Indicator Assessment

Ocean acidification

Indicator Assessment
Prod-ID: IND-349-en
  Also known as: CLIM 043
Published 09 Aug 2021 Last modified 30 Aug 2021
14 min read

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%. Reductions in surface water pH are observed across the global ocean. Ocean acidification has impacts on marine organisms and has already affected the deep ocean, particularly at high latitudes. Models project further ocean acidification worldwide. The target under United Nations Sustainable Development Goal 14.3 is to minimise the impacts of this by 2030.

Decline in ocean pH measured at the Aloha station and yearly mean surface seawater pH reported on a global scale (Copernicus Marine)

Note: A decline in pH corresponds to an increase in the acidity of ocean water. - Green line: data based on in situ measurements at Aloha station. - Blue line: pH calculated at Aloha station from dissolved inorganic carbon concentrations and total alkalinity, at in situ temperature. - Red line: global yearly mean surface seawater pH, calculated by Copernicus Marine Environment Monitoring Service.

Data source:

Over the last million years, average surface seawater pH has been relatively stable, oscillating between 8.3, during cold periods (e.g. during the last glacial maximum 20,000 years ago), and 8.2, during warm periods (e.g. just prior to the industrial revolution). Rapid increases in atmospheric CO2 levels due to emissions from human activities are now threatening this stability, as emitted CO2 is partially absorbed by the ocean, causing a decline in pH and ocean acidification. A goal of the 2030 Agenda for Sustainable Development is to address the impacts of this (UN, 2015).

The global annual mean atmospheric CO2 concentration exceeded 400 ppm in 2016 (Kahn, 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 decreased from 8.11 to below 8.06, corresponding to an approximately 30% increase in acidity. This decrease in pH occurred at a rate about 100 times faster than any change in acidity experienced during the past 55 million years (Rhein et al., 2013).

This indicator looks at the longest time series of measured pH values available, from the Aloha station, offshore of Hawaii, and also calculated data on global average surface ocean pH from the Copernicus Marine Environment Monitoring Service (CMEMS). The reduction in pH measured in surface mixed-layer depths (up to 100 m) is consistent with that calculated from atmospheric CO2 concentrations, assuming thermodynamic equilibrium between the ocean surface and atmosphere. The northernmost seas, i.e. the Norwegian and Greenland Seas, have seen significantly larger decreases in pH than the global average.

Average surface open ocean pH is projected to decline further, with the largest projected decline representing more than a doubling in acidity (Joos et al., 2011; Helcom, 2013; Bindoff et al., 2019). This will affect many marine organisms and could alter marine ecosystems and fisheries. Such rapid chemical changes are an added pressure on marine calcifiers and Europe’s sea ecosystems.

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

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 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 change in the global ocean and the impacts of climate change.

This indicator provides information on:

  • trends in ocean acidity measured at the Aloha station;
  • yearly mean surface seawater pH levels reported on a global scale is computed from monthly pH values by CMEMS.


Acidity is measured in pH.


Policy context and targets

Context description

Acidification is addressed in the 2030 Agenda for Sustainable Development (UN, 2021). One of the targets under Sustainable Development Goal (SDG) 14 (‘Conserve and sustainably use the oceans, seas and marine resources for sustainable development’), SDG 14.3, is to ‘Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels’.

On 4 March 2020, the European Commission proposed a European climate law (EC, 2020) to ensure a climate-neutral European Union by 2050 as a part of the European Green Deal (EC, 2019). This law is designed to establish a basis for adaptable management, with focus on the implementation of mitigation measures, the monitoring of progress and the improvement of management approaches if needed.



No targets have been specified

Related policy documents

No related policy documents have been specified



Methodology for indicator calculation

  • The time series are based both on direct pH measurement data from the Hawaii Ocean Time-series, obtained from the Aloha station, and gap-filling calculations using data from this station (Carter et al., 2016, 2018), and on a reconstruction of global yearly mean surface pH values from CMEMS.
  • A trend line has been added for the CMEMS data.
  • The Aloha time series data are based onin situmeasurements and calculation of pH values based on dissolved inorganic carbon concentrations and total alkalinity (Dore et al., 2009; Dore, 2012).
  • A time series of annual global mean surface seawater pH for the period 2001-2016, based on the CMEMS three-step methodology (Copernicus Marine Service, 2021), 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 SDG 14. Global average surface ocean pH values derived from CMEMS data are based on a reconstruction method usingin situand remote-sensing data, as well as empirical relationships. The indicator is available at annual resolution, and from the year 2001 onwards. The error for each yearly pH value is 0.003.
  • The estimated global mean surface seawater pH is based on alkalinity values (obtained using the 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 values based on CO2 system calculations (van Heuven et al., 2011), see (Copernicus Marine Service, 2021).


Archer, D. and Brovkin, V., 2008, ‘The millennial atmospheric lifetime of anthropogenic CO₂’,Climatic Change90(3), pp. 283-297 (DOI: 10.1007/s10584-008-9413-1).

Bindoff, N. L., et al., 2019, ‘Changing ocean, marine ecosystems, and dependent communities’, in: Pörtner, H.-O. et al. (eds),IPCC special report on the ocean and cryosphere in a changing climate, Cambridge University Press, Cambridge, UK.

Carter, B. R., et al., 2016, ‘Locally interpolated alkalinity regression for global alkalinity estimation’,Limnology and Oceanography: Methods14(4), pp. 268-277.

Carter, B. R., et al., 2018, ‘Updated methods for global locally interpolated estimation of alkalinity, pH, and nitrate’,Limnology and Oceanography: Methods16(2), pp. 119-131.

Copernicus Marine Service, 2020a,Product user manual: For global ocean surface carbon product MULTIOBS_GLO_BIO_CARBON_SURFACE_REP_015_008( accessed 1 April 2021.

Copernicus Marine Service, 2020b,Quality information document: Global ocean surface carbon product MULTIOBS_GLO_BIO_CARBON_SURFACE_REP_015_008( accessed 1 April 2021.

Copernicus Marine Service, 2021, ‘Global mean sea water pH’, Copernicus Marine Service ( accessed 1 April 2021.

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 America106, pp. 12235-12240 (DOI: 10.1073/pnas.0906044106).

Dore, J. E., 2012, ‘Hawaii Ocean Time-series surface CO₂ system data product, 1988-2008.’, SOEST, University of Hawaii, Honolulu, HI. (

EC, 2019, Communication from the Commission to the European Parliament, the European Council, the Council, the European Economic and Social Committee and the Committee of the Regions ‘The European Green Deal’ (COM(2019) 640 final of 11 December 2019).

EC, 2020, Proposal for a regulation of the European Parliament and of the Council establishing the framework for achieving climate neutrality and amending Regulation (EU) 2018/1999 (European Climate Law) (COM(2020) 80 final).

Helcom, 2013,Climate change in the Baltic Sea Area — Helcom thematic assessment in 2013, Baltic Sea Environment Proceedings No 137, Helsinki Commission — Baltic Marine Environment Protection Commission, Helsinki ( accessed 27 October 2013.

Joos, F., et al., 2011, ‘Impact of climate change mitigation on ocean acidification projections’, in:Ocean acidification, Oxford University Press, Oxford, pp. 272-288.

Kahn, B., 2016, ‘Earth’s CO₂ passes the 400 PPM threshold — maybe permanently’,Scientific American, 27 September 2016 ( accessed 19 April 2021.

Orr, J. C., et al., 2005, ‘Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms’,Nature437(7059), pp. 681-686 (DOI: 10.1038/nature04095).

Rhein, M., et al., 2013, ‘Observations: ocean’, in: Stocker, T. F. et al. (eds),Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK; New York, NY, pp. 255-316.

School of Ocean and Earth Science and Technology, 2021, ‘Hawaii Ocean Time-series (HOT)’, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa ( accessed 1 April 2021.

UN, 2015, ‘Transforming our world: the 2030 Agenda for Sustainable Development’, United Nations ( accessed 1 April 2021.

UN, 2021, ‘The Sustainable Development Agenda’, Sustainable Development Goals, United Nations ( accessed 1 April 2021.

van Heuven, S., et al., 2011,CO2SYS v 1.1: MATLAB program developed for CO2 system calculations. ORNL/CDIAC-105b, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN.

Methodology for gap filling

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

Methodology references



Methodology uncertainty

No uncertainty has been specified

Data sets uncertainty

No uncertainty has been specified

Rationale uncertainty

No uncertainty has been specified

Data sources

Other info

DPSIR: Pressure
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:
Filed under: ocean acidity, ph
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