Observed and expected future impacts of climate change on marine environment and ecosystems in the Nordic region

In its Sixth Assessment Report (AR6), the United Nations’ Intergovernmental Panel on Climate Change (IPCC) stresses that climate change is even greater and clearer than previously demonstrated, “recent changes in the climate are widespread, rapid, and intensifying, and unprecedented in thousands of years” (IPCC 2021), and that the effects on humans and nature will be very extensive (IPCC 2022). Further, IPCC (2019), states that the ocean so far has acted as a buffer against climate change, but climate change now alters the ocean and cryosphere in many ways and consequences for nature and humanity are sweeping and severe. The IPCC AR6 reports leave no doubt that marine ecosystems will be strongly affected by climate change. Climate change will in very many regions cause environmental conditions that are novel to species living in that specific area (Blenckner et al. 2021). Especially increasing sea temperature leads to a number of ecological changes, through affecting, e.g., competition between species, access to food to early life stages and migration patterns.

Although AR6 contains an enormous amount of information and has better regional resolution than earlier IPCC assessments, it is difficult to scale down to more detailed local information. Specifically, the Nordic seas are not treated as a unit or resolved well into individual seas. It is particularly challenging that the global climate models used to simulate future climate and climate effects under different emission scenarios (Coupled Model Intercomparison Project Phase 6, CMIP6) do not have sufficient resolution to provide a good basis for decision-making on a Nordic scale. This is additionally problematic because our sea areas have characteristics that distinguish them from many others and may make them especially vulnerable to future climate change. In the north (the Barents Sea and northern parts of the Greenland-Iceland-Norwegian Sea), some of the largest changes on the planet are expected, associated with increased sea temperatures and reduced ice cover. The North Sea has an extensive total burden from many other human impacts in addition to climate change. Biodiversity and productivity in the Baltic Sea already face challenges from problematically high nutrient levels (eutrophication), occasional oxygen deprivation and salinity levels that are very low for marine species (such as cod and herring). Therefore, The Nordic Council of Ministers (NCM) has asked for regional analyses and assessment for our sea areas.

Our assessment builds mainly directly on primary literature, i.e., peer reviewed scientific papers. However, we have also examined publications from several ongoing or relatively recent comprehensive activities. For Norwegian waters, from the Skagerrak to the Barents Sea, reports relating to the Norwegian integrated marine management plans have been important in our work, especially Eriksen et al. (2021). For the Arctic AMAP (2021) provides a lot of information. However, even if their southern boundary formally is set as far south the Arctic Circle at 66°32’N, the focus is further north and the overlap with our region restricted mainly to the Barents Sea, for which we use the chapter by Hausner and Trainor (2021). There exist good reviews of the knowledge of climate change and impacts for the North and Baltic Seas, for the North Sea especially the book "North Sea Region Climate Change Assessment" (NOSCCA; Quante and Colijn 2016). For the Baltic there is a lot of information in the two books “Assessment of Climate Change for the Baltic Sea Basin” (BACC Author team 2008) and “Second Assessment of Climate Change for the Baltic Sea Basin” (BACC II Author team 2015). More recently, the Baltic Earth Assessment Reports (BEAR) special issue of Earth System Dynamics includes several highly relevant review articles including Meier et al. (2022a, b) and Viitasalo and Bonsdorff (2022). These are solid sources of background knowledge, also for us, but they do not fully serve our needs. The books (despite intentions) do not really include projections for marine ecosystem impacts. Further, this is a field with rapid development and NOSCCA and BACC do not capture the most recent developments and publications.

The Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) and the Baltic Marine Environment Protection Commission (HELCOM) cover our geographical area. Since their focus is on environmental protection and only to a lesser degree climate change, we do not build upon them. We are aware of and follow the International Council for the Exploration of the Seas’ (ICES) ecosystem oriented working groups. Although we mainly refer to primary literature the work from all of the following have been useful for our overview: ICES Working Group on the Integrated Assessments of the Barents Sea (WGIBAR, ICES 2022a), ICES Working Group on the Integrated Assessments of the Norwegian Sea, (WGINOR, ICES 2022b), ICES Working Group on Integrated Assessments of the North Sea (WGINOSE, ICES 2022c), and ICES/HELCOM Working Group on Integrated Assessments of the Baltic Sea (WGIAB, ICES 2018), see https://www.ices.dk/advice/ESD/Pages/Ecosystem-overviews.aspx

This report is the first output from the project NorScen - Nordiske klimaScenarioer (Nordic climate scenarios). It presents a standalone up-to-date overview of knowledge on the present status and future development of climate impact on physical properties, organisms, and ecosystems in the seas around the Nordic countries. Throughout we further often point to that the differences in expected consequences depend on present ability to curb the release of greenhouse gases. The report also sets the scene for upcoming work within the project. NorScen aims to increase our knowledge of the effects of future climate change on the Nordic sea areas, from oceanography through biogeochemistry to marine life and ecosystems. Geographically and scale wise it is comprehensive, from the Baltic Sea via the Danish Limfjord and the North Sea to the Norwegian- and Barents seas (Fig. 1).

Abbrevations

Globally, sea levels are rising due to human-caused climate change and this current sea level rise (SLR) began at the start of the 20th century. Between 1901 and 2018, the globally averaged sea level rose by 15–25 cm, or 1–2 mm per year on average (IPCC AR6). The main driver of sea level rise so far is thermal expansion, a consequence of warm water occupying a larger volume than an equal mass of colder water at the same pressure. This can be calculated with fair precision, at least for the period from 2005 and onwards when there has been good coverage of argo floats (https://argo.ucsd.edu) observing the ocean. However, the largest potential and uncertainty exists in the melting of the ice sheets covering Antarctica and Greenland (Fox-Kemper et al. 2022). Global SLR is currently accelerating. It has been estimated to 3.6 mm per year during the last decade (Fox-Kemper et al. 2022), which is roughly twice the average rate during the 20th century.

The last ice age ended around 10 000 years ago, then great sheets of ice, 3 km thick, had covered much of Earth's Northern Hemisphere. Though the ice melted long ago, the land once under and around the ice is still rising and falling in reaction to its ice-age burden. This ongoing movement of land is called glacial isostatic adjustment . In Scandinavia it has until recently kept pace with the sea surface so that relative sea level is only now starting to increase. Both the observed and projected SLRs differ a lot between regions.

The Nordic region is very varied regarding land uplift. Forecasts for future sea level therefore vary greatly. The Baltic region has a stronger uplift than Western Norway because the ice cap eastwards was thicker and thus heavier. Projected relative changes in sea level for the Nordic region and surrounding areas can be seen in Fig. 2. For the moderate climate scenario (Fig. 2, left panel) a sea level rise of 0.5 m is, for example, projected for Copenhagen and -0.35 m for the Bothnian Bay. Within the extreme scenario the corresponding numbers are 1.0 m and +0.1 m.

In Danish waters the sea level is projected to increase 0.3 m in the moderate climate scenario and 0.5 m in the extreme climate scenarios (Fig. 3). In the shallow Limfjorden, SLR follows the general trend for Denmark.

Present and projected SLR along the Norwegian coast was reviewed after IPCC’s fifth assessment report (AR5; Simpson et al. 2015, 2017); their report is presently being revised based on later progress in AR6. AR6 based sea level projections are currently available for all Swedish municipalities through SMHI (https://www.smhi.se/klimat/stigande-havsnivaer/framtida-medelvattenstand-1.165493) .

The projected SLRs for Norway differ a lot geographically, with lowest levels at Oslo and highest levels at the west coast of southern Norway, and also depend on scenario (Tab. 1, Fig. 4). For Sweden the highest sea level rise is projected for the southernmost part of the country, where the sea level rise is almost on par with the global average. On the northern Baltic Sea coast, in contrast, the sea level is expected to fall during the current century except for in very high emission scenarios.

Mean sea levels are considerably exceeded during storm surges. Which measure is best suited for management purposes is debated as this also depends on emission scenarios and the length of the planning period. Norwegian authorities often use the 200-year return height (“200-year flood”, a large flood with a probability of 1/200 to occur in any given year, Tab. 2). The range of the 200-year return levels along the Norwegian coast is 1.2 to 2.2m.

For Sweden the storms surges typically have the highest amplitude on the west coast, while the central Baltic Sea has the smallest amplitudes. Return heights derived for a number of different tide gauge stations are available in Södling and Nerheim (2017).

Globally, the average surface temperature in the ocean has risen by 0.88 °C since before the Industrial Revolution. Heat has also reached the deep sea, and as much as 9 per cent of the amount of heat absorbed by the ocean is now found below 2000 metres depth. Generally, but not always, sea temperature fluctuations on annual and longer time scales are in synchrony through most of the Nordic region (North Sea, Norwegian Sea and Barents Sea). Sea temperature fluctuations in the three seas are largely controlled by variations in Atlantic water further south, but also modified by local heat loss and mixing in water masses while heading north. In the Norwegian Sea and the whole North Atlantic, the temperature increase since measurements started has been approximately the same as the global average.

In addition to a slow increasing trend, sea temperature in the region is also subject to large-scale and long-term natural fluctuations (Fig. 5). There was a warm North Atlantic period during the 1940s and 1950s, followed by a much colder period around 1970, before the temperature rose again until about 2006, and was at that time record high. Since then, the temperature in the North Atlantic, including the North Sea and the Norwegian Sea has stabilized and even decreased somewhat. In the Barents Sea the fluctuations followed much the same pattern, but since the 1980s there has there been a very clear increasing trend on top of the decadal-scale fluctuations (Fig. 5).

There are major differences in the predictions of regional future SST among the consortium of global models available (Alexander et al. 2018). Using a large ensemble of models (CMIP5 and CESM-LENS) they estimated rate of increase of summer mean in the Arctic to be 0.5°C per decade under the RCP8.5 "business as usual" (extreme) greenhouse gas emission scenario. The corresponding predicted summer increase rates were 0.4°C per decade for the North Sea and the Norwegian Sea, and winter increase in these regions was estimated as 0.25°C per decade.

Sandø et al (2022) conducted a risk analysis for Norwegian marine ecosystems under climate change. This included direct and indirect impacts on marine fish of three different IPCC climate scenarios within the management plan units North Sea- Skagerrak, Norwegian Sea, and Barents Sea-Lofoten. The three scenarios SSP1-2.6, SSP2-4.5 and SSP5-8.5 from the global climate model NorESM2-MM were used to cover the full scope of future climate change. These were downscaled by means of the regional ocean model NEMO-NA10km, which has a horizontal spatial resolution of 10 km and covers the North Atlantic from 39 °N and northwards, the Nordic seas, the North Sea, the Barents Sea, and the Arctic Ocean (Sandø et al. 2022). Projected future development for 2015-2100 was estimated for the physical variables sea temperature, salinity and sea ice cover, sea temperature results shown in Figs. 6 and 7.

For the Baltic Sea, the effects of climate change on the ocean are summarized in Meier et al. (2022a,b) and in the En-Clime fact sheet (Climate Change in the Baltic Sea, 2021). Results are also visualized in the SMHI climate change scenario service (SMHI, 2022) where one can interactively see maps for various parameters and for different climate and nutrient load scenarios. Meier et al. (2022b) conclude that sea surface temperatures (SST) increase between 1.1 °C (0.8 – 1.6 °C) for AR5 scenario RCP2.6 and 3.2 °C (2.5 – 4.1 °C) for RCP8.5 at the end of the century compared to the reference period 1976-2005. Large rates of temperature increase relative to other ocean basins, 0.59 °C per decade, have already been observed between 1990 and 2018, but part of that increase can be attributed to natural multidecadal temperature fluctuations over the North Atlantic rather than climate change.

For Kattegat and Skagerrak, the number of downscaled projections is smaller than for the Baltic Sea since many of the regional ocean models used for the Baltic Sea assessments have used outer boundary conditions at the Kattegat/Skagerrak border. Results from downscaling of five global models to the North Sea and the Baltic Sea for RCP8.5, Fig. 8, show that summer SST generally increases most in the Gulf of Bothnia and least in the Kattegat and Skagerrak regions, but also that the increase differs between the different global models (Gröger et al. 2019). According to the SMHI scenario service (SMHI 2022) that is based on the same data for Kattegat and Skagerrak, Tab. 3, the ensemble mean SST increases in Skagerrak and Kattegat are 2.9 °C and 3.0 °C for RCP8.5, i.e. only slightly less than the 3.2 °C projected for the Baltic Sea.

Marine heatwaves (MHWs) are episodes that potentially can severely change marine ecosystems with far-reaching ecological and socio-economic consequences. MHWs can generally be defined as a prolonged discrete anomalously warm water event that can be described by its duration, intensity, rate of evolution, and spatial extent, several more specific definitions have been made (Hobday et al., 2016). IPCC has warned that increasing frequency and severity of MHWs attributable to global warming is a worldwide threat to marine ecosystems (Collins at al. 2019). In the Pacific such heatwaves have reached enormous proportions and duration (Laufkötter et al. 2020). Huang et al. (2021) analyzed the historical occurrence since 1982 of MHWs north of the 60° latitude, and showed that in the Norwegian Sea east of the 0° meridian the frequency has been 1-2 year ^-1, and slightly higher between the 0° meridian and Iceland. The frequency around Svalbard was 2-3 year ^-1. The annual mean MHW frequency over the entire Barents Sea increased by 62 % from the pre- to the post-2004 period, the number of MHW days with 73 %. More than half of all MHW days from 1982-2020 occurred in the last decade (Mohamed et al., 2022).

Other literature relevant to the Nordic regions was reviewed and evaluated in a report ordered by the Norwegian Environment Agency (Borgman et al. 2022). They pointed out that MHW literature focused on the Nordic Sea is scarce, but that satellite information since 1982 can be utilized for many purposes. Moreover, there exist a substantial number of time series on temperature data from water profiles collected by research vessels that can be added to the suite of data for a reanalysis of historical MHWs. Borgman et al. (2022) also suggested a list of recommendations for scientific efforts. They recommend documentation of the historical record of MHW, and to strengthen biological monitoring and understanding of biological responses to future climate. In accordance with recommendations by IPCC the need for more marine protected areas was emphasized.

It has been demonstrated that the hydrographical and meteorological mechanisms involved in the formation and stabilization of MHWs are diverse and not completely constrained (Holbrook et al. 2019). Therefore, predictions of future regional patterns are rather imprecise, but all studies indicate increased frequency, severity, and duration concomitant with severity of global warming. At the present global mean temperature 1°C above preindustrial value, the North Sea and Norwegian Sea experience a frequency of MHWs 2-4 times the preindustrial probability (Borgman et al. 2022). In a world with 3.5°C warming, the relative probability is estimated to increase to 20-30 times that of preindustrial times.

Salinity is an important parameter for the marine ecosystems, especially in the Baltic Sea where the species have adjusted to the low salinities present there. Salinity projections in the Baltic Sea have large uncertainty due to problems with the representation of the hydrological cycle in global climate models. Early projections showed a clear future decrease in Baltic Sea salinities due to increased runoff of freshwater, but the most recent assessments (Climate Change in the Baltic Sea, 2021, Meier et al. 2022b) state that there are no robust estimates of future salinity changes for the Baltic Sea. The main reason for this changed conclusion regarding salinity is that the most recent projections also include sea level rise in the entrance region, which will tend to increase salinities due to larger inflows of saline water.

Projections for the North Sea do show decreased salinities due to increased runoff. For the extreme RCP8.5 scenario the projected decrease in sea surface salinity by the end of the century is –1.5 g kg ^-1 in Skagerrak and –1.2 g kg ^-1 in Kattegat based on downscalling of 5 global climate models (Fig. 8 and Table 3; Gröger et al. 2019).

The ocean has absorbed about a third of the human emissions of CO₂ into the atmosphere. Increasing concentration of CO₂ in the atmosphere affects the acidity of seawater, generally reducing pH and saturation state of carbonate minerals. When pH decreases, the saturation states of carbonate minerals (aragonite and calcite) decrease. The degree of saturation decreases downwards in the water column and the saturation horizon is the boundary between supersaturated conditions above this depth and undersaturated conditions below, i.e., the level below which carbonate minerals begin to dissolve. Acidification, causing decreasing saturation rate and the saturation horizon moving upwards in the water column, is potentially a major challenge for several forms of marine life, especially corals and calcifying algae, but also sea snails with “hard parts”.

In our waters, we find the lowest pH in the warm Atlantic water. However, mainly since more CO₂ can be dissolved in cold than in warmer water, the lowest saturation states for carbonate minerals are in the colder Arctic waters to the north and west (Fransner et al. 2022). Currently the saturation horizon for aragonite in our region is at around 2000 m, i.e. all water deeper than this is undersaturated with this carbonate mineral. Calcite is less soluble than aragonite, and the entire water column is, so far, supersaturated.

The mean global surface seawater pH has decreased by 2 * 10^-3 pH units per year since the 1980ies. In the Nordic Seas the ocean acidification has been faster than the global average. Since the onset of the Industrial Revolution, the pH level in our region has decreased by around 0.1 units. Measurement series since the 1990s show a significant decrease in pH in almost all locations in the North Sea and the Norwegian Sea where we have sufficient data to investigate trends. Tab. 4 shows the development in different parts of the Nordic (Greenland-Iceland-Norwegian) Seas and Tab. 5 for three Danish fjords.

Future development of ocean acidification in the Nordic Seas as a function of CO₂ emission scenarios has been projected by Fransner et al. (2022; Fig. 9). In the rather optimistic RCP-2.6 scenario the upper ocean pH will continue to decrease for the next decennium but will stabilize close to pH=8 after 2050. In the business-as-usual RCP8.5, pH steadily continues to decrease throughout the century and will average 7.67 in 2100. Within this scenario large parts of our sea areas will be undersaturated with aragonite before the end of this century.

Fransner et al. (2022) also present a comprehensive review of observations and model results from the beginning of the industrial revolution until present. The geographical distribution of acidification, measured as the saturation state of calcium carbonates in the upper waters, decreased more rapidly closer to land in the Norwegian Sea compared to further west

In addition to physical variables (described above) Sandø et al. (2022) also estimated the development of biogeochemical variables using the model system NORWECOM.e2e (Skogen et al., 1995). The physical model results were used as input to NORWECOM.e2e, which delivered pH (and also oxygen content, primary, and secondary production). For pH the projected decrease during the period 2015-2100 was, as expected, largest under scenario SSP5-8.5. For SSP1-2.6 the pH was stable or even increased slightly (Fig. 10). The spatial pattern in projected change in pH from 2015 to 2100 for each of three IPCC scenarios is shown in Fig. 11.

For coastal areas, ocean acidification does not only depend on atmospheric CO₂ levels, but also on changes in the catchment areas and internal cycling of carbon and nutrients. Increased river runoff will lower salinity and thereby the alkalinity, while weathering on land and eutrophication may increase alkalinity. For the Baltic Sea there will most likely be a pH reduction, but the degree of this reduction is still highly uncertain (Climate Change in the Baltic Sea, 2021, Gustafsson and Gustafsson, 2020). In our northernmost regions, runoff from (mainly the large Russian) rivers is increasing and river water generally has a low calcium content.

Biological effects of ocean acidification have been hard to ascertain and scientists disagree on species’ sensitivity and scope for adaptation to this pressure (e.g., IPCC 2019). This is reviewed by Hurd et al. (2020).

Model and observation results demonstrate that global warming leads to an expansion of ocean water bodies with low oxygen (hypoxia) and no oxygen (anoxia), and consequently the volume of habitats for organisms that are dependent on oxygen will shrink (Schmidtko et al. 2017). Moreover, reversal of the present development may take millennia, because of the slow turnover of bottom waters.

The rate of deoxygenation is slow in the North Atlantic compared to other regions of the world’s oceans (Schmidtko et al. 2017). Therefore, impacts will be small in most European coastal waters, where local mechanisms dominate the 21st century oxygen balance. However, in the Arctic Ocean, oxygen concentration will decrease mainly because the predicted warming reduces O₂ solubility (Schmidtko et al. 2017; Sandø et al. 2022). Projected temporal development is shown in Fig 12, spatial pattern of change in O₂ concentration in Fig. 13 (both from Sandø et al., 2022).

The Baltic Sea deep oxygen conditions are influenced by a permanent salinity stratification that limits ventilation of the deep water to the rare occasions when saline surface water from the North Sea enters though the shallow and narrow Danish Straits. Large amounts of nutrients that have accumulated in the system during the last century have caused a situation with large algae blooms that sink to the bottom and consume the oxygen with hypoxic and anoxic bottom areas as a consequence. Today the situation is worse than ever (Almroth-Rosell et al. 2021) although the nutrients loads have been drastically reduced since the 1980s (Gustafsson and Arneborg 2020). Projections for a combination of climate and nutrient load scenarios have been performed with a coupled physical-biogeochemical ocean model to study the relative influence of climate change and nutrient loads on future oxygen conditions (Saraiva et al. 2019, Fig. 14). The nutrient load scenarios were (i) the Baltic Sea Action Plan (BSAP) where nutrient loads followed the BSAP recommendations after 2020, (ii) a reference case, where land use in the catchment area was assumed to be similar to today, and (iii) a worst-case scenario with increased nutrient loads from cities and farmlands. It was found that nutrient load scenarios were more important for future oxygen conditions than climate change, although climate change also has a negative influence as it amplifies oxygen depletion. For all scenario except the worst climate scenario (RCP8.5) combined with the worst nutrient scenario, oxygen conditions will be better at the end of the century than they are today.

Future changes in temperature, precipitation, freshwater runoff, wind, and (in the northern parts) ice cover, may, through affecting nutrient availability and seasonal dynamics, lead to alterations in primary production in the Nordic region (phytoplankton and ice algae; Eriksen et al. 2021).

Primary production is dependent on nutrients mixed up from deeper water to the upper layer of the sea where light is available for photosynthesis. Warming of surface seawater and increased freshwater from precipitation leads to stronger stratification, and reduced vertical advection between surface and deeper water layers. This will again lead to a decrease in primary production and total biomass, which further can cause reduced energy flow upwards in the food web. On the other hand, stronger winds and an increase in the number and/or intensity of storms can have a positive effect on plankton production through weaker layering and thus increased mixing of nutrients. This is, however, complicated as really strong mixing of the water masses may transport the planktonic algae down to depths with insufficient light for photosynthesis. Further, changes in stratification may affect the balance between the different phytoplankton groups.

The Nordic Seas are situated in the transition zone of waters south of Iceland with predicted decrease in production, and Arctic waters with predicted increase. Data from satellites show no increase in the Norwegian Sea driven by global warming, a small increase in the Greenland Sea, and a clear increasing trend in the Polar Ocean (Børsheim 2017; Lewis et al. 2020; ICES 2022b).

In temperate European ocean and shelf Sea areas, including the North Sea, future climate is predicted to lead to reduced primary production (Laufkötter et al. 2015; Sandø et al. 2022). Community production is expected to decrease by up to 30 % at the end of the century (Carozza et al. 2019 ; Kwiatkowski et al. 2019). Fish and commercial fisheries are expected to be affected by this projected decrease in total production through the food web.

At high latitudes, including the northern parts of the Nordic seas, the effect of climate change on primary production is opposite to many other areas. The extent of the ice has already been significantly reduced, and the period of ice cover is shorter – a process that is expected to continue. Here, total primary production is expected to increase due to more light reaching into the sea and the warming of these previously very cold waters. Contrary, ice algae, with their entire life cycle within or connected to sea ice, are in danger of disappearing or being greatly reduced.

For the arctic and polar regions of the Barents Sea Sandø et al. (2022) project a near doubling of net primary production (NPP) from 2015-2100 for SSP5-8.5, while little or no change is expected under SSP1-2.6 (Figs. 15 and 16).

In the Baltic Sea, primary production depends more on nutrient loads than on climate change. Climate change and warmer water will tend to increase primary production, but the expected long-term reductions due to reduced nutrient loads for the BSAP and reference scenarios are expected to cause reduced primary production at the end of the century (Saraiva et al. 2019). Primary production is only expected to increase for the worst case nutrient scenario.

Biogeographic changes in zooplankton communities may have important impact on ecosystem structure and functioning. This is the case not least because they can lead to significant alterations in nutrient content and fatty acid profiles, which in turn affects the survival and growth of fish larvae. There is a clear example of this in our region. Here, climate change, through temperature increase, has caused redistribution of zooplankton species especially in the North Sea (Beaugrand et al. 2002, Edwards et al. 2016). The calanoid copepod Calanus finmarchicus has over a period of several decades been outcompeted by its “cousin”, the more warm-water affiliated species Calanus helgolandicus . Although only experts can see the difference, this change has had major implications for higher trophic levels. Nauplii (early stages) of Calanus finmarchicus are normally the main food for early life stages of North Sea cod. Cal helgolandicus nauplii have less nutritional value and appear too late in the season to match the need of the small cod. This mechanism is likely one of the main reasons (in addition to heavy fishing) for the abundance of cod in the North Sea being at worryingly low levels. This is an example of warming leading to a reduction of reproductive success, because early life stages no longer are present at the same time as food is available (the match-mismatch hypothesis (Cushing 1990, Durant et al. 2007).

Relatively recent changes in the zooplankton community have also been seen in the Barents Sea (Eriksen et al. 2017), while no significant changes in zooplankton biomass or secondary production have been observed in the Norwegian Sea (ICES 2020, Kjesbu et al. 2021). The recent major modelling work of Sandø et al. (2022) also covered the anticipated future development of gross secondary production (GSP). Projected development of GSP at different locations is shown in Fig. 17, while Fig. 18 shows spatial development for three climate change scenarios.

Marine fish in the Nordic region are significantly affected by variability in their environment, both directly on their physiology or indirectly through predators or prey. Impacts on individual fish may lead to responses at the population level, affecting distribution, seasonal migration patterns, and recruitment success (Fig. 19). Although it yet often is difficult to distinguish between impacts of natural variability and anthropogenic climate change, there are now developments we ascribe to the latter and we project clearer and stronger effects in the future.

Higher temperature, retreating sea ice, and changes in prey availability is affecting marine fish in the northern Barents Sea. Here higher temperatures have expanded suitable feeding areas for boreal/subarctic species towards the previously cold north and east and has contributed to increased Atlantic cod production (Kjesbu et al., 2014). In contrast, arctic species are expected to be affected negatively by a shortened ice covered season and reduced sea ice extent through loss of spawning habitat and shelter, increased predatory pressure, reduced prey availability, and impaired growth and reproductive success. Until recently, the northern Barents Sea was dominated by these small-sized, slow-growing arctic fish species with specialized diets, except for polar cod mostly living in close association with the sea floor. Thus, simultaneous with rising sea temperatures and retreating sea ice, these Arctic fishes are being replaced by boreal, fast-growing, large-bodied generalist fish moving in from the south. These large, migratory predators take advantage of increased production while the Arctic fish species suffer from higher competition and predation and are retracting northwards and eastwards. Consequently, climate change is inducing structural change over large spatial scales, leading to a borealisation or ‘Atlantification’ of the European Arctic biological communities (Fossheim et al., 2015; IPCC 2019).

An even more pronounced recent range expansion is that of the summer feeding distribution of the temperate Atlantic mackerel in the Nordic Seas. From 1997 to 2016 the total area occupied by this large stock expanded from 0.4 to 2.5 million km 2 and the centre-of-gravity of distribution shifted westward by 1650 km and northward by 400 km, far into Icelandic and Greenland waters and even up to Svalbard. This range expansion was linked to a combination of a pronounced increase in stock size and warming of the ocean (Fig. 20; Olafsdottir et al., 2019; IPCC 2019). The mechanism may be more complex though. The analyses of Pacariz et al (2016) suggest that declining silicate concentrations along the northern European continental slope reduces primary and thus secondary production, intensifying food scarceness in the east and adding to the incentive for the mackerel to migrate westward. Under the RCP4.5 and RCP8.5 scenarios further range expansions of mackerel are projected in Greenland waters (Jansen et al., 2016). However, further northwards expansion of planktivorous species may generally be restricted by them not being adapted to lack of primary production during winter (Sundby et al., 2016), while lack of light during the long winter season may seriously hamper the expansion of visual predators in general.

As described under Secondary production, cod are not doing well in the North Sea. Fortunately, other species are increasing their abundance there. Hake is a warm-temperate codfish that for long has been a popular food fish further south in Europe. Hake is expanding quite rapidly in the northern North Sea and should be well suited for the present conditions there. The amount fished here is still moderate; the recent years around 5000 tons have been caught annually by Norwegian fishers.

In the Baltic Sea, species that already live on the edge of their tolerance limits for salinity will have fundamental challenges if salinity decreases further, while some other species will have an advantage of warmer water and less ice (e.g. Blenckner et al. 2021, Turkia et al. 2022). Nutrient and bottom oxygen conditions also influence the future Baltic ecosystem. Cod is an example of a species that today lives close to several of its tolerance boundaries in the Baltic. Cod require deep water with salinity levels above 11 g/kg and more dissolved oxygen than 2 ml/l to reproduce. The volumes of such waters will probably diminish in the future, putting even larger pressure on the Baltic Cod (Wåhlström et al. 2020).

In IPCC SROCC (2019) future projections of total animal biomass and maximum fisheries catch potential were made on a global scale (Fig. 21). If one zooms in a decrease is seen for most of the Nordic region, except the very northern part. However, this is very rough and more geographically focused model projections are needed to provide a better foundation for understanding the expected future situation in our seas. This has partly been done, and will be followed up in NorScen.

The most likely development towards 2050 of 39 fish resources in the North, Norwegian and Barents Seas under the intermediate climate emission scenario RCP4.5 has been assessed by Kjesbu et al. (2022). Contrary to most results for other regions most of the assessed stocks were found to respond positively. However, the stocks will develop entirely different depending upon the encountered stressors: cold-temperate stocks at the southern fringe (like North Sea saithe and cod) and Arctic stocks at the northern fringe (like Polar cod) are expected to be severely negatively impacted, beyond what already is presently observed. On the other hand, warm-temperate stocks expanding from the south (like hake and sprat in the North Sea) were found to do well along with cold-temperate stocks as Barents Sea cod and haddock currently inhabiting below-optimal temperature (Fig. 22).

These projections were confirmed and strengthened by an even more recent and thorough study of the same areas by Sandø et al. (2022). They assessed the accumulated directional development for marine resources, taking into account projected development of sea temperature, sea ice, O₂, pH, primary and secondary production. Further, they undertook separate evaluations for each of the three most recent climate emission scenarios SSP1-2.6, SSP2-4.5, and SSP5-8.5 (Fig. 23).

The potential effects of climate change on aquaculture range from changes to production capacity in existing cultivation areas to changes in the areas themselves. Locations may become unsuitable for particular species, but also suitable for new species (Cubillo et al. 2021). The prediction of where and how such changes may occur is challenging, not least because the cultivated species may themselves exhibit plasticity, which makes it difficult to forecast the degree to which different locations and culture types may be affected (Cubillo et al. 2021).

Climate change projections of six key aquaculture species in Europe showed different responses depending on the species and geographic region (Tab. 6). Most of the studied finfish species were projected to have increased feed conversion ratio (FCR, i.e. decreased feeding efficiency) in both mid-century and end-of-century scenarios (Cubillo et al. 2021). Hence, fish will need more feed to reach harvest size and feed costs will be higher (Tab. 6). In Norway, due to colder baseline (current) temperatures, the temperature under RCP4.5 does not reach such high values as to observe a negative effect on salmon growth. In fact, growth and profit increased in both mid- and end-of-century scenarios when compared to present-day results (Tab. 6). The RCP 8.5 scenarios do not cover Norwegian salmon so for them no conclusion can be drawn for more extreme temperatures.

Shellfish bivalve species showed a decreasing trend in relation to most productivity parameters (e.g. average harvest size, production and profit) as climate change progressed under both emission scenarios (Tab. 6). The Limfjorden is currently the most productive area for both mussel fishery and aquaculture (bottom and suspended cultures) in Denmark. The longline culture of blue mussel in Denmark was projected to be most deleteriously impacted in the study. Increased number of hypoxia events and ocean acidification were, however, not considered in the projections and could amplify the negative impacts on shellfish even further.

Global population and seafood demand are both increasing, while fisheries decline during this period of rapid climate change. Aquaculture is well positioned to help meet the world’s future demand for protein and food security needs but is also heavily dependent on the environment and thus highly vulnerable to climate change effects since it relies on specific species that are sufficiently domesticated and takes place at fixed locations (Cubillo et al. 2021) . There are, however, opportunities for adaptation. In offshore-suspended cultivation systems, the effects of climate change seem to be ameliorated by oceanic conditions. The Norwegian salmon sector is already investing in moving production systems offshore. In Norway a more northerly distribution of coastal salmon farms is also an expected response to increasing sea temperatures. Steeves et al. (2018) also concluded that offshore aquaculture should be considered as a cultivation method to avoid increased temperature-related mortality of blue mussels from thermal stress.

For marine mammals, the warming has been negative for a number of particularly sensitive Arctic seal and whale species (Eriksen et al., 2021). Climate change generally has a negative impact on the grazing areas and breeding grounds of a number of Arctic marine mammals (Eriksen et al., 2021). Some of the Barents Sea species that have proven particularly sensitive to this are described here. Beluga whales around Svalbard face challenges with decreasing ice cover. Killer whales are an increased threat to them in areas with little sea ice. In addition, the areas in front of glaciers where beluga whales mainly hunt are becoming smaller and smaller as the glaciers melt and retreat. Nevertheless, beluga whales seem to be able to adapt to the new conditions to some extent. They now spend more time out in the fjords away from glacier fronts on the west side of Svalbard and probably hunt for Atlantic fish species. Narwhal is considered to be one of the Arctic marine mammal species most sensitive to climate change. They prefer cold Arctic waters (0.6–1.5°C) and studies on the effects of past climate variations suggest that this species responded strongly to changes in temperature and ice extent and will probably do so again as a result of current changes. The challenges are particularly related to their specialised deep-diving eating behaviour, ice dependence, limited distribution area, as well as disturbances from increased shipping. This ongoing increases in sea temperature and reduction in ice extent may also be perilous for bowhead whales , both due to habitat loss and more direct thermal stress. Moreover, also they may be exposed to increased predation from killer whales (this is already a serious problem for the bowhead whale population in the Sea of Okhotsk, off the western Pacific).

Less ice and poorer ice quality can lead to a further decline in the populations of greenland seals and hooded seals (Eriksen et al., 2021). After the drastic changes in ice conditions that started in 2006, which could be termed a “tipping point”, the behaviour of ringed seals around Svalbard has changed a great deal. This applies to both those who migrate out and those who remain close to the coast. They now spend much more time diving and less for resting; everything indicates they are working harder to find food (Hamilton et al. 2015).

One assumes that seals and whales in the North Sea and Skagerrak are somewhat less exposed to climate change as they typically have a wide temperature tolerance range, and there are no clear trends there. Nevertheless, there are major uncertainties here, especially in that they may be negatively affected by reduced food availability (Eriksen et al., 2021).

In the Baltic Sea four marine mammal species are resident: the grey seal, harbour seal, ringed seal, and harbour porpoise. Being at the top of the marine food web, these predators are sensitive to changes throughout the ecosystem, including those related to climate. The overall status of marine mammal species in the Baltic Sea is unfavourable. However, at species level, grey seals and harbour seals show increasing population sizes. Of particular concern is the local population of harbour porpoise in the Baltic Proper, with a population size recently estimated at around 500 animals. Ringed seal is in a critical state in the Gulf of Finland, where it is currently only represented by around 100 animals and has a decreasing abundance (HELCOM 2018).

Future changes in temperature and strat­ification, prey distribution, quality and quantity will affect all marine mammals, but aggregated effects on their abun­dance and distribution are unpredictable and the relation of the former to haul-out sites are not well known (HELCOM/Baltic Earth, 2021). Furthermore, the extent and quality of sea ice is important particularly to the ice-breeding ringed seals and also the ice-breeding grey seals.

In some areas, seals are dependent on low-lying haul-outs (land areas for rest­ing, breeding, foraging etc.). Both the ice cover and duration of the ice season have already been mark­edly reduced. The changes are most prominent in the southern Baltic areas where the duration of ice cover during breed­ing season of ringed and grey seals has increasingly often been either too short or completely lacking. Disappearance of ringed seals from southern areas is possible and transfer of grey seal breeding to land sites probable. Sea level rise causing flooding of haul-outs in the southern Baltic may force out breeding seals. This will likely cause reduction of harbour and grey seal occurrence to foraging individuals (HELCOM/Baltic Earth 2021).

There are no direct studies of the effects of climate change on harbour porpoises in the Baltic Sea (Meier et al. 2022b). There are a multitude of ways that changes in one parameter can affect others, and we do not currently have the knowledge to predict the cumulative effects this might have on the Baltic Sea harbour porpoise population (Meier et al. 2022b).

Seabirds are typically at the top of the marine food web and are the most numerous and visible of marine top predators. Predicting the effects of climate variability on and through the different trophic levels is a major challenge, and one that increases in complexity at successively higher levels of the food web up to seabirds. Worldwide, seabirds have declined faster than terrestrial bird groups with comparable numbers of species, with most trends consistent with climate change.

Seabirds can be affected by changing climate both directly, for example, if extreme weather becomes more frequent, or indirectly, through changes in their food supply. There is a substantial body of evidence suggesting that in most cases indirect effects are the more important of the two, but with fluctuations in seabird demography and population dynamics caused in part by climate fluctuations acting through the availability and distribution of food.

Effects of climate on life history traits have been documented across many species and populations in Nordic waters (e.g., Brander et al., 2016, Eriksen et al., 2021). Seabirds are vulnerable to changes in food availability. Especially in high latitude areas like the Barents and Norwegian Seas synchrony in food demand and production is important, causing match or mismatch. If zooplankton, fish larvae and fry do not appear at the right time for the seabirds' breeding season, this will have a negative effect and low juvenile survival. The future prospects for the various seabird species and populations are therefore closely linked to zooplankton and fish spawning grounds and times. One particular clear case seems to be Svalbard (Spitsbergen). Increased sea temperature to the west and north of Svalbard has resulted in less sea ice in the fjords, which has led to changes in the ecosystem. Arctic prey has largely been replaced by Atlantic species (herring, capelin and krill), which typically contains less energy. This is likely the reason that several seabird species now have reduced growth and juvenile survival. The consequence is a decline in the populations of these species in the Svalbard area.

The breeding results of seabirds will thus be negatively affected if climate change causes fish species to change preferences for spawning grounds and pull away, or if they spawn asynchronously with the birds' nesting (Eriksen et al., 2021). It is also conceivable that the availability of fish larvae is increasing in some areas as a result of changing spawning grounds and current conditions. Still, the decrease already observed for quite some time in in many seabird species in the Norwegian Sea is expected to continue (Fauchald et al. 2015).

Changes in plankton communities will affect the composition of seabird diet, for example, of specialized seabirds in the Barents Sea such as the little auk (“alkekonge”; Fig. 24). They need access to large energy-rich plankton organisms, like the arctic copepod species Calanus glacialis (“ isåte” ), and Calanus hyperboreus (“feitåte”) for better survival of the young (Eriksen et al., 2021). However, with increasing temperatures and expansion of Atlantic water masses, they are in large areas becoming out-competed by the smaller and less nutritious Calanus finmarchicus (“raudåte”; we have not been able to find English common names for these species).

Seabirds in the other parts of the Nordic region; the Baltic, Skagerrak and North Sea face similar challenges. They too are of course dependent on access to food. If plankton or fish populations weaken or change from what the seabirds are adapted to eating, this will have a negative impact on breeding success and winter survival (Eriksen et al., 2021).

The amount and species composition of bottom-dwelling animals (benthic fauna) varies geographically, particularly in relation to water mass impacts and sediment composition. Depth, temperature variation and current conditions also affect species composition, among other things because most bottom-dwelling species have larvae that are transported by the water masses. For example, sediment composition, depth, food availability and water temperature are the main environmental factors governing the large-scale distribution of benthic species in the North Sea (Brander et al., 2016) . This suggests that also benthos will be affected by climate change, something that indeed has been well documented (e.g., Brander et al., 2016).

In a future Arctic characterized by warmer waters and reduced sea-ice it is expected that species new to the region will expand or invade Arctic seafloor habitats. Historically, a warmer Arctic is more readily invaded or transited by boreal species than it is during cold periods and inflow shelves like the Barents Sea is among the most likely locations for expansion. Increasing human activities in the region could speed introductions of boreal fauna.

Although the link to climate change is not always clear, there are strong indications for also bottom-living animals being affected. A northward shift has been seen in the amount of benthic animals and temperate benthic communities, particularly in the Barents Sea. However, also along the coast of Norway more temperate species are spreading northwards (Eriksen et al., 2021). Concretely, a significant shift in boreal bottom-dwelling animals and changes in communities have been observed in the Arctic part of the Barents Sea during the last 20 years, with a decline in biomass of bottom surface living animals after 2014 (Eriksen et al., 2021). For instance, changes in biodiversity, particularly the increase in boreal species and the withdrawal of Arctic species have been registered in the Svalbard area.

Further south in the Nordic region, distributional shifts in benthic species related to increasing water temperatures have also been well documented. For example, t he northern range edge of many benthic invertebrate species in the North Sea has expanded with increased temperature. The impacts of climate change on benthic invertebrates, at least in the North Sea region, seem to arise from changes in temperature, nutrients and hydrodynamics affecting food supply and hence reproduction (Brander et al. 2016). Looking forward, Weinert et al. (2016) projected distributional changes in 75 North Sea benthic species to 2099, based upon the IPCC scenario SRES A1B (an older intermediate release level scenario). The results were complex, but revealed distributional shifts predominantly to the north. Further, the projected changes in species distribution resulted in a habitat loss for more than 60% of the modelled species (Weinert et al., 2016).

Species and ecosystems are challenged not only by climate change, but also other anthropogenic drivers. For example, in the Oslofjord the coastal cod populations are weak and show no sign of recovery. Many negative factors are suggested to contribute to this including professional and recreational fishing, overloading of nutrients through increased run-off from land, pollution in general, and dredging. Still, the additional pressure by increased temperatures may be decisive.

Another case illustrates the singular importance of climate change. The cumulative impact from a large number of pressures on many ecosystem components was assessed for Swedish waters using the marine spatial management tool SYMPHONY. Pressure from changing salinities, temperatures and ice cover under future climate scenarios was included (Wåhlström et al. 2022). It was concluded that the effects of climate change on the marine environment were as large as or larger than the cumulative impacts of all other present pressures (Fig. 25). Considering that the Barents and Norwegian Seas are more remote from major cities and towns and the level of human activities far less, the relative importance of climate change is expected to be even larger in these regions.

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