Indicator Assessment

Ocean acidification

Indicator Assessment
Prod-ID: IND-349-en
  Also known as: CLIM 043
Published 20 Dec 2016 Last modified 11 May 2021
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  • Ocean surface pH has declined from 8.2 to below 8.1 over the industrial era as a result of the increase in atmospheric CO2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %.
  • Ocean acidification in recent decades has been occurring 100 times faster than during past natural events over the last 55 million years.
  • Observed reductions in surface water pH are nearly identical across the global ocean and throughout continental European seas, except for variations near the coast. The pH reduction in the northernmost European seas, i.e. the Norwegian Sea and the Greenland Sea, is larger than the global average.
  • Ocean acidification already reaches into the deep ocean, particularly at the 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 21st century, depending on future CO2 emissions 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

Data sources:

Projected change in global ocean surface pH

Data sources:

Past trends

The annual mean atmospheric CO2 concentration reached 397 ppm in 2014, which is 40 % above the pre-industrial level (280 ppm); half of that increase has occurred since the 1980s. Over the same time period, ocean pH has been reduced from 8.2 to below 8.1, which corresponds to an increase of about 30 % in ocean acidity (defined here as the hydrogen ion concentration). This change has occurred at rates ranging between –0.0014 and –0.0024 per year, which is about a hundred times faster than any change in acidity experienced during the last 55 million years [i]. 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 [ii]. 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 [iii].

Figure 1 shows the decline in ocean surface pH over the period 1988–2014 from a station offshore of Hawaii, for which the longest time series is available [iv]. The changes observed at two other ocean stations suitable for evaluating long-term trends (offshore of the Canary Islands and Bermuda) are very similar [v].


Average surface water pH is projected to decline further to between 8.05 and 7.75 by 2100, depending on future CO2 emissions (Figure 2). Similar declines are also expected for enclosed, coastal seas such as the Baltic Sea [vi]. The largest projected decline represents more than a doubling in acidity [vii].

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 under-saturation will become widespread in these regions at atmospheric CO2 levels of 500–600 ppm [viii]. The waters of the Baltic Sea will also become more acidic before the end of the century [ix]. Such changes affect many marine organisms and could alter marine ecosystems and fisheries. These rapid chemical changes are an added pressure on marine calcifiers and 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 [x] and millions of years for coral reefs to return, based on palaeo-records of natural coral reef extinction events [xi].

[i] M. Rhein et al., ‘Observations: Ocean’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge; New York: Cambridge University Press, 2013), 255–316,

[ii] Robert H. Byrne et al., ‘Direct Observations of Basin-Wide Acidification of the North Pacific Ocean’,Geophysical Research Letters 37 (2010): L02601, doi:10.1029/2009GL040999; Rhein et al., ‘Observations: Ocean’.

[iii] Skjelvan Ingunn et al., ‘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’ (Bergen: Uni Research, Havforskningsinstituttet og Universitetet i Bergen, 2014),

[iv] J.E. Dore et al., ‘Physical and Biogeochemical Modulation of Ocean Acidification in the Central North Pacific’,Proceedings of the National Academy of Sciences 106 (2009): 12235–12240, doi:10.1073/pnas.0906044106; J.E. Dore, ‘Hawaii Ocean Time-Series Surface CO2 System Data Product, 1988-2008’ (Honolulu: SOEST, University of Hawaii, 2012),

[v] Rhein et al., ‘Observations: Ocean’.

[vi] HELCOM, ‘Climate Change in the Baltic Sea Area: HELCOM Thematic Assessment in 2013’, Baltic Sea Environment Proceedings (Helsinki: Helsinki Commission, 2013).

[vii] F. Joos et al., ‘Impact of Climate Change Mitigation on Ocean Acidification Projections’, inOcean Acidification (Oxford: Oxford University Press, 2011), 272–90,; L. Bopp et al., ‘Multiple Stressors of Ocean Ecosystems in the 21st Century: Projections with CMIP5 Models’,Biogeosciences Discussions 10, no. 2 (27 February 2013): 3627–76, doi:10.5194/bgd-10-3627-2013; P. Ciais et al., ‘Carbon and Other Biogeochemical Cycles’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge; New York: Cambridge University Press, 2013), 465–570,; IGBP, IOC, and SCOR, ‘Ocean Acidification Summary for Policymakers — Third Symposium on the Ocean in a High-CO2 World’ (Stockholm: International Geosphere-Biosphere Programme, 2013),

[viii] B. I. McNeil and R. J. Matear, ‘Southern Ocean Acidification: A Tipping Point at 450-Ppm Atmospheric CO2’,Proceedings of the National Academy of Sciences 105, no. 48 (20 November 2008): 18860–64, doi:10.1073/pnas.0806318105; M. Steinacher et al., ‘Imminent Ocean Acidification in the Arctic Projected with the NCAR Global Coupled Carbon Cycle-Climate Model’,Biogeosciences 6, no. 4 (6 April 2009): 515–33, doi:10.5194/bg-6-515-2009; Ciais et al., ‘Carbon and Other Biogeochemical Cycles’.

[ix] HELCOM, ‘Climate Change in the Baltic Sea Area: HELCOM Thematic Assessment in 2013’.

[x] David Archer, ‘Fate of Fossil Fuel CO₂ in Geologic Time’,Journal of Geophysical Research 110 (2005): C09S05, doi:10.1029/2004JC002625; Toby Tyrrell, John G. Shepherd, and Stephanie Castle, ‘The Long-Term Legacy of Fossil Fuels’,Tellus B 59, no. 4 (September 2007): 664–72, doi:10.1111/j.1600-0889.2007.00290.x; David Archer and Victor Brovkin, ‘The Millennial Atmospheric Lifetime of Anthropogenic CO₂’,Climatic Change 90 (4 June 2008): 283–97, doi:10.1007/s10584-008-9413-1.

[xi] James C. Orr et al., ‘Anthropogenic Ocean Acidification over the Twenty-First Century and Its Impact on Calcifying Organisms’,Nature 437, no. 7059 (2005): 681–86, doi:10.1038/nature04095; J. E. N Veron,A Reef in Time: The Great Barrier Reef from Beginning to End (Cambridge, MA: Belknap Press of Harvard University Press, 2008).

Supporting information

Indicator definition

  • Decline in ocean acidity measured at the Aloha station
  • Projected change in global ocean surface acidity


  • Acidity (pH)


Policy context and targets

Context description

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 Better informed decision-making, which will be achieved by bridging the knowledge gap and further developing the European climate adaptation platform (Climate-ADAPT) as the ‘one-stop shop’ for adaptation information in Europe. Climate-ADAPT has been developed jointly by the EC and the 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 vulnerablesectors 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. The EC also supports adaptation in cities through the Covenant of Mayors for Climate and Energy initiative.

In September 2016, the EC presented an indicative roadmap for the evaluation of the EU Adaptation Strategy by 2018.

In November 2013, the European Parliament and the European Council adopted the 7th EU 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.


No targets have been specified.

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.


Methodology for indicator calculation

The time series shows both direct measurement data from the Aloha station pH as well as calculations for gap filling (see methodology reference below).

A trend line has been added.

Methodology for gap filling

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

Methodology references



Methodology uncertainty

Not applicable

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 is subject to 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 every 4 years
EEA Contact Info


Geographic coverage

Temporal coverage


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