Seas under pressure – Ocean acidification due to CO₂

Besides soils and forest ecosystems, oceans and seas are among the largest global carbon reservoirs. They absorb approximately one-third of anthropogenic carbon dioxide (CO₂) emissions from the atmosphere. Due to the increasing energy-related CO₂ emissions into the atmosphere, the constant gas exchange between seawater and the atmosphere also directly increases the CO₂ concentration in the seawater near the surface. The CO₂ dissolves in the water, is biologically sequestered (for example by algae) or deposited as inorganic matter (for example lime).

The following processes determine CO₂ storage in the oceans:

a) Biogenic carbon storage (formation of organic matter), which converts CO₂ into organic compounds through photosynthesis by phytoplankton, seagrasses, and seaweed.

b) CO₂ storage in the form of bound lime through biomineralisation during the formation of bone and shell structures (production of ‘composite materials’ in so-called calcifiers such as corals, calcareous algae, mussels and snails).

Without this storage function of the oceans, climate change would progress much faster and more intensively. Oceans can therefore slow down climate change, but this comes at a high ecological price.

As the concentration in the air increases, the oceans also absorb more CO_₂. Due to the increased dissolution of CO_₂ in seawater, the pH value in the oceans decreases, and the oceans slowly acidify. The pH value of water is a measure of the acid-base equilibrium system and is predominantly controlled in most natural waters by the presence of CO_₂, bicarbonate (HCO_₃^-) and carbonate (CO_₃ ²^-).

Acidification process in the sea

Illustration of the chemical equilibrium reaction for carbon dioxide (CO₂) in the sea.

Source: FG II 2.3 / UBA

CO₂ uptake depends on various factors. The most important factors are atmospheric pressure (partial pressure, p) and temperature (shown in the figure by the increase in reddish colouring). Upon contact with the sea surface, CO₂ from the atmosphere dissolves in the water (H₂O), and consequently the partial pressure of CO₂ (pCO₂) in the water is increased. Carbonic acid (H₂CO₃) is formed briefly. However, it immediately decomposes into HCO₃^- and a hydrogen cation (proton, H₃O⁺).

The formation of HCO₃⁻ is part of the chemical buffer systems, which absorb pH fluctuations and maintains a constant pH value. The increase in free protons reduces the pH value of the water and fewer CO₃²^- ions are available to react further with calcium cations to form calcium carbonate (CaCO₃). These various chemical compounds in the water derived from CO₂ are known as dissolved inorganic carbon (DIC).

Before industrialisation, the pH value of the oceans was 8.2 - primarily due to the soluble salts (salinity) - and therefore in the slightly alkaline range. Today, the CO₂ content in the oceans is as high as it has been in the last 20 million years and continues to rise. Long-term measurement series have shown that the pH value - parallel to the atmospheric increase in CO₂ - has already decreased by an average of 0.1 units. It is important to note that the pH scale is logarithmic, not linear. A decrease of 0.1 pH units corresponds to an increase in acidity of nearly 30 %.

In scientific circles, the effect of ocean acidification is referred to as the ‘evil twin‘ of global warming. Although seawater currently remains slightly alkaline with a pH value of 8.1, it is becoming increasingly acidic. By the end of the 21st century, the pH value could decrease by a further 0.3 to 0.4 units - which would correspond to an increase in acidity of 100 to 150 %.

The temperature of the water plays a role in CO₂ absorption. In addition to CO₂, oceans store the majority of the heat caused by the greenhouse gas effect. However, rising temperatures reduce the CO_₂ absorption capacity of seawater and, thus, the amount of CO_₂ already dissolved. CO_₂ outgasses from warmer seas and the buffering capacity of the oceans decreases as a result. This feedback effect leads to changes in the pH value. Due to the high spatial and temporal variability of pCO_₂ in seawater, it is currently unclear which seas represent a net carbon sink or a source of CO_₂.

Ocean acidification is not progressing at the same rate in all oceans. The speed at which ocean acidification occurs varies geographically and throughout the water column. Overall, colder waters absorb more CO_₂ from the atmosphere than warmer waters. This is why the oceans in the polar regions are acidifying faster than the global average. Even though melting ice, i.e. fresh water, might slightly raise the pH value, the increased partial pressure of the CO_₂ concentration in the atmosphere leads to more CO_₂ being absorbed by the sea in the cold regions of the Arctic and Antarctic. Polar ecosystems are particularly vulnerable to the rapid progression of ocean acidification, as the organisms there can only adapt slowly to changing conditions. The problem of acidification also affects so-called upwelling areas, which provide a good food source (including for commercially utilised fish species) due to the high nutrient supply. These upwelling areas occur in the open ocean in the subpolar regions and along the equatorial ocean. As the water from the deep is generally more acidic than the water near the surface, a further decrease in the pH value in upwelling areas could negatively impact the food supply. The resulting changes to the ecological system would also have an economic impact, such as affecting fisheries.

Coastal regions naturally experience fluctuations in pH value. Species inhabiting these areas are adapted to these fluctuations and can cope with daily or seasonal changes.

It is already measurable that the pH value of the North Sea, like the average pH value of seawater worldwide, has decreased by 0.1 units. Scientists predict that the value will decrease by a further 0.27 units by the end of the century. Acidification has demonstrable effects on marine life in the North Sea - for example, on the composition of plankton or on the development and behavior of fish. As crabs and mussels particularly suffer from ocean acidification, the economically important shellfish fishery in the North Sea will be affected. The assessment of the state of the North-East Atlantic illustrates such changes and their effects on marine ecosystems, among other things.

In contrast to the North Sea, the Baltic Sea has so far hardly been affected by measurable acidification, as measurement series by various research groups since 1980 have shown (Climate Change in the Baltic Sea, 2021 Fact Sheet and Baltic Earth Assessment Reports 2023). This is partly due to the fact that the Baltic Sea is an inland sea that is largely separated from the world's oceans, representing a complex and very heterogeneous ecosystem. The special hydrographic conditions (for example large river discharge, rare and occasional inflows of saline water and permanent stratification of the water column) and the extreme pressure from external nutrient and pollutant inputs are among the dominant factors. Furthermore, weathering products from rocks carried in via rivers have a buffering effect on the CO₂ content in the Baltic Sea. These continental inputs contribute to an increased pH value in the Baltic Sea, compensating for acidification.

Acidifying oceans inhibit calcification

The main problem with ocean acidification is the decreasing concentration of carbonate ions. Carbonate, CO_₃ ²^-, in combination with calcium, is the main component of calcareous skeletons and shells of many marine organisms. Corals, echinoderms such as sea urchins and starfish, as well as some phytoplankton species (the so-called coccolithophores), for example, have a calcareous skeleton (see figure below), while mussels, shelled snails and crabs have calcareous shells.

Calcareous algae (coccolithophore) Emiliana huxleyi (coccolithophores)

Calcareous algae use carbon to build their skeleton. Skeletal dissolution occurs in an acidic environment.

Source: Prof. Dr. André Scheffel / Universität Greifswald

Due to ocean acidification, these organisms have less CO_₃ ²^- available, impairing and weakening calcification (biomineralisation). Two types of CaCO_₃ play a crucial role. The more easily soluble aragonite (a modification of CaCO_₃) is a main component of mother-of-pearl and is therefore relevant for the structure of mussel shells. The less soluble calcite, also a form of CaCO_₃ belonging to the mineral class of ‘carbonates and nitrates’, is an essential building block for stony coral skeletons, mussel shells, sea urchins and starfish. The cold-water coral reefs in the Northeast Atlantic are also affected.

Most marine organisms have to adapt their bodily functions to the increasingly acidic conditions in the water. They do this by adjusting their acid-base balance to the changed pH value. This adaptation costs energy, which they then lack for vital processes such as growth or reproduction. In many marine animal species, it has already been demonstrated that their ability to survive, their population size, or their larval development decreases under acidic conditions. This applies to juvenile fish, for example, which feed on a yolk sac during the first few days of life. The size and amount of energy contained in the yolk sac depend on the temperature and acidity of the seawater. A lack of sufficient nutrition can affect larval growth and later reproduction. Marine organisms are also exposed to other environmental stresses - such as rising temperatures, eutrophication, or pollution from waste and pollutants. More information can be found in the results of the international ‘Bioacid’ project.

There are also organisms that benefit from the increasing CO_₂ levels in the oceans. These include organisms that perform photosynthesis, converting CO_₂ and water into oxygen and energy. Examples of photosynthetically active organisms include seagrasses, algae, and cyanobacteria (blue-green algae). They show a higher photosynthetic capacity and stronger growth under increased CO_₂ levels. However, they suffer from rising temperatures. Jellyfish and some toxic algae are among the beneficiaries of ocean acidification. Due to the reduction in fish stocks, they have fewer food competitors and predators and are appearing more and more frequently in masses.

Algal bloom in the North Sea

ESA satellite image of the algal bloom taken by Copernicus Sentinel-3A in September 2018.

Source: European Space Agency 2018 / https://www.esa.int/esearch?q=algae+north+sea)

In addition to CO_₂ as the main cause of ocean acidification, the direct input of sulphur and nitrogen oxides from the atmosphere (known as aerosols) also influences the pH value in the sea. Chemical reactions, such as the formation of sulphuric acid in the atmosphere, lead to a reduction in the pH value in precipitation (sometimes to a pH value below 5), creating acid rain. Other gases such as ammonia (NH_₃), which mostly originate from agricultural sources and are transported to the sea via the air, can in turn neutralise acidic aerosols in the atmosphere. The acidification potential of the sea can therefore be strongly influenced by humans through air pollution.

In addition to aerosols from natural and anthropogenic terrestrial sources, the sea surface itself is also an important source of a large number of trace gases. In addition to carbon, marine aerosols also contain sulphur, nitrogen and halogen compounds, which are produced by macrophytes and phytoplankton, among others. Volatile organic compounds (VOCs) are also produced and released back into the atmosphere.

Thus, there is a continuous gas exchange over the sea (see the following figure). Atmospheric oxidation products of some trace gases, such as dimethyl sulfide (DMS), methylamine, and a range of biogenic volatile organic compounds, have impacts on marine aerosols, which in turn affect cloud formation processes, ozone formation, and global radiative forcing. Therefore, aerosols are considered the natural antagonists of greenhouse gases. Given the known and predicted effects of ocean acidification on biological processes, it is likely that the net production of many biogenic trace gases will also be influenced by ocean acidification.

Increasing ocean acidification thus has a direct impact on the formation of gaseous and aerosol-like substances. The formation of marine aerosols and their feedback on atmospheric chemistry and potential climatic effects is complex and requires further research (GESAMP Report, IMPACT ON AIR–SEA CHEMICAL EXCHANGE, 2019).

Gas exchange of aerosols

The sea surface is an important source for the formation of trace gases and aerosols.

Source: FG II 2.3 / UBA

Our food sources, regional coastal fisheries, aquaculture, and the seafood industry are also affected by ocean acidification. In the long term, they will have to adapt to other species and declining catches.

In tropical regions, the dying coral reefs will result in the loss of an important component of coastal protection by reducing wave energy. The death of corals also leads to decreasing biodiversity in the affected regions, which negatively impacts the tourism industry in coastal areas while reducing the stressors tourism imposes on the ocean. However, the long-term consequences of acidification on marine life are not foreseeable. What is certain is that many people worldwide depend on the ocean and will need to adapt to these changes.

1. Reduce emissions at the source

Ocean and marine protection must be closely linked to climate protection: Global CO_₂ emissions as well as air pollutants must be reduced, and international climate protection goals must be achieved in order to mitigate the impacts of climate-related environmental stressors, such as ocean acidification. To reduce greenhouse gas emissions, the German Government has developed a climate action program in 2023 and has enshrined national greenhouse gas reduction targets in the climate action law. The Federal Environment Agency supports the development of the climate protection plan and reduction targets.

1. Marine conservation

Seas and oceans must be protected from further harmful influences, such as pollution. The fewer stressors acting on the oceans, the more likely nature can adapt to the inevitable changes (‘Marine conservation concerns us all’). Germany is committed to protecting at least 30 percent of the oceans worldwide. Protected areas in the Antarctic and Arctic aim to safeguard marine life in the polar regions, that are particularly affected by ocean acidification. In the protected areas of the North Sea, the Federal Ministry for the Environment (BMUV) supports measures for the restoration of European oyster reefs, among other initiatives. Additionally, Germany is a member of the International Coral Reef Initiative (ICRI) and supports developing countries in coral reef conservation and restoration efforts within this framework.

1. Utilise research results more efficiently

The problem of ocean acidification is complex and its impacts and interactions with other environmental stressors have not yet been fully researched and understood. Science has been focusing on these issues for around 20 years. In Germany, the nationally-funded Bioacid project investigated the effects of ocean acidification on marine life between 2009 and 2017.

Further research is needed on the interactions of ocean acidification with other environmental stressors – for example, with temperature increases, eutrophication, ocean deoxygenation, and various types of pollution.

1. Setting guard rails

With its special report on the future of the oceans, the German Advisory Council on Global Change (WBGU) proposed in 2006 to use ocean acidification as a kind of ‘guardrail‘. Specifically, this means that the pH value in the surface layers of the oceans should not decrease by more than 0.2 units compared to the pre-industrial value of 8.18 on average. The WBGU also emphasises the need to further specify the spatial and temporal averaging relating to this threshold value, as the pH value is strongly influenced by natural variations. Continuous measurements and further research are necessary for this purpose.

1. Adjust international framework conditions

Seas and oceans must be protected comprehensively and with all diligence through international treaties and agreements, as envisaged by the European Marine Strategy Framework Directive (MSFD). Regional agreements focus on the protection of the North Sea and the North-East Atlantic (Oslo and Paris Convention; OSPAR) and on the protection of the Baltic Sea (Helsinki Commission; HELCOM).
Due to the global pressure to act on global warming, the United Nations (UN) also established the Oceans Compact initiative in 2012, which includes a strategic vision for the protection of the seas and the people whose livelihoods depend on them.

With the adoption of the Global Agenda 2030 in 2015, 17 Sustainable Development Goals (SDGs) were established as a global plan to promote sustainable peace and prosperity and to protect our planet. ‘SDG 14’ addresses the agenda item of ‘Conserve and sustainably use the oceans, seas, and marine resources for sustainable development.’ The main objective is to minimise and address the impacts of ocean acidification.

Due to the far-reaching and accelerating climate-induced changes to marine ecosystems, the Intergovernmental Panel on Climate Change (IPCC) issued a special report on the ocean and cryosphere. This report summarises observations and scientific findings and describes the effects of climate change on the oceans (e.g. warming, acidification, sea level rise) in more detail.

In March 2023, the United Nations was able to agree on an international treaty for marine protection for the High Seas after many years of negotiations. With the ‘Biodiversity beyond national jurisdiction (BBNJ)‘ agreement, conditions are now set for establishing protected areas beyond national waters and for conducting effective environmental impact assessments of human activities to protect the biodiversity of the High Seas and address the climate crisis.

1. Actively removing CO_₂ emissions

There are various ocean-based strategies for mitigating climate change that are intended to contribute to achieving net zero emissions. These are categorised into natural (blue carbon) and technical marine geoengineering solutions. Many of them are still at a conceptual or early stage of development, and their effectiveness and feasibility on a large scale has not yet been clearly demonstrated.

The research mission ‘Marine Carbon Storage as a Pathway to Decarbonisation’, CDRmare (CDR, Carbon Dioxide Removal), addresses the issue of dealing with rising CO_₂ concentrations in the atmosphere and possible solutions. The three-year project has been running since 2021. The environmental risks of CDR approaches, including the potential to mitigate rather than exacerbate ocean acidification, need to be further explored.