Summary

Climate change is expected to significantly impact the Baltic Sea by increasing surface temperatures, shortening ice cover periods, intensifying eutrophication, and lowering salinity and oxygen levels. These changes are likely to partially counteract and undermine the positive effects of ongoing and planed nutrient reduction measures. The physical and biogeochemical conditions in the Baltic Sea are diverse, so are the ecosystem response to climate change expected to be. 

Phytoplankton are already experiencing earlier spring blooms and prolonged growing seasons. Future projections indicate a decrease in spring blooms and an increase in summer cyanobacteria blooms. However, if the primary production stays high or decreases compared to today´s conditions depend largely on the interplay of climate change impacts on the sea and the level of nutrient load from land. It is likely that zooplankton will be affected by temperature and salinity changes, leading to shifts in species composition.

In the Baltic Sea the benthic animals and plants (such as bladderwrack and eelgrass) are generally expected to be more affected by eutrophication due to nutrient overload than direct effects of a warming climate. The BS’ deep bottom areas, particularly in the Southern and Central regions, suffer from chronic lack of oxygen, which affects benthic biodiversity. The area which such problems may expand significantly with increased water stratification and decreased ventilation of deeper waters. Especially if also nutrient loads continue to be high, even larger areas of the Baltic Sea may become void of benthic macrofauna. Simulation studies suggest that benthic responses to environmental change are hard to estimate, being nonlinear and decoupled from pelagic responses, with varying outcomes depending on nutrient loads and climate scenarios.

Climate change is expected to alter the fish species composition by the end of the century. In general, climate change-driven changes in temperature, ice-cover, salinity, and river-discharge will affect coastal and migratory fish (fish from freshwater origin) in particular, whereas the pelagic and demersal fish (fish of marine origin) mainly respond to changes in water temperature, salinity, and oxygen conditions. The effects on commercially valuable species like cod, sprat, and herring remain uncertain. Still, potential benefits for sprat reproduction are anticipated, while cod and herring will be challenged by yet lower salinity and oxygen levels. Invasive species like goby may benefit from climate change.

Baltic seabirds are experiencing altered wintering patterns and breeding times, with mixed responses among species. Bottom feeders benefit from reduced ice cover, while fish-feeders are less favoured.  There are four marine mammal species resident in the Baltic Sea: Baltic grey seal, ringed seal, harbour seal and harbour porpoise. They are generally expected to be negatively affected by climate change, both directly through habitat loss due to reduced ice-cover, rising sea levels, and decreasing salinities, and indirectly via changes in prey.

Overall, future climate projections suggest significant changes in the Baltic Sea’s biogeochemical conditions, likely affecting many plant and animal species through distribution, growth, behaviour, and interactions. Future conditions will depend on which climate and nutrient management scenario that will be realized.

Future projections indicate a decline in North Sea primary production by up to 30% by the end of the century. This reduction could negatively impact the marine ecosystem's food resources, but may also mitigate eutrophication, improving water quality and reducing harmful algal blooms. The development of primary production depends on biogeochemical processes and future climate scenarios, with varying impacts under different nutrient load conditions. Overall, water quality in the NS is expected to improve within the scenarios explored. Because of rising temperatures and reduced nutrient availability, the size of the typical NS zooplankton will decrease. This both because of a shift towards smaller species and within-species size reductions. A significant regime shift in the 1980s altered zooplankton dynamics, with warmer waters favouring the temperately adapted Calanus helgolandicus over Calanus finmarchicus. Projections indicate future northward shifts for both these species and further decrease in C finmarchicus abundance, especially under strong warming scenarios. 

Climate change is also significantly affecting bottom-dwelling species in the NS, altering their distribution and composition due to factors like changing sediment composition and rising temperatures. Many invertebrates have expanded their northern range boundaries, including the Pacific oyster, which has invaded Nordic waters from the south, with a recent population explosion along the Norwegian Skagerrak coast. Projections through 2099 under a medium climate scenario suggest continued northward movement of benthic species, while more than 60% of the 75 studied species are expected to experience habitat loss.

NS fish populations face mounting pressures from climate change, with rising sea temperatures affecting both physiology and ecosystem dynamics. Many species have shifted distributions northward or into deeper waters over the past 20 years as an adaptation to warming conditions. Populations of cold-temperate species like cod, saithe, and haddock, already occasionally close to their upper thermal tolerance limits, are expected to decline, while warm-temperate species like European hake may benefit.

Climate change is also significantly impacting NS seabirds. Most species are projected to experience further declining breeding success. Key impacts also include changes in prey availability, particularly the decline of important species like C. finmarchicus and sandeels. Increasing temperatures and storminess are directly affecting breeding conditions and chick survival, especially for shoreline nesting species. Some species may adapt by shifting distribution, but species' ability to adapt to rapid climate changes will determine their long-term survival.

Marine mammals in the NS will experience both direct impacts, including physiological stress from temperature changes affecting metabolism and reproduction, and indirect effects involving changes in prey availability and habitat loss. Most NS mammal species have broad temperature tolerance and varied diets, making them generally quite resilient. Already observed impacts include range shifts, with warm-water animals becoming more common and cold-water species less frequent. Future climate challenges include sea level rise affecting seal breeding grounds in the southern NS, while also the harbour porpoise, the most abundant marine mammal in the NS, is identified as potentially vulnerable. 

Climate change is expected to have pronounced holistic ecosystem effects on the North Sea. A key impact is the projected decrease in primary production, which forms the foundation of the marine food web. This reduction affects zooplankton, fish, seabird and mammal populations, and overall marine biodiversity.

The Norwegian and Iceland seas are nutrient-rich and support diverse marine life such as zooplankton, fish, whales, and seabirds. Despite their high latitude, these seas remain ice-free year-round due to the continuation of the Gulf Stream. The region plays a crucial role in global ocean circulation, with the Atlantic Meridional Overturning Circulation transporting warm, saline water northwards and cold, dense water southwards. For this area we give a more integrated ecosystem description, focusing on bottom-up, food driven mechanisms and energy flowing throughout the environment and ecosystem. Climate is a main driver, influencing physical, biogeochemical and biological processes.

Bottom-up processes, such as changes in zooplankton and forage fish populations, are critical to understanding the declines in fish stocks and seabird populations in the subpolar North Atlantic over the past few decades. The strong atmospheric jet stream and subpolar gyre is an important driver, influencing temperature, salinity, and nutrient content, impacting the food web from zooplankton to commercial fish and seabirds.

Pulses of intensified atmospheric activity, often proxied by a high North Atlantic Oscillation (NAO) index, increase nutrient upwelling and primary production on the south Iceland and Faroes shelves, benefiting zooplankton and fish populations. This was the situation during the late 1980s and early 1990s, but a sudden weakening in 1995–1996 changed the size and circulation of the Subpolar Gyre, negatively affecting sandeel abundance, while benefiting species like blue whiting that thrive in warmer, stratified waters.

Major shifts in gyre circulation and ocean currents, such as those in the mid-1990s, have led to changes in water properties and marine species distributions. Long-term trends, including declining silicate levels, suggest that climate change will fundamentally alter the functioning of marine ecosystems in the coming decades. Understanding these processes is essential for predicting future ecological changes in the Nordic Seas.

Greenland has during the recent decades experienced rapid warming, affecting ice cap melt rates, ocean temperatures, and ecosystem structures. These changes are projected to become more pronounced over the coming century. CMIP6 models project air temperature increases over 5 °C and significant precipitation increases by the end of the century under the high emission SSP5-8.5 scenario. Within this scenario glacier volume loss could reach 67%, leading to increased freshwater runoff. This will have significant marine implications as glaciers retreat inland. Changes in mixed layer depth and nutrient distribution will impact marine life and ecosystem dynamics around Greenland. 

Currently, marine primary productivity around Greenland is higher in the warmer, ice-free southern regions. Projections suggest a decline in productivity in these southern areas due to stronger stratification and nutrient limitations. Conversely, in northern areas like Baffin Bay and the eastern Greenland Sea increased productivity is expected due to reduced ice cover and increased light availability. Also, CO₂ uptake will lower pH levels, likely negatively affecting organisms with calcium carbonate shells, with more severe effects in the south.

Projections give a decrease in zooplankton biomass in the ice-free southerly Greenland waters. Further, the important copepod Calanus glacialis is being replaced by the smaller, less fatty C. finmarchicus due to changes in sea ice cover and temperature. C. glacialis has been retreating northward in Baffin Bay, and this trend is expected to continue. The commercially vital northern shrimp is similarly shifting its habitat northward in response to temperature changes. However, the overall impact on the Greenlandic shrimp stock and industry remains uncertain.

In the Arctic, pelagic and benthic productivity are closely linked, leading to a decrease in benthic biomass and species variety with increasing latitude, ice cover, and depth along the west Greenland shelf. Projected decreases in ice cover and increases in primary productivity in poleward areas around Greenland are expected to enhance organic matter flux to the seafloor, benefiting northern benthic communities. Bottom temperatures have risen by over one degree since 1990, prompting poleward expansion of communities, especially west of Greenland. Factors like acidification, bottom trawling, and increased walrus feeding due to reduced ice cover may negatively impact Greenlandic benthic communities.

The fish around Greenland are already affected by increasing temperatures and decreasing ice cover, leading to changes in habitats and food availability. Boreal species, like mackerel, are moving northward, while abundance of Arctic bottom dwelling benthivores and demersal fish is declining. By 2100, biomass is projected to increase by up to 50% (under SSP5-8.5) in areas currently ice-covered during winter, like Baffin Bay and the east Greenland Sea. Overall, continued warming and reduced sea ice are expected to drive further changes in fish distributions, with consequences for fisheries.

Greenlandic seabirds are facing significant changes due to climate change. Specialized Arctic species, like the ivory gull, are particularly vulnerable to shifts in sea ice and prey availability, potentially leading to population declines. In contrast, generalist species may adapt or even thrive under new conditions. The little auk, the most abundant seabird in the Atlantic Arctic, dependent on Arctic copepods, is also at risk due to habitat shifts. However, some seabirds, such as the Common Eider and Great Black-Backed Gull, benefit from warmer conditions and have increasing populations and habitats expanding northward.

Climate change is affecting the habitats also of Greenlandic marine mammals, especially ice-dependent species like narwhals, walruses, and seals. Studies show a major loss of summer habitats for the Arctic ice-dependent whale species bowhead whale, beluga, and narwhal. Walruses rely on ice, and their numbers have dropped in some areas due to sea ice decline. On the other hand, walruses have a good heat tolerance, and longer ice-free periods may also have a positive effect, giving prolonged access to foraging and access to terrestrial haul-outs. In southeast Greenland, boreal cetaceans like pilot whales and dolphins are increasing in number as summer ice cover decreases, leading to major habitat changes. 

In general, the Arctic marine ecosystem around Greenland is changing rapidly due to climate change, with rising temperatures and decreasing sea ice altering habitats and species distributions. For Greenlandic waters several positive impacts are expected from moderate climate change. Projections indicate increased primary and secondary production in northern Greenland, leading to an increase in fish biomass. Diverse species, including Calanus glacialis and narwhals will move northward. Also economically important species, such as shrimp and halibut, are moving further north, benefiting local fisheries. In southern Greenland, warmer waters are attracting boreal species like mackerel and tuna, which could enhance future fisheries also there.

In the northern, seasonally ice-covered, part of the Barents Sea, limited light has hindered primary production. This has resulted in short and geographically limited phytoplankton blooms. Recently, the BS has seen a dramatic increase in net primary production due to reduced sea ice coverage. Spring phytoplankton blooms are occurring up to a month earlier and expanding northward at approximately 1° per decade. Model projections through 2100 under various climate scenarios suggest continued increases in primary production, with the most pronounced changes expected under higher emission scenarios. The effects vary by region, with northern parts of the Barents Sea benefiting from increased light exposure due to ice reduction, while southern areas are more influenced by wind-induced mixing and nutrient availability. Further, as ice coverage diminishes, a shift from ice-algal to open-water phytoplankton production is expected.

Ice-restricted primary production has until recently led to low food availability for zooplankton. Ongoing and future decreases in ice coverage will significantly expand and prolong secondary production, especially under the SSP8.5 scenario. Also, the fundamental transition from ice-algae to open-water plankton blooms will affect zooplankton community composition and size distribution. These changes in zooplankton communities are expected to continue and intensify with ongoing climate warming.

Benthic animals in the BS adapted to warmer temperatures may expand their range northward, while cold-adapted species may struggle as they cannot easily relocate. Many benthic animals are sessile and cannot move away from warmer waters, relying on gradual larval dispersal.  Active movers like the red king crab and snow crab are expanding their range. While the expansion is not directly linked to climate change, rising temperatures may facilitate further spread northward and offshore. Northern prawns demonstrate a notably positive response to warming scenarios in model projections, especially the SSP5-8.5 high-emission scenario. Increasing sea temperatures and retreating ice will likely expand fisheries northward, putting additional pressure on previously unaffected benthic species. 

Fish in the Barents Sea have historically been significantly affected by temperature variability. Higher temperatures and retreating sea ice, recently in 2004–2014, have expanded feeding areas for boreal species like Atlantic cod, while negatively impacting small Arctic fish species. The cod population has since decreased to “normal” levels, but this period may serve as an indicator of potential permanent changes under continued climate warming. Distinguishing between natural variability, climate change, and direct human pressures on harvested fish stocks is challenging. Future climate change will likely play a dominant role but impacts on fish stocks will vary. Recent modelling studies present contrasting projections for species like Atlantic cod and capelin, with some showing benefits from increased production and expanded distribution, while others suggest negative responses through the food chain.

Seabirds, which like mammals generally are at the top of the marine food web, are affected by climate change directly via extreme weather, but indirect effects through changes in food supply are likely more important. Changes in the timing of food availability, like the mismatch between ice melt and plankton blooms, can negatively impact Arctic seabird breeding. The islands around the Barents Sea host approximately 20 million seabirds during spring and summer, with 90% belonging to just five species. Recent population trends vary notably, with an important species like black-legged kittiwakes showing stability in Svalbard but declining in mainland Norway. Model projections suggest a general pattern of continued negative climate change impact on Arctic seabird populations throughout the Nordic- and Barents Seas.

Marine mammals in the Arctic including the northern BS are quite robust to direct climate change effects. However, reductions in sea ice are likely to negatively impact especially seals and walruses by directly reducing or removing their established breeding habitats and more indirectly by shifting the general location and timing of lower trophic level productivity. Also, several seal species already exhibit heightened vulnerability to rising temperatures. Model projections indicate increased negative responses to warming temperatures for various seal species, especially under higher emission scenarios. The effects of warming on whale species are more uncertain, but some, like bowheads, are expected to lose significant habitat under high emission scenarios.

The Barents Sea ecosystem is undergoing substantial transformations, leading to shifts in species distribution and community structure. While the differences in impact between emission scenarios are quite moderate towards 2050, they are pronounced toward the end of the century. Rising temperatures are, and will continue to, reduce ice cover, modify the mixed layer depth and alter nutrient mixing, impacting primary production and propagating through the food web. Marine heat waves are expected to become more frequent and intense, posing challenges for Arctic ecosystems. Further northward expansions of Atlantic communities, both pelagic and benthic, displacing Arctic species, are expected. Model projections and expert evaluations point towards mixed responses to future climate change, with some populations likely being negatively affected and others positively. However, changes will undoubtedly occur at the ecosystem level. A complicating factor is that for some central species (e.g., Atlantic cod) different models project contrasting developments also within the same climate scenario. Thus, there admittedly remains considerable uncertainty in projections for higher trophic levels.

Policy makers and managers where asked to point out what they consider to be the largest climate change related threats to marine ecosystems in the Nordic region. These pressures could be physical or more related to biogeochemical alterations. The following were highlighted, although not all are relevant for all the Nordic sea areas: Rising sea temperatures, more frequent and intense marine heat waves (MHWs), retreating sea ice, sea level rise, changes in freshwater runoff, coastal browning, eutrophication, and oxygen depletion. Secondary consequences included increasing number of harmful algal blooms, and general loss of biodiversity.

Rising sea temperatures is a relevant issue for all our areas, brought up by “everyone”. However, both mechanisms and consequences differ. It was also said that a gradual, slow, temperature rise may be less troublesome for marine life than more intense episodes, like MHWs. Negative ecosystem effects are generally considered most likely in areas that already are warm, but the most fundamental change will be in regions that today are (seasonally or more permanently) covered by ice. This naturally covers the very northern parts of the Nordic sea areas, but as pointed out, also large parts of the Baltic. The average length of what is called ice winter is projected to shorten 6 days per decade in the Bothnian Bay during this century (based on moderate RCP4.5 scenarios). Sea level rise was mentioned, but by a physicist, not the biologically oriented managers. Still, some areas will suffer 30–40 cm rise until 2100 (also within RCP4.5) and this will affect near-coast life. Eutrophication and biogeochemical changes are of major concern, especially in the Baltic. Here land-based eutrophication and atmospheric climate change are sources, which in turn intensify changes in marine biogeochemistry including reducing oxygen levels.

The respondents were also charged to suggest means to further develop Nordic cooperation across national borders aiming to sustainably solve common environmental challenges. This as a follow-up to questions to a panel debate at the final conference of the Nordic Council of Ministers Vision Project Marine Management and Climate in Gothenburg, November 2024.

The importance of marine protected areas (MPAs) was highlighted by respondents. It was said that by including MPAs more actively into marine management, one would better provide for safeguarding of important ecological functions and resilient ecosystems.  Main suggestions for expanding from today’s MPAs is i) to better take expected climate change impacts into consideration when designating MPAs, ii) to undertake more coordinated work towards establishing cross-border MPAs in shared Nordic waters, including iii) to develop common management plans for larger cross-border MPAs. 

It is also important for successful Nordic cooperation to establish a platform where all necessary data is accessible to relevant countries, so well-balanced decisions between harvesting and conservation can be made. This will also be enhanced by closer collaboration between managers and scientists. With regards to restoration of damaged ecosystems some see it as a last resort, due to its cost compared to not deteriorating, others view it as a needed and positive means that should improve marine nature's resilience against climate change.