Indicator Specification

Ocean acidification

Indicator Specification
  Indicator codes: CLIM 043
Published 20 Dec 2016 Last modified 04 Dec 2019
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Decline in ocean acidity measured at the Aloha station Projected change in global ocean surface acidity

Assessment versions

Published (reviewed and quality assured)
  • No published assessments


Justification for indicator selection

Across the ocean, the pH of surface waters has been relatively stable for millions of years. Over the last million years, average surface water pH oscillated 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 concentration due to emissions from human activities are now threatening this stability, as the CO2 is subsequently partially absorbed in the ocean. Currently, the ocean takes up about one-quarter of the global CO2 emissions coming from human activities, e.g. combustion of fossil fuels. The uptake of CO2 in the sea causes ocean acidification, as the pH of sea water declines, even though ocean surface waters will remain alkaline.

When CO2 is absorbed by the ocean, it reacts with water, producing carbonic acid. Carbonic acid dissociates to form bicarbonate ions and protons, which further react with carbonate ions. The carbonate ions act as a buffer, helping to limit the decline in ocean pH; however, they are being used up as more and more anthropogenic CO2 is added to the ocean. As carbonate ion concentrations decline, so does the ocean’s capacity to take up more anthropogenic CO2. Hence, the ocean’s ability to moderate atmospheric CO2 and thus climate change comes at the cost of substantial changes in its fundamental chemistry.

Ocean acidification can have wide-ranging impacts on biological systems by reducing the availability of carbonate. Decreasing carbonate ion concentrations reduce the rate of calcification of marine calcifying organisms, such as reef-building corals, mussels and plankton. pH also affects biological molecules and processes, e.g. enzyme activities and photosynthesis. Thus, anthropogenic acidification could affect entire marine ecosystems. Organisms appear to be increasingly sensitive to acidification when they are concurrently exposed to elevated seawater temperature. Of equal importance is the effect of acidification on primary producers, as it changes the bioavailability of essential nutrients, such as iron and zinc. Primary producers are responsible for a significant part of global carbon fixation, thereby forming the basis of marine food webs.

Scientific references

  • IPCC, 2013: 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., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.

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.

Key policy question

What is the trend in the acidity of ocean surface water?



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


Data specifications

EEA data references

  • No datasets have been specified here.

External data references

Data sources in latest figures



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

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Long term work

Work specified here will require more than 1 year (from now) to be completed.

General metadata

Responsibility and ownership

EEA Contact Info

Monika Peterlin


European Environment Agency (EEA)


Indicator code
CLIM 043
Version id: 2

Frequency of updates

Updates are scheduled every 4 years


DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)


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