Indicator Assessment

Ocean acidification

Indicator Assessment
Prod-ID: IND-349-en
  Also known as: CLIM 043
Published 15 Nov 2019 Last modified 11 May 2021
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  • 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.
  • 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 already affected the deep ocean, particularly at high latitudes.
  • 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.
  • Ocean acidification is affecting marine organisms and this could alter marine ecosystems.

Decline in ocean pH measured at the Aloha station

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Data sources:

Yearly mean surface sea water pH reported on a global scale

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Data sources:

Past trends

The global annual mean atmospheric CO2 concentration exceeded400 ppm in 2016, which is more than 40 % above the pre-industrial level (280 ppm); half of that increase has occurred since the 1980s. Over the same period, ocean pH reduced 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 CO2 concentrations, 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, which is clearly higher than elsewhere in the open ocean (Touratier and Goyet, 2011; Hassoun et al., 2015).

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).


Average surface open ocean pH is projected to decline further with 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 increase in atmospheric CO2 may be partly mitigated by various processes that cause increased alkalinity (Müller et al., 2016)

Surface waters are projected to become seasonally corrosive to aragonite in parts of the Arctic within a decade and in parts of the Southern Ocean within the next three decades in most scenarios. Aragonite is a less stable form of calcium carbonate and undersaturation will become widespread in these regions at atmospheric CO2 levels of 500-600 ppm (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 (
  • 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.
  • 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 basisContribution 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 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 Copernicus Marine Service (CMEMS), with the long-term decreasing trend estimate.


  • Acidity is measured as pH.


Policy context and targets

Context description

The environmental pillar and main driver for clean, healthy and productive European seas within the Integrated Maritime Policy is the 2008 Marine Strategy Framework Directive (MSFD; Directive 2008/56/EC). The MSFD is aimed at protecting and restoring the marine environment and phasing out pollution, so that there are 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 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 beside 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 efforts of the European Union 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 turn the Earth system into states detrimental or catastrophic for human development.

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

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 Union 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 set basis for adaptable management, with focus on 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 (March, 2020). 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.
  • 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. 


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 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), is 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 (SD Goal  14). Global average surface ocean 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 surface sea water 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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