How climate change impacts marine life

How does climate change impact marine life?

Climate change’s ‘deadly trio’

The main factors of climate change influencing Europe’s seas are increasing levels of carbon dioxide in the atmosphere, rising global temperatures and lower oxygen levels in the water. So far, the ocean globally has absorbed 91% of the heat generated by increased greenhouse gas emissions to the atmosphere, and around 30% of carbon emissions (IPCC, 2021; UNFCC 2021).

Carbon dioxide and increasing temperatures contribute to creating climate change’s ‘deadly trio’ for marine biodiversity: ocean acidification, sea warming and deoxygenation (Box 1).

The increasing trajectories of ocean acidification and sea warming are putting more stress on marine organisms (Figure 1).

Figure 1. Ocean acidification and increasing mean sea surface temperature in Europe’s seas

Notes: a) Decline in ocean pH measured at the Aloha station in the Pacific Ocean and yearly mean surface seawater pH reported on a global scale (Copernicus Marine Service; EEA, 2022a).

b) The past, present and future annual mean sea surface temperature for Europe’s seas. The black line shows the annual values for 1979-2018 from reanalysis data, and the dashed horizontal line shows the mean for 1986-2005 (for further details, see EEA, 2021a).

Combined, the ‘deadly trio’ make marine biodiversity more vulnerable and reduce ecosystems’ resilience. This can be through either direct impacts that create lethal conditions (e.g. too high temperatures) or through indirect effects exacerbating impacts from the other drivers of change (e.g. increased nutrient loads leading to low oxygen conditions).

The deadly trio leads to biodiversity loss, and degraded ecosystem functions and structures – often through non-linear, cascading effects. In parallel, ecosystem services suffer. For instance, there are reduced catches of commercially important fisheries, and blue carbon ecosystems have less capacity to store carbon.

Observations indicate that anthropogenic climate change has exposed ocean and coastal ecosystems to unprecedented conditions over millennia, leading to significant impacts for marine life and coastal regions.

The observed effects include alterations in ecological communities due to warming, acidification and deoxygenation, which create unfavourable conditions for many marine fish and invertebrates. These changes have resulted in habitat loss, population declines, increased risks of species extirpations and extinctions, and rearrangements of marine food webs (Pörtner et. al., 2022).

Ocean acidification

Ocean acidification, primarily caused by carbon dioxide emissions, reduces calcium carbonate availability for organisms. This makes it more difficult for organisms such as corals, molluscs and some plankton to build and maintain their structural integrity i.e. shells and skeletons. Ocean acidification has been increasing rapidly with pH dropping by approximately 30% since the pre-industrial era (IPCC, 2023; Figure 1). In the North Atlantic Ocean, the potential impacts on cold water corals are expected to be severe due to acidification and losses of carbonate skeleton (Fransner et al., 2022). When ocean acidification starts impacting organisms, its effects cascade throughout the food web — affecting ecosystem services such as fisheries.

Warmer waters

Due to the uptake of excess heat caused by global warming, seas have constantly warmed since the 1970s (Figure 1). This trend is expected to continue with the average temperature 0.88°C higher in 2011-2020 compared to 1850-1900 (IPCC, 2023). The world’s average sea surface temperature reached a new record of 21.1°C in April 2023 (NOAA, 2023).

Warmer water alters organisms’ metabolisms. For example, warmer water can increase oxygen demand. It can cause mobile species to move and change their distribution range, leading to changes in food webs and ecosystem dynamics, as seen for many fish species (EEA, 2022b).

Marine heat waves are extreme events that can outright kill native species, especially if they happen during the summer months. Such an event happened in the Western Mediterranean Sea in 2003 (von Schuckmann et. al., 2019).

Besides being lethal to native species, heatwaves open habitats to other pressures like non-indigenous species. These can rapidly take over the ‘ecological space’ left open by the disappearance of native species. Non-indigenous species can thus become invasive by overtaking the ecological niche (Garrabou et. al., 2009).

Ocean deoxygenation

Deoxygenation — the loss of oxygen in sea water — is a result of ocean warming. It occurs as warmer waters hold less dissolved oxygen. This occurs alongside increased oxygen consumption by various organisms and increased stratification and changes in ventilation (IPCC, 2019).

Coupled with a rising nutrient concentration that can occur due to more precipitation or runoff from intense farming, deoxygenation can lead to an expansion of hypoxic or anoxic conditions. These conditions will be lethal to many species, especially sessile (non-mobile) marine organisms.

Coastal areas influenced by hypoxia have increased four-fold since the 1950s (Breitburg et al., 2018). In Europe, the information available to assess oxygen loss is limited to a few areas. At the stations monitoring the EU’s marine regions, 9% showed a deteriorating trend and 3% an improving trend. Yet for the remaining 88%, no trend could be established (EEA, 2022c). It is projected that the global ocean will lose 3-4% of its oxygen by 2100 (Bopp et. al, 2013).

How is Europe’s marine biodiversity doing?

Overall, Europe’s seas are in a state of degradation (Figure 2). Most species groups are in a degraded state across Europe’s seas. Bony fish are potentially a positive exception (Vaughan et al., 2019). Across the four major seas — North-east Atlantic Ocean, Baltic Sea, Mediterranean Sea and Black Sea — many marine ecosystems are in a poor to bad state (EEA, 2020; ICES, 2022a, 2022b). The only exceptions are offshore areas in the North-east Atlantic Ocean where less activities occur (EEA, 2020).

Figure 2. Overall condition and trends of marine biodiversity in Europe’s seas

Note: For details on sources and methodology, please refer to Vaughan (2019).

The overall degraded state of Europe’s marine ecosystems cannot be ascribed to one stand-alone factor. Rather, it represents the sum of interlinked effects from past and present use and other impacts from human activities. These activities are related to the direct drivers of change. This includes climate change, pollution with nutrients and contaminants, changes in land and sea use, overexploitation and the introduction of non-indigenous species (EEA, 2020).

A recent study from Sweden indicates that climate change may be the biggest single threat to marine ecosystems today, with modelled impacts comparable to the combined impact of all other current stressors (Wåhlström et. al., 2022).

However, some species, such as seals, are starting to show improving trends. Such trends may tentatively indicate that national, regional and EU-wide policies and actions, such as a ban on hunting, may be beginning to work (EEA, 2020; EEA 2021).

Moreover, some biodiversity trends are improving. This suggests that it may be possible to help certain ecosystem components recover by reducing the pressures that impact them. Such components include habitats and species. If enough components are restored, it may even be possible to recover structures and functions and thus maintain ecosystem resilience despite the ever-present and growing effects of the ‘deadly trio’. 

Understanding the combined impacts of climate-related stressors

Map 1 depicts marine areas’ relative vulnerabilities to climate change using a new exploratory approach. It does this by combining a range of climate change stressors (sea surface temperature, salinity and water), chemistry (acidification and oxygen content), physical elements (currents, wind exposure and sea level rise), ice cover and ecosystem components from across Europe’s seas (Map 1; Murray et. al, forthcoming).

The European Marine Climate Change Index indicates the relative magnitude of the effects of climate change stressors (e.g. sea surface temperature and acidification) on the ecosystem components mapped in the assessment.

In general, semi-enclosed seas, shallower shelf areas and coastal areas are more vulnerable than open sea areas. Vulnerability is particularly high in parts of the Baltic Sea and the Adriatic Sea, and in parts of the North Sea. There are also vulnerable areas in the North-east Atlantic Ocean close to Svalbard, Norway.

Map 1. Exploratory approach to mapping the European Marine Climate Change Index (EMCCI) for Europe’s seas

Note: The index shows areas where ecosystem components are subject to relatively greater impacts from climate-related stressors. It is specific to a particular study and cannot be compared directly with effect or impact indices from other similar studies. 

Sources: ETC/ICM; see Murray et al. (2023) on methodology.

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Benthic (bottom-dwelling) species are the most impacted by climate stressors in Europe's seas as a whole and in the Baltic Sea, Mediterranean Sea and Black Sea. However, in the North-east Atlantic, fish species account for 34% of the indexed score sum (most impacted feature), while benthic communities account for 31% (Figure 3a).

Water chemistry stressors, specifically pH and dissolved oxygen concentrations, make the largest contributions to Europe’s seas’ total effect index. These impacts are greater in offshore waters but still have the most significant impacts in coastal waters across Europe (Figure 3b).

Figure 3. a) Relative sums of estimated effect index within Europe’s seas, grouped by ecosystem component, and b) Relative sums of contributions of stressor groups to the total sums of estimated effect index within Europe’s seas

Note: NIS: Non-Indigenous Species. 

Sources: ETC/ICM; see Murray et al. (2023) on methodology.

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The current trends of the ‘deadly trio’ will take decades, if not centuries, to stall and reverse (IPCC, 2023). However, even if we manage to stabilise global warming at a certain point in time, the warming, the acidification and the deoxygenation will continue to increase due to time lag of achieving a homeostasis between the global atmosphere and the ocean. In addition to the need to reduce greenhouse gases to limit long term climate impacts, shorter term actions to reduce other stressors are crucial to help mitigate damage to marine ecosystems.

The insights provided by the spatial description (Map 1) and the linkages between stressors and ecosystem components (Figure 3 a and b) may inform viable management actions to maintain and restore ecosystem resilience. Specifically, these insights inform stakeholders about the areas and associated ecosystem components that are most vulnerable to a given stressor.

This spatial approach also enables a further integration of climate change with future assessments of cumulative impacts and their effects on the marine environment (EEA, 2021). This approach better distinguishes the relative contributions of individual pressures at a given locality and thus can better inform management measures. It can also inform the development of stronger spatial management measures for human activities impacting Europe’s seas.

Actions to restore marine ecosystem health and services

With the European Green Deal, the EU has set ambitious targets for maintaining and restoring Europe’s seas.

The MSFD aimed to achieve ‘good environmental status’ by 2020 by introducing an ecosystem-based approach to managing human activities (EC, 2008). The MSFD includes a holistic approach to reducing cumulative pressures and effects and setting threshold values for good environmental status as of 2018. The objective to achieve ‘GES’ by 2020 has failed.

To improve this situation, the MSFD is being revised with a focus on achieving environmental targets and GES threshold values. The revisions aim to (EC, 2022):

• strengthen implementation and enforcement;
• improve regional cooperation and coordination;
• improve policy coherence, including with climate policy;
• ensure more effective data management.

The EU Strategy on Adaptation to Climate Change (2021) explicitly recognises the need for ecosystem restoration and management to minimise the risks and improve resilience. It includes measures to close knowledge gaps on climate impacts and resilience, including those gaps relating to oceans. This strategy will continue to incentivise and assist Member States to rollout nature-based solutions. The EU will continue to be strongly engaged in international ocean governance and observation.

As part of the EU Biodiversity Strategy for 2030, the proposed Nature Restoration Law (NRL) aims to strengthen the EU’s biodiversity objectives, which are the protection, restoration and increased resilience of marine ecosystems. Both the MSFD and NRL have strong spatial elements regarding linking the marine environment and the pressures on that environment. However, to properly link them to the management of human activities, a strong integration with the Maritime Spatial Planning Directive will be needed. This includes careful consideration of where and how to use the seas’ limited space and resources.

A key measure for restoring ecosystem resilience includes expanding marine protected areas to cover 30% of Europe’s seas. This is a central target of the EU Biodiversity Strategy, which has now also been adopted globally in the Kunming-Montreal Global Biodiversity Framework. The EU and 195 countries adopted the Kunming-Montreal Global Biodiversity Framework in 2022. The expansion of MPAs in the EU could take place under either the Natura 2000 network or as part of the MSFD’s implementation.

Moreover, stronger implementation is needed to link nature conservation and sustainable use, for example through the EU’s action plan to protect and restore marine ecosystems for sustainable and resilient fisheries.

These policies are further supported by EU research funded through Horizon Europe, notably the EU missions to restore oceans and waters by 2030 and on adaptation to climate change. A key task will be to take a more holistic approach to the design of MPA networks and management of individual MPAs by considering all habitats and species rather than only vulnerable and sensitive elements.

A sharp reduction of greenhouse gas emissions to limit global warming to a maximum of between 1.5°C to 2°C is key and a prerequisite for the ability to take measures to make marine ecosystems more resilient. This way, these ecosystems can adapt as much as possible to the unavoidable impacts of climate change.

In addition, it may be possible for oceans to become more resilient by stepping up EU efforts to ensure synergies between these policy measures while taking action to mitigate climate change.

EU actions may enable marine areas to adapt to climate change, while conserving and restoring biodiversity. Marine areas may continue to provide society with much-needed ecosystem services, including carbon sequestration, food and biogenic material supply, and recreation and tourism.

On the other hand, a business-as-usual scenario will have devastating consequences on marine ecosystems as more and more tipping points would be crossed. This scenario would result in even more losses of ecosystem resilience, further impacting communities across Europe.