Indicator Assessment

Ocean acidification

Indicator Assessment
Prod-ID: IND-349-en
  Also known as: CLIM 043
Published 24 Jun 2020 Last modified 09 Aug 2021
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  • Currently, the ocean takes up about one quarter of global CO2 emissions from human activities. The uptake of CO2 in the sea causes ocean acidification, as the pH of sea water declines.
  • Ocean surface pH declined from 8.2 to below 8.1 over the industrial era as a result of an increase in atmospheric CO2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %.
  • In recent decades, ocean acidification has been occurring 100 times faster than during natural events over the past 55 million years. These rapid chemical changes are an added pressure on marine ecosystems.
  • Observed reductions in surface water pH are nearly identical across the global ocean and throughout European seas, except for variations near coasts. The reduction in pH in the northernmost European seas, i.e. the Norwegian Sea and the Greenland Sea, is larger than the global average.
  • Ocean acidification has wide-ranging impacts on marine ecosystems. A reduction in carbonate availability reduces the rate of calcification of marine calcifying organisms, such as reef-building corals, shellfish and plankton. Ocean acidification has already affected the deep ocean, particularly at high latitudes.
  • Changes in pH affect biological processes, e.g. enzyme activities and photosynthesis, which in turn affects primary production. These changes may be exacerbated by rising seawater temperatures.
  • Changes in marine primary production will have an impact on the global carbon cycle and the absorption of atmospheric CO2 in the ocean, as well as on the overall capacity of ocean to mitigate climate change.
  • Models consistently project further ocean acidification worldwide. Ocean surface pH is projected to decrease to values between 8.05 and 7.75 by the end of the 21st century, depending on future CO2 emission levels. The largest projected decline represents more than a doubling in acidity.
  • The combined effects of elevated seawater temperatures, deoxygenation and acidification are expected to have negative effects on entire marine ecosystems and cause changes in food webs and marine production, and will also result in economic losses.

Decline in ocean pH, as shown by measured pH and pH calculated from dissolved inorganic carbon concentrations and total alkalinity, at in situ temperature at the Aloha station (Hawaii), and global annual average pH

Data sources:
Data sources:

Yearly mean surface seawater pH reported on a global scale

Data sources:
Data sources:

Past trends

The global annual mean atmospheric CO2 concentration exceeded 400 ppm in 2016, which is more than 40 % above the pre-industrial level (280 ppm). Half of the increase in CO2 concentrations has occurred since the 1980s. Over the same period, the ocean pH decreased from 8.11 to below 8.06, which corresponds to an approximately 30 % increase in ocean acidity (defined here as the hydrogen ion concentration). This decrease in pH occurred at a rate of approximately 0.02 pH units per decade (Bindoff et al. 2019), which is about 100 times faster than any change in acidity experienced during the past 55 million years (Rhein et al., 2013). The measured reduction in surface pH in the surface mixed layer (depths to 100 m) is consistent with that calculated on the basis of increasing atmospheric CO2concentrations, assuming thermodynamic equilibrium between the ocean surface and the atmosphere (Byrne et al., 2010). The northernmost seas, i.e. the Norwegian Sea and the Greenland Sea, have experienced surface water pH reductions of 0.13 and 0.07, respectively, since the 1980s, both of which are larger than the global average (Ingunn et al., 2014). In the Mediterranean Sea, all waters have been acidified by values ranging from −0.156 to −0.055 pH units since the beginning of the industrial era. This is clearly higher than elsewhere in the open ocean (Touratier and Goyet, 2011; Hassoun et al., 2015). These conclusions apply to open ocean waters, acidification in coastal waters is not assessed in this indicator.

Fig. 1 shows the decline in ocean surface pH over the period 1988-2017 from a station offshore of Hawaii (the Aloha station), for which the longest time series is available (Dore et al., 2009). The changes observed at two other ocean stations suitable for evaluating long-term trends (offshore of the Canary Islands and Bermuda) are very similar (Rhein et al., 2013).

The global average of surface ocean pH from the Copernicus Marine Environment Monitoring Service (CMEMS) is also used for the indicator. The indicator is available at annual resolution, and from the year 1985 onwards (Fig. 2). The global mean surface sea water pH estimated by CMEMS shows a trend closely following the in situ measurements in Fig. 1. According to the estimated global mean surface sea water pH (Fig. 2), there has been a decrease in pH since 1985 of 0.0016 pH units per year, with an error on each yearly value of 0.0006 (Gehlen et al., 2019).


The average surface open ocean pH is projected to decline further, in the range 0.04 to 0.29 pH units by 2081-2100, relative to 2006-2015, depending on future CO2 emissions (Bindoff et al. 2019). The largest projected decline represents more than a doubling in acidity (Joos et al., 2011). Similar declines are also expected for enclosed, coastal seas such as the Baltic Sea (Helcom, 2013).  However in the Baltic Sea, acidification caused by an increase in atmospheric CO2 varies from north to south. Difference in acidification trends are related to bedrock in sub-basins and terrestrial runoff from these areas. The bedrock of the northern part mainly consists of granite and is covered only with a thin layer of soil, while the bedrock of the southern part consists of limestone. Limestone dissolves under acidic condition and therefore increases acidification; runoff from these areas cause increased alkalinity and by that partly mitigate acidification (Müller et al., 2016).

Ocean acidification caused by anthropogenic CO2 uptake is expected to be detrimental to multiple calcifying plankton species by lowering the concentration of carbonate to levels where calcium carbonate shells begin to dissolve. Research by McNeil and Matear (2008) indicates that more sensitive phytoplankton species are expected to experience detrimental conditions much earlier than previously thought and recognised by IPCC. Disturbance to key phytoplankton species such as the Pteropod species (Limacina helicina) may cause deleterious impacts on the wider Southern Ocean marine ecosystem (McNeil and Matear, 2008).

Ocean acidification will affect many marine organisms and could alter marine ecosystems and fisheries. These rapid chemical changes are an added pressure on marine calcifiers and the ecosystems of Europe’s seas.

Without substantial reductions in CO2 emissions, recovery from human-induced acidification will require thousands of years for the Earth system to re-establish roughly similar ocean chemical conditions (Archer, 2008) and millions of years for coral reefs to return, based on palaeo-records of natural coral reef extinction events (Orr et al., 2005).

References in specific assessment text

  • Archer, D. and Brovkin, V., 2008, ‘The millennial atmospheric lifetime of anthropogenic CO2’, Climatic Change 90, pp. 283-297, doi:10.1007/s10584-008-9413-1.
  • Dore, J. E., et al., 2009, ‘Physical and biogeochemical modulation of ocean acidification in the central North Pacific’, Proceedings of the National Academy of Sciences of the United States of America 106, pp. 12235-12240, doi:10.1073/pnas.0906044106.
  • Dore, J. E., 2012, 'Hawaii ocean time-series surface CO2 system data product, 1988-2008’ (
  • Gehlen, M., et al., 2019, 'Global mean sea water pH', EU Copernicus Marine Service Information (
  • Hassoun, A.E.R. et al., 2015, Acidification of the Mediterranean Sea from anthropogenic carbon penetration. Deep Sea Research Part I: Oceanographic Research Papers. Volume 102: 1-15.
  • Helcom, 2013, ‘Climate change in the Baltic Sea area: Helcom thematic assessment in 2013’, Baltic Sea Environment Proceedings 137.
  • Ingunn, S., et al., 2014, ‘Havforsuring Og Opptak Av Antropogent Karbon I de Nordiske Hav, 1981-2013 [Ocean Acidification and Uptake of Anthropogenic Carbon in the Nordic Seas, 1981-2013]’, Uni Research, Havforskningsinstituttet og Universitetet i Bergen, Bergen (
  • Joos, F., et al., 2011, ‘Impact of climate change mitigation on ocean acidification projections’, in: Ocean acidification, Oxford University Press, Oxford, pp. 272-290 ( 
  • McNeil, B. I. and Matear, R. J., 2008, ‘Southern ocean acidification: a tipping point at 450-ppm atmospheric CO2’,Proceedings of the National Academy of Sciences of the United States of America 105, pp. 18860-18864, doi:10.1073/pnas.0806318105.
  • Müller, J.D. et al., 2016. Long‐term alkalinity trends in the Baltic Sea and their implications for CO2‐induced acidification. Limnology and Oceanography 61: 1984-2002
  • Orr, J. C., et al., 2005, ‘Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms’, Nature 437, pp. 681-686, doi:10.1038/nature04095.
  • Rhein, M., et al., 2013, ‘Observations: Ocean’, in: Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Stocker, T. F., et al. (eds), Cambridge University Press, Cambridge, UK, and New York, NY, pp. 255-316 (
  • Touratier F. and Goyet C., 2011. Impact of the Eastern Mediterranean Transient on the distribution of anthropogenic CO2 and first estimate of acidification for the Mediterranean Sea. Deep-Sea Research, 58:1-15.

Supporting information

Indicator definition

This indicator illustrates the global mean average rate of ocean acidification, quantified by decreases in pH, which is a measure of acidity and is defined here as the hydrogen ion concentration. A decrease in pH value corresponds to an increase in acidity.

The observed decrease in ocean pH resulting from increasing concentrations of CO2 is an important indicator of global change.

This indicator provides information on:

  • - trends in ocean acidity measured at the Aloha station;
  • - yearly mean surface sea water pH levels reported on a global scale, based on information from the Copernicus Marine Service (CMEMS), with the estimated long-term decreasing trend.


  • Acidity is measured as pH (dimensionless)


Policy context and targets

Context description

The 2008 Marine Strategy Framework Directive (MSFD; Directive 2008/56/EC) is the environmental pillar of the Integrated Maritime Policy and its main driver towards clean, healthy and productive European seas. The MSFD aims to protect and restore the marine environment and phase out pollution, leading to no significant impacts on or risks to marine biodiversity, human health and the legitimate use of marine resources. The MSFD requires the achievement of 'good environmental status' (GES) for EU marine waters by 2020. Acidification is addressed under MSFD Descriptor 7 (Hydrographic conditions).

In April 2013, the European Commission (EC) presented the EU Adaptation Strategy Package. This package consists of the EU Strategy on adaptation to climate change (COM/2013/216 final) and a number of supporting documents. The overall aim of the EU Adaptation Strategy is to contribute to a more climate-resilient Europe.

One of the objectives of the EU Adaptation Strategy is to enable better-informed decision-making, which will be achieved by bridging the knowledge gap and further developing the European climate adaptation platform (Climate-ADAPT) as a ‘one-stop shop’ for adaptation information in Europe. Climate-ADAPT has been developed jointly by the EC and the European Environment Agency (EEA) to share knowledge on (1) observed and projected climate change and its impacts on environmental and social systems and on human health; (2) relevant research; (3) EU, transnational, national and subnational adaptation strategies and plans; and (4) adaptation case studies.

Further objectives include promoting adaptation in key vulnerable sectors through climate-proofing EU sector policies and promoting action by Member States. Most EU Member States have already adopted national adaptation strategies and many have also prepared action plans on climate change adaptation.

Staff working document SWD(2013) 133 Climate change adaptation, coastal and marine issues was published alongside the EU Strategy, The paper provided an overview of the main impacts of climate change on coastal zones and marine issues, including environmental, economic and social systems aspects. The document also pointed out knowledge gaps and existing EU efforts to best adapt to the impacts of climate change on coastal zones and marine issues.

In November 2013, the European Parliament and the European Council adopted the Seventh Environment Action Programme (7th EAP) to 2020, ‘Living well, within the limits of our planet’. The 7th EAP is intended to help guide EU action on environment and climate change up to and beyond 2020. It highlights that ‘Action to mitigate and adapt to climate change will increase the resilience of the Union’s economy and society, while stimulating innovation and protecting the Union’s natural resources.’ Consequently, several priority objectives of the 7th EAP refer to climate change adaptation. The planetary boundary framework identified nine processes that regulate the stability and resilience of the Earth system — ‘planetary life support systems’. The framework proposes precautionary quantitative planetary boundaries within which humanity can continue to develop and thrive, also referred to as a ‘safe operating space’. It suggests that crossing these boundaries increases the risk of generating large-scale abrupt or irreversible environmental changes that could shift the Earth system to states that are detrimental to or catastrophic for human development.

Ocean acidification is identified as one of the nine planetary boundaries.

The EC published an Evaluation of the EU Adaptation Strategy in November 2018. The evaluation package comprises a Report on the implementation of the EU Strategy on adaptation to climate change (COM(2018)738), the Evaluation of the EU Strategy on adaptation to climate change (SWD(2018)461), and the Adaptation preparedness scoreboard Country fiches (SWD(2018)460). The evaluation found that the EU Adaptation Strategy has been a reference point to prepare Europe for the climate impacts to come, at all levels.

The European Green Deal, communicated by the Commission on 11 December 2019, sets out a new growth strategy that aims to transform the EU into a fair and prosperous society, with a modern, resource-efficient and competitive economy, where there are no net emissions of greenhouse gases in 2050 and where economic growth is decoupled from resource use. It also aims to protect, conserve and enhance the Union's natural capital, and protect the health and well-being of citizens from environment-related risks and impacts. At the same time, this transition must be just and inclusive, leaving no one behind.

On 4 March 2020, the Commission proposed a European climate law to ensure a climate neutral European Union by 2050. The law is designed to be the basis for adaptable management, with a focus on the implementation of mitigation measures, monitoring of progress and improvement of management approaches if needed.

Acidification is also one of the topics addressed in the 2030 Agenda for Sustainable Development ( One of the targets under SDG 14 ('Conserve and sustainably use the oceans, seas and marine resources for sustainable development’), is SDG 14.3 (‘Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels’).


No binding targets have been specified. Under SDG 14.3, the target to minimise and address the impacts of ocean acidification by 2030 was formulated.

Related policy documents

  • 7th Environment Action Programme
    DECISION No 1386/2013/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 20 November 2013 on a General Union Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. In November 2013, the European Parliament and the European Council adopted the 7 th EU Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. This programme is intended to help guide EU action on the environment and climate change up to and beyond 2020 based on the following vision: ‘In 2050, we live well, within the planet’s ecological limits. Our prosperity and healthy environment stem from an innovative, circular economy where nothing is wasted and where natural resources are managed sustainably, and biodiversity is protected, valued and restored in ways that enhance our society’s resilience. Our low-carbon growth has long been decoupled from resource use, setting the pace for a safe and sustainable global society.’
  • Climate-ADAPT: Adaptation in EU policy sectors
    Overview of EU sector policies in which mainstreaming of adaptation to climate change is ongoing or explored
  • Climate-ADAPT: Country profiles
    Overview of activities of EEA member countries in preparing, developing and implementing adaptation strategies
  • DG CLIMA: Adaptation to climate change
    Adaptation means anticipating the adverse effects of climate change and taking appropriate action to prevent or minimise the damage they can cause, or taking advantage of opportunities that may arise. It has been shown that well planned, early adaptation action saves money and lives in the future. This web portal provides information on all adaptation activities of the European Commission.
  • EU Adaptation Strategy Package
    In April 2013, the European Commission adopted an EU strategy on adaptation to climate change, which has been welcomed by the EU Member States. The strategy aims to make Europe more climate-resilient. By taking a coherent approach and providing for improved coordination, it enhances the preparedness and capacity of all governance levels to respond to the impacts of climate change.
  • EU Biodiversity Strategy for 2030
    The European Commission has adopted the new  EU Biodiversity Strategy for 2030 and an associated Action Plan (annex)  - a comprehensive, ambitious, long-term plan for protecting nature and reversing the degradation of ecosystems. It aims to put Europe's biodiversity on a path to recovery by 2030 with benefits for people, the climate and the planet. It aims to build our societies’ resilience to future threats such as climate change impacts, forest fires, food insecurity or disease outbreaks, including by protecting wildlife and fighting illegal wildlife trade. A core part of the  European Green Deal , the Biodiversity Strategy will also support a green recovery following the COVID-19 pandemic.
  • European Climate Law proposal
    aims to enshrine the 2050 climate-neutrality objective into EU law
  • Evaluation of the EU Adaptation Strategy Package
    In November 2018, the EC published an evaluation of the EU Adaptation Strategy. The evaluation package comprises a Report on the implementation of the EU Strategy on adaptation to climate change (COM(2018)738), the Evaluation of the EU Strategy on adaptation to climate change (SWD(2018)461), and the Adaptation preparedness scoreboard Country fiches (SWD(2018)460). The evaluation found that the EU Adaptation Strategy has been a reference point to prepare Europe for the climate impacts to come, at all levels. It emphasized that EU policy must seek to create synergies between climate change adaptation, disaster risk reduction efforts and sustainable development to avoid future damage and provide for long-term economic and social welfare in Europe and in partner countries. The evaluation also suggests areas where more work needs to be done to prepare vulnerable regions and sectors.
  • Marine Strategy Framework Directive 2008/56/EC
    Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive)
  • Paris Agreement
    The Paris Agreement. Report of the Conference of the Parties on its twenty-first session, held in Paris from 30 November to 11 December 2015.
  • Regulation (EU) 2018/1999
    Regulation (EU) 2018/1999 of the European Parliament and of the Council of 11 December 2018 on the Governance of the Energy Union and Climate Action
  • Resolution adopted by the General Assembly on 25 September 2015. Transforming our world: the 2030 Agenda for Sustainable Development
    The United Nations General Assembly ad opted  the Resolution 70/1, Transforming our World: the 2030 Agenda for Sustainable Development on 25th September 2015. This document lays out the 17 Sustainable Development Goals , which aim to end poverty and hunger, protect human rights and human dignity, to protect the planet from degradation, and foster peace. 
  • Sustainable Development Goal 14
    Aim: conserve and sustainably use the oceans, seas and marine resources
  • The European Green Deal
    is roadmap for making the EU's economy sustainable; aims to turn climate and environmental challenges into opportunities across all policy areas


Methodology for indicator calculation

  • •  The time series are based on both direct pH measurement data from the Aloha station in the Hawaii Ocean Time-series as well as gap-filling calculations for this station (see Methodology references section below), and on a reconstruction of global yearly mean surface pH values by the Copernicus Marine Service (CMEMS).

    • A trend line has been added to the CMEMS data.

    • The Aloha time series is based on in situ measurements and calculation of pH from DIC concentrations and total alkalinity (Dore et al., 2009)

    • A time series of annual global mean surface sea water pH over the period 2001-2016, based on the CMEMS three-step methodology (Gehlen et al., 2019), has been used for the indicator for the first time. The aim of future CMEMS work is to deliver pan-EU and regional assessments of acidification. This indicator will also be used for reporting under the Sustainable Development Goal Agenda (SDG 14). Global average ocean surface pH values derived from Copernicus Marine Service data are based on a reconstruction method using in situ and remote-sensing data, as well as empirical relationships. The indicator is available at annual resolution, and from the year 2001 onwards. The error on each yearly value is 0.003.

    •  The estimated global mean ocean surface pH is based on alkalinity values (locally interpolated alkalinity regression (LIAR), method after Carter et al., 2016, 2018), surface ocean partial pressure of CO2 (pCO2) (CMEMS product) and an evaluation of a gridded field of ocean surface pH, based on CO2 system calculations (van Heuven et al., 2011). Data sets used for the analysis were sea surface salinity, temperature and height; mixed-layer depth and chlorophyll CMEMS products; and atmospheric CO2 from the Max Planck Institute for Biogeochemistry ( and pCO2 from the Surface Ocean CO2 Atlas (SOCAT) database (Bakker et al. (2016,, see Gehlen et al., 2019).

Methodology for gap filling

The methodology for gap filling is described in the methodology references below.

Methodology references



Methodology uncertainty

For CMEMS data: The total uncertainty of yearly mean surface sea water pH is 0.003 pH unit. It is evaluated from the contributions of (1) speciation uncertainty; (2) mapping uncertainty; (3) uncertainty due to spatial averaging; and (4) measurement uncertainty. See

Data sets uncertainty

In general, changes related to the physical and chemical marine environment are better documented than biological changes because links between cause and effect are better understood and often time series of observations are longer. Ocean acidification occurs as a consequence of well-defined chemical reactions, but its rate and biological consequences on a global scale are still matters for research.

Rationale uncertainty

No uncertainty has been specified

Data sources

Other info

DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CLIM 043
Frequency of updates
Updates are scheduled once per year
EEA Contact Info


Geographic coverage

Temporal coverage



Filed under: ph, acidification, sea surface
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