Indicator Assessment

Ocean acidification

Indicator Assessment
Prod-ID: IND-349-en
  Also known as: CLIM 043
Published 19 Jun 2014 Last modified 11 May 2021
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  • Surface-ocean pH has declined from 8.2 to below 8.1 over the industrial era due to the growth of atmospheric CO2 concentrations. This decline corresponds to an increase in oceanic acidity of 26%.
  • Observed reductions in surface-water pH are nearly identical across the global ocean and throughout Europe’s seas.
  • Ocean acidification in recent decades is occurring a hundred times faster than during past natural events over the last 55 million years.
  • Ocean acidification already reaches into the deep ocean, particularly in the high latitudes.
  • Models consistently project further ocean acidification worldwide. Surface ocean pH is projected to decrease to values between 8.05 and 7.75 by the end of 21st century depending on future CO2 emission levels. The largest projected decline represents more than a doubling in acidity.
  • Ocean acidification may affect many marine organisms within the next 20 years and could alter marine ecosystems and fisheries.

Decline in pH measured at the Aloha station

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Data sources:

Projected change in global ocean surface pH

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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 in the last 30 years. Over the same time period ocean pH has been reduced from 8.2 to below 8.1, which corresponds to an increase of 26% 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 metre) are consistent with that calculated on the basis of increasing atmospheric CO2 concentrations, assuming thermodynamic equilibrium between the surface ocean and the atmosphere [ii].

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


Average surface-water pH is projected to decline further to between 8.05 and 7.75 by the year 2100, depending on future CO2 emissions (Figure 2). The largest projected decline represents more than a doubling in acidity [v].

Surface waters are projected to become seasonally corrosive to aragonite, which is a less stable form of calcium carbonate, in parts of the Arctic and in some coastal upwelling systems within a decade and in parts of the Southern Ocean within the next three decades in most scenarios. Aragonite undersaturation becomes widespread in these regions at atmospheric CO2 levels of 500–600 ppm [vi]. Surface waters of the Baltic Sea will also become corrosive well before the end of the century. These changes affect many marine organisms and could alter marine ecosystems and fisheries. In the Black Sea and Mediterranean Sea there is no danger of surface waters becoming corrosive to calcium carbonate before 2100, but they will suffer sharp reductions in carbonate ion concentrations (Med Sea -37 %; Black Sea -45 %). These rapid chemical changes are an added pressure on marine calcifiers and ecosystems of the European seas that are already heavily suffering from other anthropogenic influences.

Without dramatic actions to curb CO2 emissions, recovery from human-induced acidification will require thousands of years for the Earth system to re-establish roughly similar ocean chemical conditions [vii] and millions of years for coral reefs to return, based on palaeo-records of natural coral reef extinction events [viii].

[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, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 3,

[ii] Robert H. Byrne et al., “Direct Observations of Basin-Wide Acidification of the North Pacific Ocean,”Geophysical Research Letters 37, no. 2 (2010): n/a – n/a, doi:10.1029/2009GL040999; Rhein et al., “Observations: Ocean.”

[iii] 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): 12235–40., doi:10.1073/pnas.0906044106; J.E. Dore, “Hawaii Ocean Time-Series Surface CO2 System Data Product, 1988-2008.” (SOEST, University of Hawaii, Honolulu, HI., 2012),

[iv] Rhein et al., “Observations: Ocean.”

[v] F. Joos et al., “Impact of Climate Change Mitigation on Ocean Acidification Projections,” inOcean Acidification (Chapter 14) (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 (February 27, 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, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 6,; IGBP, IOC, SCOR,Ocean Acidification Summary for Policymakers - Third Symposium on the Ocean in a High-CO2 World (Stockholm: International Geosphere-Biosphere Programme, 2013),

[vi] 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 (November 20, 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 (April 6, 2009): 515–33, doi:10.5194/bg-6-515-2009; Ciais et al., “Carbon and Other Biogeochemical Cycles.”

[vii] David Archer, “Fate of Fossil Fuel CO₂ in Geologic Time,”Journal of Geophysical Research 110, no. C9 (2005), 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 (June 4, 2008): 283–97, doi:10.1007/s10584-008-9413-1.

[viii] Orr et al., “Anthropogenic Ocean Acidification over the Twenty-First Century and Its Impact on Calcifying Organisms”; J. E. N Veron,A Reef in Time : The Great Barrier Reef from Beginning to End (Cambridge, Mass.: Belknap Press of Harvard University Press, 2008).

Supporting information

Indicator definition

  • Decline in ocean acidity


  • acidity (pH)


Policy context and targets

Context description

In April 2013 the European Commission presented the EU Adaptation Strategy Package ( This package consists of the EU Strategy on adaptation to climate change /* COM/2013/0216 final */ and a number of supporting documents. One of the objectives of the EU Adaptation Strategy is Better informed decision-making, which should occur through Bridging the knowledge gap and Further developing Climate-ADAPT as the ‘one-stop shop’ for adaptation information in Europe. Further objectives include Promoting action by Member States and Climate-proofing EU action: promoting adaptation in key vulnerable sectors. Many EU Member States have already taken action, such as by adopting national adaptation strategies, and several have also prepared action plans on climate change adaptation.

The European Commission and the European Environment Agency have developed the European Climate Adaptation Platform (Climate-ADAPT, to share knowledge on observed and projected climate change and its impacts on environmental and social systems and on human health; on relevant research; on EU, national and subnational adaptation strategies and plans; and on adaptation case studies.


No targets have been specified.

Related policy documents

  • 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 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.

Further information on uncertainties is provided in Section 1.7 of the EEA report on Climate change, impacts, and vulnerability in Europe 2012 (

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