (until 2100) caused by natural
variability, climate change and other human activities. The division of
topics among articles follows the grand challenges and selected themes of
the Baltic Earth Science Plan, such as the regional water, biogeochemical
and carbon cycles; extremes and natural hazards; sea-level dynamics and
coastal erosion; marine ecosystems; coupled Earth system models; scenario
simulations for the regional atmosphere and the Baltic Sea; and climate
change and impacts of human use. Each review article contains an
introduction, the current state of knowledge, knowledge gaps, conclusions
and key messages; the latter are the bases on which recommendations for future research are
made. Based on the BEARs, Baltic Earth has published an information leaflet
on climate change in the Baltic Sea as part of its outreach work, which has
been published in two languages so far, and organised conferences and
workshops for stakeholders, in collaboration with the Baltic Marine
Environment Protection Commission (Helsinki Commission, HELCOM).
IntroductionBALTEX/Baltic Earth history
Baltic Earth (https://baltic.earth, last access: 4
February 2023) is an international research network dealing with Earth
system sciences in the Baltic Sea region (Fig. 1). It is politically
independent and focuses on research about the water and energy cycles, climate variability and climate change, water management and extreme events, and associated impacts on marine and terrestrial biogeochemical cycles. Research about the human impact on the Earth system in more general terms, i.e. the
anthroposphere, defined as the part of the environment created or modified
by humans for use by human activities, was also included in the Baltic Earth
Science Plan (2017) (https://baltic.earth/grandchallenges, last access: 4 February 2023).
The Baltic Sea and its catchment area with climatological mean sea
surface salinity (in g kg-1) and river discharge (in mm yr-1).
(Source: Meier et al., 2014; their Fig. 1 distributed under the terms of the
Creative Commons CC-BY 4.0 License, http://creativecommons.org/licenses/by/4.0/, last access: 4 February 2023.)
Baltic Earth is the successor of the Baltic Sea Experiment (BALTEX)
programme, which was founded in 1993 as a GEWEX (Global Energy and Water Exchanges) continental-scale experiment (a core project of the World Climate
Research Programme) (Reckermann et al., 2011). In the first phase
(1993–2002), BALTEX was primarily devoted to hydrological, meteorological
and oceanographic processes in the Baltic Sea drainage basin and thus
focused on physical aspects of the Earth system. In the second phase
(2003–2012), the programme was expanded to include regional climate
research, biogeochemical cycles including carbon, engagement with
stakeholders and decision-makers via assessment reports; and
communication and education, i.e. the organisation of summer and winter
schools and international master courses.
In 2013, Baltic Earth was launched with a new science plan to strengthen
efforts to address grand challenges on (1) salinity dynamics in the Baltic
Sea, (2) biogeochemical linkages between land and sea, (3) natural hazards
and extreme events, (4) sea-level and coastal dynamics, (5) regional
variability in water and energy exchanges, and (6) multiple drivers of
regional Earth system changes (Meier et al., 2014). Working groups on
coupled Earth system models; the Baltic Sea Model Intercomparison Project
(BMIP); uncertainty of scenario simulations for the Baltic Sea; and
education, outreach and communication have been established.
Baltic Earth and its predecessor BALTEX have produced three comprehensive
regional assessment reports since 2008. The first two (The BACC (Assessment of Climate Change in the Baltic Sea Basin) Author Team,
2008, and The BACC II Author Team, 2015) focused on climate change and its
impacts in the Baltic Sea region; they were published as textbooks. The
third, the Baltic Earth Assessment Reports (BEARs), was published in the
format of a special issue of the journal Earth System Dynamics in 2022. The BACC reports (https://baltic.earth/bacc, last access: 4 February 2023) and BEARs fill a
gap compared to the assessment reports of the Intergovernmental Panel on
Climate Change (IPCC), as the latter focus on global scales and do not
provide detailed local to regional information about the current state of
knowledge on climate change and its impacts in the Baltic Sea region. The
BEARs provide a comprehensive and up-to-date overview of the
state-of-the-art research on the compartments of the Earth system in the
Baltic Sea region that encompasses processes in the atmosphere, on land and in
the sea, including the marine and terrestrial ecosystems as well as
processes and impacts related to the anthroposphere.
The BEARs summarise the published scientific knowledge currently available
and update the second BACC report (The BACC II Author Team, 2015) based on
the latest scientific literature. The BEAR special issue includes 10
articles on the Baltic Earth grand challenges and Baltic Earth special
topics (Baltic Earth Science Plan, 2017), including a summary of current
knowledge on past, present and future climate change in the Baltic Sea
region. The articles encompass contributions from 109 authors from 14
countries and reference 2822 scientific articles and institutional reports.
Baltic Sea region characteristics
The Baltic Sea is a semi-enclosed, shallow sea with limited water exchange
with the World Ocean and small tidal amplitudes. Located in northern Europe,
the climate of the region is highly variable as it is in the transition zone
between maritime and continental climates and is influenced by the North
Atlantic and Arctic. River discharges from the large catchment area cause a
pronounced gradient in sea surface salinity from about 20 g kg-1 in the region of the Danish straits to about 2 g kg-1 or even less in the northern
and eastern reaches of the Baltic Sea. Hence, the Baltic Sea is brackish,
with habitats of marine species in the south-west and freshwater species in
the north-east. The Baltic Sea catchment area is about 4 times the
surface area of the Baltic Sea and covers an area of almost 20 % of the
European continent (Fig. 1). It stretches from the temperate, densely
populated south to the subarctic wilderness in the north; it is home to
approximately 85 million people in 14 countries, namely Belarus, the Czech
Republic, Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Norway,
Poland, Russia, Slovakia, Sweden and Ukraine.
Episodically, large amounts of saline water flow from the North Sea over the
sills in the Danish straits into the Baltic Sea and ventilate the deep
waters of the Baltic Sea. These events require a period of about 20 d
with easterly winds that lower the sea level in the Baltic Sea, followed by
a period of about the same length with strong westerly winds that push
saline water into the Baltic Sea. These events are called Major Baltic
Inflows (MBIs) and are important for the water exchange between the North
Sea and the Baltic Sea. Mixing is low compared to other seas, with an origin
at the lateral boundaries because tidal amplitudes are very small and
energetically insignificant.
In recent decades, environmental conditions in the Baltic Sea have changed
considerably. For instance, the Baltic Sea has been warming more than any
other coastal sea since 1980 (Fig. 2), which has led to a reduction in sea
ice and snow cover over the land in winter. Furthermore, increasing nutrient
input from the land in the 1950s and 1960s, caused by population growth and the
discharge of sewage into the Baltic Sea, as well as the increased use of
fertilisers in agriculture, led to eutrophication and the spread of hypoxic
and anoxic areas. Since the 1980s, nutrient inputs into the Baltic Sea have
been steadily decreasing, but this has not yet led to a significant
improvement in oxygen conditions. Recent trends in acidification are lower
than in the World Ocean, especially in the northern Baltic Sea, as positive
trends in alkalinity input counteract acidification.
Annual mean sea surface temperature anomalies relative to the
reference period 2002–2018 from de-seasonalised measurements at the Arkona
Deep monitoring station and the MARNET stations (Darss Sill and Arkona Basin)
in the period 1980–2018. (Source: Meier et al., 2022b; their Fig. 20
distributed under the terms of the Creative Commons CC-BY 4.0 License,
http://creativecommons.org/licenses/by/4.0/, last access: 4
February 2023.)
Methods
Succeeding The BACC Author Team (2008) and The BACC II Author Team (2015)
assessments, the BEAR project is an attempt to summarise the scientific
knowledge on climate change and other drivers of Earth system changes and
their impacts on the Baltic Sea region. The two BACC books have a format
inspired by the IPCC assessment reports. This special issue in Earth System Dynamics is the third
assessment. It has a new format of BEARs, encompassing 10 peer-reviewed
scientific journal articles. The knowledge assessed was extracted from the
scientific literature such as peer-reviewed articles, reports from research
institutions and published datasets. Importantly, literature from
non-governmental organisations with political or economic interests,
political parties and other stakeholder organisations was excluded from the
assessment to ensure that only scientific knowledge was included in the
assessment. The BEARs focus on publications after 2013, the year of the
editorial deadline of the second assessment report. Whenever possible, the
uncertainty levels of the BEAR results are ranked based on a matrix of
consensus within the scientific literature as well as documented evidence of
detected changes and their attributed drivers such as climate change and
human use. A high level of scientific consensus and evidence is required for
high confidence in a particular statement. Disagreements and gaps in
knowledge are documented and discussed to prioritise future research.
Together with the intergovernmental Baltic Marine Environment Protection
Commission (Helsinki Commission, HELCOM), Baltic Earth has established an Expert Network on
Climate Change (EN CLIME). The aim of the expert network is to regularly
produce a climate change fact sheet (CCFS, 2021, https://helcom.fi/wp-content/uploads/2021/09/Baltic-Sea-Climate-Change-Fact-Sheet-2021.pdf,
last access: 4 February 2023) based on the BEAR and BACC material. In 2021,
it was published for the first time (http://helcom.fi/ccfs, last access: 4 February 2023). The CCFS contains
some background information; a map showing regional future climate changes
for selected parameters under the greenhouse gas concentration scenario
RCP4.5; and information on 34 variables, directly and indirectly affected by
climate change. For each parameter, a general description, past and
prospective future changes, drivers other than climate change (only for the
indirect parameters), knowledge gaps, policy relevance and references are
presented. More than 100 scientists contributed to the compilation of the
first fact sheet, which was coordinated by the HELCOM secretariat. Updated
versions are planned at 7-year intervals. Like the BEARs, the fact sheet
was peer-reviewed and quality-assured. So far, it has only been translated into
German (Klimawandel in der Ostsee: 2021 Faktenblatt, 2022, https://baltic.earth/ccfs, last access: 4 February 2023), but
translations into other languages are planned to improve accessibility to
stakeholders.
In this overview article, we highlight the key findings and knowledge gaps as
described by the BEARs and propose future work.
Results
Some of the key findings of the 10 BEARs are selected and highlighted below.
Salinity dynamics of the Baltic Sea, Baltic Earth Grand Challenge 1 (Lehmann et al.,
2022). Salinity is an important parameter for the circulation and the marine ecosystem in the Baltic Sea. Any changes in salinity are caused by changes in either the freshwater inflow from rivers and net precipitation over sea
or the water exchange between the Baltic Sea and the adjacent North Sea.
Although long-term records of salinity and its drivers suffer from data
gaps, these records starting in the 19th century are globally unique.
Major research efforts focused on the MBI event in 2014 and its consequences
for water masses, oxygen concentration and biogeochemical cycling. During
the event, an unexpected large contribution of oxic intrusions at
intermediate depth and essentially nonturbulent conditions in the deep
interior were found, emphasising the importance of boundary mixing. A
revised reconstruction of the long-term record of MBIs showed no trend but a
pronounced multidecadal variability with a period of about 30 years.
Despite intense research activities, observed variations in the intensity
and frequency of MBIs and related large volume changes (LVCs) could not be
attributed to atmospheric circulation variability. Hence, on timescales
larger than the synoptical timescale, MBIs are not predictable. As an
advance over the previous assessments, salinity dynamics of the various
subbasins and lagoons mainly based on observations have been discussed,
documenting large regional differences.
Biogeochemical functioning of the Baltic Sea, Baltic Earth Grand Challenge 2
(Kuliński et al., 2022). The review addresses the following topics: (1)
terrestrial biogeochemical processes and nutrient inputs to the Baltic Sea,
(2) the transformation of C, N and P in the coastal zone, (3) the production
and remineralisation of organic matter, (4) oxygen availability, (5) the
burial and turnover of C, N and P in sediments, (6) the Baltic Sea CO2
system and seawater acidification, (7) the role of certain microorganisms in
the biogeochemistry of the Baltic Sea, and (8) the interactions between
biogeochemical processes and chemical pollutants. It was found that oxygen
depletion and the area of anoxic bottoms still increased despite the
reductions in nutrient inputs from land since the 1980s. Hence, the nitrogen
pool has declined due to denitrification, whereas the phosphorus inventory
has increased. Estimates suggest that about 1 % and 4 % of the annual
nitrogen and phosphorus loads, respectively, have accumulated in the Baltic
Sea, while the remainder are either exported to the North Sea or lost via
biogeochemical processes such as denitrification and burial. Furthermore, it
was discovered that in the central and northern subbasins, the uptake of C,
N and P during primary production does not correspond to the Redfield ratio,
which strongly affects the relationship between primary production, export
of organic matter and oxygen demand of the deep sea. While it is clear that
the Baltic Sea is a CO2 sink in summer and a CO2 source in winter,
the annual net balance remains unknown. The past increase in total
alkalinity of unknown origin has entirely mitigated ocean acidification in
the northern Baltic Sea and significantly reduced it in the central Baltic
Sea. In the future, a doubling of atmospheric pCO2 would still result
in lower pH in the entire Baltic Sea, even if alkalinity should further
increase.
Natural hazards and extreme events in the Baltic Sea region, Baltic Earth Grand Challenge
3 (Rutgersson et al., 2022). Existing knowledge is summarised about extreme
events in the Baltic Sea region with a focus on the past 200 years with
instrumental data as well as future projections. Considered events are
windstorms, extreme waves, high and low sea levels, hot and cold spells in
the atmosphere, marine heat waves, droughts, sea-effect snowfall, sea-ice
ridging, extremely mild and extremely severe sea-ice winters, heavy
precipitation events, river floods, and extreme phytoplankton blooms.
Furthermore, the knowledge about implications of these extreme events for
society, such as forest fires, coastal flooding, offshore infrastructure and
shipping, was assessed. With respect to the impacts of climate change,
terrestrial and marine heat waves, extremely mild sea-ice winters, heavy
precipitation and high-flow events are expected to increase, while cold
spells, severe sea-ice winters and sea-ice ridging are expected to decrease
due to the increase in mean atmospheric temperature. Changes in relative sea-level extremes will depend on the competing impacts of the rising global
mean sea level, the gravitational effect of the melting of the Greenland and
Antarctic ice sheets, changes in wind fields, and the regionally differing
glacial isostatic adjustment (GIA) resulting in land uplift or subsidence.
Furthermore, projections suggest an increase of droughts in the southern and
central parts of the Baltic Sea region, mainly in summer. Significant future
changes in windstorms, extreme waves and sea-level extremes relative to the
mean sea level have not been found, suggesting that these changes will
likely be small compared with natural variability.
Sea-level dynamics and coastal erosion in the Baltic Sea region, Grand
Challenge 4 (Weisse et al., 2021). In this study, the current knowledge
about the diverse processes affecting mean and extreme sea-level changes,
coastal erosion and sedimentation with impacts on coastline changes and
coastal management is assessed. Such processes are GIAs, contributions from
global sea-level changes, windstorms, wind waves, seiches or meteotsunamis.
During 1886–2020, the mean absolute sea level in the Baltic Sea corrected
for the GIA increased by about 25 cm or ∼ 2 mm yr-1 on
average. Land uplift in the north is still faster than the absolute sea-level rise, while in the south, the opposite is true with potential impacts on
changes in coastal erosion and inundation. The current acceleration of sea-level rise is small and could only be determined by spatially averaging
observations at different tide gauge locations. Future sea-level rise in the
Baltic Sea is expected to further accelerate, probably somewhat less than
the global mean sea-level rise. The Baltic sea level is substantially more
sensitive to melting from the Antarctic than from the Greenland ice sheet.
Concerning sediment transports, the dominance of mobile sediments makes the
southern and eastern Baltic Sea coasts susceptible to wind-wave-induced
transports, particularly during storms. Due to the global sea-level rise,
future sediment transports can be expected to increase in these coastal
areas, with a large spatial variability depending on the angles of incidence
of incoming wind waves.
Human impacts and their interactions in the Baltic Sea region, Baltic Earth Grand
Challenge 6 (Reckermann et al., 2022). An inventory and discussion of the
various man-made factors and processes affecting the environment of the
Baltic Sea region and their interrelationships are presented. In total, 19
factors are addressed (Table 1). Some of the factors are natural and are
only modified by human activities (e.g. climate change, coastal processes,
hypoxia, acidification, submarine groundwater discharges, marine ecosystems,
non-indigenous species, land use and land cover); others are entirely
man-made (e.g. agriculture, aquaculture, fisheries, river regulation,
offshore wind farms, shipping, chemical contamination, dumped ammunition,
marine litter and microplastics, tourism and coastal management). All
factors are interconnected to varying degrees. The knowledge of these
linkages was assessed and analysed in depth. The main finding was that
climate change has an overarching, integrating effect on all other factors
and can be interpreted as a background effect that affects the other factors
differently. After climate change, shipping and land use/agriculture are the
factors affecting most other factors, while fisheries, marine ecosystems and
agriculture in turn are the most affected. The results of the assessment
depend on the region and may be different for other coastal seas and their
catchments in the world, where different human activities prevail.
The matrix of factors studied by Reckermann et al. (2022). +=
evidence for a connection; -= no evidence for a connection; ?= no
evidence but a connection is plausible (according to the author's assessment).
The table is read from left to right, i.e. if you go to the right in the
first row “Climate change”, you see the factors on which climate change has
an effect (or not), etc. (Source: Reckermann et al., 2022; their Table 2a
distributed under the terms of the Creative Commons CC-BY 4.0 License,
http://creativecommons.org/licenses/by/4.0/, last access: 4
February 2023.)
Global climate change and the Baltic Sea ecosystem: direct and indirect effects on species, communities and ecosystem functioning, Baltic Earth
Special Topic (Viitasalo and Bonsdorff, 2022). Climate change has multiple
impacts on species, communities and ecosystem functioning in the Baltic Sea
through changes in physical and biogeochemical parameters such as
temperature, salinity, oxygen, pH and nutrient levels. The associated
secondary effects on species interactions, trophic dynamics and ecosystem
function are also likely to be important. Climate change (warming, recent
brightening, decrease in sea ice) has led to shifts in the seasonality of
primary production, with a prolonged growing season of phytoplankton, an
earlier onset of the spring bloom and a delayed autumn bloom. However, the
development of cyanobacteria varies from species to species, and a clear
causal relationship between temperature or salinity and the abundance of
cyanobacteria has not been demonstrated. An increase in water temperature
and river input of dissolved organic matter (DOM) could reduce primary
production while favouring bacterial growth. If nutrient reduction
continues, the improvement in oxygen conditions could initially increase
zoobenthos biomass, but the subsequent decrease in sedimenting organic
matter would likely disrupt the pelagic–benthic coupling and result in lower
zoobenthos biomass. Sprat and some coastal fish species could be favoured by
a rise in temperature. Regime shifts and cascading effects have already been
observed in both pelagic and benthic systems as a result of climate change.
Coupled regional Earth system modelling in the Baltic Sea region, Baltic
Earth Special Topic, with relevance to Baltic Earth Grand Challenge 5
(Gröger et al., 2021). Recent progress in the development of coupled
climate models for the Baltic Sea region is assessed. Feedback mechanisms
are important to simulate the response of the Earth system to external
forcing such as greenhouse gas and aerosol emissions. In this review
article, the couplings between (1) atmosphere, sea ice and ocean; (2)
atmosphere and land surface including dynamic vegetation; (3) atmosphere,
ocean and waves; and (4) atmosphere and hydrological components to close the
water cycle are discussed. Adding surface waves to coupled atmosphere–ocean
system models is becoming more important with increasing resolution, in
particular when detailed information is required, for instance, for offshore
wind-energy applications in the coastal zone. Furthermore, the wave
information is essential for the calculation of ocean mixing and
resuspension. While long-term climate simulations using coupled atmosphere,
sea-ice and ocean models, or coupled atmosphere and dynamic vegetation models
have successfully been performed and their added value demonstrated, the
impact of aerosols on the climate of the Baltic Sea region has not been
considered. Coupling hydrology models to close the hydrological cycle is
also still problematic, as the precipitation accuracy provided by the
atmospheric models is, in most cases, insufficient to realistically simulate
river discharge into the Baltic Sea without bias adjustments.
Atmospheric regional climate projections for the Baltic Sea region until 2100, Baltic Earth Special Topic (Christensen et al., 2022). Current climate
projections based on regional climate atmosphere-only models of the
EURO-CORDEX (Coordinated Regional Climate Downscaling Experiment) project with a horizontal resolution of 12.5 km under the
scenarios RCP2.6, 4.5 and 8.5 are presented. As the number of simulations
(124) is relatively large compared to previous assessments, the
uncertainties can be better estimated than before. These projections
indicate strong warming, especially in the north in winter, where warming
approaches twice the average global warming. Precipitation is projected to
increase throughout the Baltic Sea region, except in the southern half in
summer, where the results are inconclusive. Extreme precipitation, here the
10-year return value, is projected to increase systematically throughout the
study area, especially in summer. The large ensemble of simulations does not
indicate a significant change in wind speed. Surface solar radiation is
projected to remain unchanged in summer but to decrease slightly in winter
due to increased cloud cover and possibly less snow in the future. Snow
cover is projected to decrease dramatically, especially in the south of the
Baltic Sea catchment. The comparison between the uncoupled model simulations
of the EURO-CORDEX project and a small ensemble of scenario simulations
performed with a coupled atmosphere–sea-ice–ocean model driven by a subset
of global climate models indicates stronger warming in the coupled model
during winter, mainly in areas that are seasonally affected by sea ice
today. In summer, the coupled model shows weaker warming compared to the
uncoupled models.
Oceanographic regional climate projections for the Baltic Sea until 2100,
Baltic Earth Special Topic (Meier et al., 2022a). New projections of the
future Baltic Sea climate with a coupled physical–biogeochemical ocean model
were compared with previous projections. The differences are mainly due to
different scenario assumptions and model setups. For example, the impact of
future global sea-level rise on salinity was previously neglected but taken
into account in the latest projections. Although the number of projections
for the Baltic Sea is still small compared to regional atmospheric
projections such as the EURO-CORDEX model ensemble, a relatively large
ensemble of 48 scenario simulations allowed the assessment of uncertainties
related to greenhouse gas emissions, global climate model differences,
global sea-level rise, nutrient inputs and natural variability. In the
future climate, higher water temperatures, a shallower mixed layer with a
sharper thermocline in summer, lower sea-ice cover and stronger mixing in
the northern Baltic Sea during winter compared to the current climate could be
expected. The assessment of marine heat wave changes is new. Both the
frequency and duration of marine heat waves are projected to increase
significantly, especially in the coastal zone of the southern Baltic Sea.
Due to uncertainties in the projections regarding regional winds,
precipitation and global sea-level rise, no robust and statistically
significant changes in salinity could be identified. The impacts of a
changing climate on the biogeochemical cycle are projected to be
significant but still less than the plausible changes in nutrient inputs.
Implementation of the proposed Baltic Sea Action Plan, a basin-wide nutrient
input reduction plan, would lead to a significant improvement in the
ecological status of the Baltic Sea, including a reduction in the size of
the hypoxic area in a future climate as well.
Climate change in the Baltic Sea region: a summary, Baltic Earth Special
Topic (Meier et al., 2022b). In this comprehensive study, the recent
knowledge on past (palaeoclimate), present (historical) and projected future
(<2100) climate change in the Baltic Sea region, based upon all
BEARs and >800 scientific articles, is summarised. It focuses on
the atmosphere, the land surface, the cryosphere, the ocean and its
sediments, and the terrestrial and marine biospheres. Thirty-three parameters
characterising the state of these components of the Earth system were
analysed (Fig. 3, Table 2). The anthroposphere is not part of this
assessment by Meier et al. (2022b) but is discussed in detail by
Reckermann et al. (2022) instead. The main findings concerning changes of the 33
selected state parameters attributed to climate change are summarised in
Fig. 3. The prevailing causal relationships of climate change with
sufficiently high confidence suggest a clear impact of global greenhouse gas
emissions on regional heat cycles including all parameters of the
cryosphere. However, changes caused by global warming of the water, momentum
and carbon cycles are less clear because of either the large natural
variability at regional scales or the impact of drivers other than global
warming. For further details, the reader is referred to Meier et al. (2022b). Overall, it was concluded that the results from the previous BACC
assessments are mainly still valid. However, new long-term, homogenous
observational records, such as those for Scandinavian glacier inventories,
sea-level-driven saltwater inflows (MBIs) or phytoplankton species
distributions, and new scenario simulations with improved models, such as
those for glaciers, lake ice or marine food webs, have become available,
resulting in a revised understanding of observed changes. Compared to
previous assessments, observed changes in air temperature, sea ice, snow
cover, and sea level were shown to have accelerated. However, natural
variability is large, challenging our ability to detect observed and
projected changes in the climate of the Baltic Sea region. As the ensembles of
scenario simulations for both the atmosphere and the ocean became larger,
uncertainties can now be better estimated, although coordinated scenario
simulations for the Baltic Sea based on ensembles of different regional
ocean models are still missing. Furthermore, with the help of coupled
models, feedback loops between several components of the Earth system have been
studied, and multiple driver studies were performed, e.g. projections of
the marine food web that include fisheries, eutrophication and climate
change. Intensive research on the land–sea interface, focusing on the
coastal filter, has been performed, and nutrient retention in the coastal
zone was estimated for the first time. However, a model for the entire
Baltic Sea coastal zone is still missing, and the effect of climate change
on the coastal filter capacity is still unknown. More research on changing
extremes was performed, acknowledging that the impact of changing extremes
may be more important than that of changing means (see also Rutgersson et
al., 2022). However, many observational records are either too short or too
heterogeneous for statistical studies of extremes due to data gaps.
Variables of the Meier et al. (2022b) assessment and further
references to the BEARs (1: Lehmann et al., 2022; 2: Kuliński et al.,
2022; 3: Rutgersson et al., 2022; 4: Weisse et al., 2021; 5: Reckermann et
al., 2022; 6: Gröger et al., 2021; 7: Christensen et al., 2022; 8: Meier et
al., 2022a; 9: Viitasalo and Bonsdorff, 2022). The third column lists the
subsection in the study by Meier et al. (2022b) that contains further
information. (Source: Meier et al., 2022b; their Table 2 distributed under
the terms of the Creative Commons CC-BY 4.0 License, http://creativecommons.org/licenses/by/4.0/, last access: 4 February 2023.)
NumberVariablePast and present climates Future climate Atmosphere 1Large-scale atmosphericcirculation3.2.1.133.3.1.13, 72Air temperature Warm spell Cold spell3.1.2, 3.1.3, 3.1.4 3.2.1.2 3 33.3.1.27 3 33Solar radiation and cloudiness3.2.1.33.3.1.374Precipitation Heavy precipitation Drought3.1.2, 3.1.3, 3.1.4 3.2.1.43 33.3.1.47 3 35Wind Storm3.2.1.533.3.1.57 36Air pollution, air quality and atmospheric deposition3.2.1.63.3.1.6Land 7River discharge High flow3.2.2.133.3.2.18 38Land nutrient inputs3.2.2.23.3.2.28Terrestrial biosphere 9Land cover (forest, crops,grassland, peatland, mires)3.2.363.3.310Carbon sequestration3.3.3Cryosphere 11Snow Sea-effect snowfall3.2.4.133.3.4.17 312Glaciers3.2.4.23.3.4.213Permafrost3.2.4.33.3.4.314Sea ice Extreme mild winter Severe winter Ice ridging3.2.4.43 3 33.3.4.48 3 3 315Lake ice3.2.4.53.3.4.5Ocean and marine sediments 16Water temperature Marine heat wave3.2.5.133.3.5.18 317Salinity and saltwater inflows3.2.5.213.3.5.2818Stratification and overturningcirculation3.2.5.313.3.5.38
Continued.
NumberVariablePast and present climates Future climate 19Sea level Sea-level extreme3.2.5.44 33.3.5.48 320Waves Extreme waves3.2.5.54 33.3.5.5321Sedimentation and coastal erosion3.2.5.643.3.5.622Oxygen and nutrients3.1.4 3.2.5.7.123.3.5.7.1823Marine CO2 system3.2.5.7.223.3.5.7.2Marine biosphere 24Pelagic habitats: microbialcommunities3.2.6.1.12, 93.3.6.1.1925Pelagic habitats: phytoplankton and cyanobacteria3.2.6.1.22, 3, 93.3.6.1.23, 926Pelagic habitats: zooplankton3.2.6.1.393.3.6.1.3927Benthic habitats: macroalgaeand vascular plants3.2.6.2.193.3.6.2.1928Benthic habitats: zoobenthos3.2.6.2.293.3.6.2.2929Non-indigenous species3.2.6.393.3.6.3930Fish3.2.6.493.3.6.4931Marine mammals3.2.6.593.3.6.5932Waterbirds3.2.6.693.3.6.6933Marine food web3.2.6.793.3.6.79
Synthesis of knowledge on present and future climate change. Shown
are anthropogenic climate changes in 33 Earth system variables (bubbles) of
the atmosphere (yellow), land surface (brown), terrestrial biosphere (dark
green), cryosphere (grey), ocean and sediments (blue) as well as marine biosphere
(light green). The abbreviation NIS stands for non-indigenous species. The
sign of a change (plus/minus) is shown together with the confidence level
indicated by the number of signs, i.e. one to three signs correspond to a
low, medium and high confidence level as a result of the literature
assessment reflecting consensus and evidence according to IPCC definitions.
The colours of the signs indicate the direction of past (black) and future
(red) changes according to Meier et al. (2022b). Uncertain changes are not
shown. The external anthropogenic drivers of the Earth system studied are
shown as red squares, i.e. greenhouse gases, especially CO2, and
aerosol emissions. The predominant climate change linkages with sufficiently
high confidence are shown by arrows (black: thermal cycle, blue:
hydrological cycle, orange: momentum cycle including sea-level change, pink:
carbon cycle). Projections of carbon sequestration of Arctic terrestrial
ecosystems for the 21st century first show an increased uptake and
later a carbon source, marked by “1. + 2. –”. Future changes in mean
sea level are dominated by the thermal expansion of the global ocean and the
melting of ice sheets outside the Baltic Sea region. (Source: Meier et al.,
2022b; their Fig. 35 distributed under the terms of the Creative Commons
CC-BY 4.0 License, http://creativecommons.org/licenses/by/4.0/,
last access: 4 February 2023.)
Discussion
One of the main objectives of the BEAR project was to identify knowledge
gaps in the Earth system science of the Baltic Sea region so that these can
be further addressed in future research. For specific knowledge gaps that
have been identified during the project, the reader is referred to the
individual assessment reports. However, as an overarching result, three new
research topics are identified.
Small-scale processes and their impact on large-scale climate dynamics and biogeochemical cycles. The number of observations in the sea is smaller than that on land. This is also true for the Baltic Sea, although the
international long-term monitoring programme in the Baltic Sea started more
than a century ago, with measurements of temperature, salinity and oxygen
concentration in the central parts of the different subbasins. Nowadays,
monitoring data are available from all subbasins with a resolution of up to
1 month. Recently, many new observational systems for temporally and
spatially high-resolution data have been developed or are under development,
including remotely operated vehicles (ROVs) and autonomous underwater
vehicles (AOVs) as well as remote-sensing data. Examples of such systems
operating in the Baltic Sea are continuously profiling moorings, ARGO
floats, Gliders, ScanFish and echo sounders. In addition to traditional
physical parameters, measurements of turbulence, biogeochemical and
biodiversity (e.g. environmental DNA) parameters are now available. Another
area of research that is developing rapidly is numerical modelling of the
Earth system, also on a regional scale, e.g. eddy- and submesoscale
resolving multiyear simulations for the Baltic Sea. Similar arguments apply
to the atmosphere, e.g. cloud-resolving simulations to cope with heavy
precipitation events. A novel research topic for Baltic Earth would
therefore be a better understanding of the dynamics of small-scale
atmospheric and oceanic processes that are not yet resolved in
state-of-the-art numerical models or conventional observations and their
role in the large-scale circulation on short and long timescales. Such
research activities would help to fill some of the gaps in knowledge that
have been raised by Lehmann et al. (2022), Kuliński et al. (2022),
Rutgersson et al. (2022), Weisse et al. (2021), Viitasalo and Bonsdorff
(2022), and Gröger et al. (2021). Furthermore, a realistic consideration
of small-scale processes would improve the projections for the atmosphere
(Christensen et al., 2022) and the ocean (Meier et al., 2022a).
Attribution of regional climate variability and change to anthropogenic radiative forcing and other drivers. In order to unambiguously disentangle
the impacts of anthropogenic climate change and other human influences from
the natural climate variability of the regional Earth system, more knowledge
about internal variations and feedback mechanisms is needed. For example,
climate models have recently shown that multidecadal variability emanating
from the North Atlantic and the Arctic significantly controls the climate of
the Baltic Sea region by means of teleconnection patterns (Lehmann et al.,
2022; Meier et al., 2022a, b). For example, observations of
precipitation and wind in the Baltic Sea region, total river discharge from
the catchment, individual river flows, water temperature, sea level, MBIs
and salinity in the Baltic Sea show a pronounced multidecadal variability
with a quasi-periodicity of about 30 years (Meier et al., 2022b). It is
assumed that the Atlantic multidecadal variability (AMV) and, as part of it,
the variations of the North Atlantic overturning circulation are the source
of these variations, although the exact mechanisms, cause-and-effect chains
and feedback processes are still unknown. Knowledge about the
teleconnectivity of the Baltic Sea region with the North Atlantic and the
Arctic is essential for the development of climate prediction models.
Factors discussed by Reckermann et al. (2022) sorted by related
economic sectors or state variables of the Earth system.
Human activities Economic sectorsFactorsCommentsPrimary (natural) sector (e.g. food production)FisheriesAgricultureMarine and coastal ecosystem servicesFactor belongs to several sectorsBlue carbon storage capacityMitigation of greenhouse gasesSecondary (industrial) sector (e.g. energy production)River regulationOffshore wind farmsGreenhouse gas and aerosol emissionsEmission are largest from industriesDumped warfare agentsFactor is an industrial productTertiary (service) sector (e.g. transportation, tourism, healthcare)ShippingChemical contaminationContamination is a result of several sectorsMarine noiseMarine noise is a result of several sectorsMarine litter and microplasticsEmission mainly by offshore platforms, shipping, lost containers, fisheries, aquaculture, agriculture, municipal waste and tourismTourismCoastal protection and managementAlso relevant for the other sectorsQuaternary (information) sector (e.g. information technology, media,research and development)–Earth system Environmental state variablesCoastal processesHypoxiaSubmarine groundwater dischargeMarine ecosystemsLand use and land coverNon-indigenous speciesIndirect parameters such as carbon and nutrient cycles, biota and ecosystemsClimate state variablesClimate change, acidification, direct parameters of the climate systemSuperordinated concept (large scale)Direct parameters of the climate systemAcidification
Development of integrated Earth system models accounting for anthropogenic changes in the Baltic Sea region. The BEAR study by Reckermann et al. (2022)
on human influences and their interactions in the Baltic Sea region is part
of the relatively new Baltic Earth Grand Challenge 6 on the multiple drivers
of Earth system change in the Baltic Sea region and represents an important
step towards an integrated understanding of the Earth system that
encompasses all traditionally considered climate compartments such as
atmosphere, cryosphere, hydrosphere, lithosphere (including the pedosphere),
biosphere (marine and terrestrial) and anthroposphere. Such a holistic
view is urgently needed, as in many cases, several reasons are responsible
for the observed changes in the Earth system, and attributing them to only
one factor, e.g. climate change, would be an inadmissible simplification. One
example is the oxygen depletion and the large hypoxic area in the Baltic Sea
caused by anthropogenic nutrient inputs from land and exacerbated by rising
water temperatures (Kuliński et al., 2022). Of course, the factors
discussed by Reckermann et al. (2022) cannot exhaustively consider the
entire Earth system and all interactions, and the selection of factors is
biased towards ocean-related parameters and activities. Moreover, the
analysis is based on an extensive literature review by experts who reflect
their subjective interpretations of the results. Nevertheless, this is the
first time such an assessment has been conducted, which is a major step
forward. To continue and deepen this research, the factors discussed by
Reckermann et al. (2022) could be subdivided by either human activities
(e.g. food production, energy production, transport, tourism, healthcare) or environmental and climate state variables of the Earth system (e.g.
hypoxia, acidification) (Table 3). Such a breakdown of parameters would
allow the development of an integrated Earth system model that includes the
anthroposphere at the regional scale. This type of research is timely, and
such efforts are already underway (e.g. Korpinen et al., 2019; references in
Reckermann et al., 2022).
The fact sheet on climate change in the Baltic Sea (CCFS, 2021) was
positively received by various stakeholders and decision-makers. Although
uncertainties regarding observed and projected future climate change and the
other drivers remain high, our experience in engaging with stakeholders
confirms that scientific uncertainties are taken into account in different
ways in management and decision-making. This is an important reason for
investing in the above key issues. They have the potential to reduce
uncertainties that currently hamper decision-making in the region.
Concluding remarks
We conclude that (1) the BEARs have been useful to identify research progress
and knowledge gaps and to initiate new research foci as, for example,
suggested in the Discussion section; (2) regional assessments, such as the BEARs,
complement the IPCC climate change assessments by adding a greater depth and
scope of regional information about the specific situation of the Baltic Sea
region; and (3) the BEARs provided useful information for the Expert Network
on Climate Change, that produced the Baltic Earth–HELCOM climate change
fact sheet for stakeholders. Since the information summarised by the BEARs
is used extensively in science and management, it is recommended that a new
update of the reports will be conducted in about 7 years.
Data availability
All datasets presented here have previously been published and are publicly available. Figure 1 is adopted from Meier et al. (2014). Data to reproduce Fig. 20 are available from the Oceanographic Data Center at the Federal Maritime and Hydrographic Agency (dod@bsh.de).
Author contributions
HEMM wrote the first draft of the overview paper. All co-authors, who acted
as guest editors of the special issue in Earth System Dynamics, contributed with important
comments and editing of the paper, read and approved the submitted
manuscript version.
Competing interests
All co-authors are guest members of the editorial board of Earth System Dynamics for the special issue “The Baltic Earth Assessment Reports (BEAR)”. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “The Baltic Earth Assessment Reports (BEAR)”. It is not associated with a conference.
Acknowledgements
During 2019–2022, the Baltic Earth Assessment Reports were produced under
the umbrella of the Baltic Earth programme (Earth System Science for the
Baltic Sea region; see http://baltic.earth, last access: 2
February 2023). The contribution of 109 co-authors from 14 countries to 10 articles in the international scientific journal Earth System Dynamics is acknowledged. A total of 2822 different references have
been assessed. We thank the reviewers of all 10 articles of the special
issue for their constructive comments that helped to improve the review
articles. In particular, we thank Jouni Räisänen, Donald Boesch and Andris Andrusaitis for their advice and many excellent
comments on individual articles and this overview article.
Financial support
The publication of this article was funded by the Open Access Fund of the Leibniz Association.
Review statement
This paper was edited by Gabriele Messori and reviewed by Andris Andrusaitis, Jouni Räisänen, and Donald Boesch.
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