European Environment Agency – Ocean acidification


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  • Justification for indicator selection
  • Key policy question
  • Data specifications
  • Further work
  • General metadata
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Assessment versions

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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). Human activities are threatening this stability by adding large quantities of CO2 to the atmosphere, which is subsequently partially absorbed in the ocean. This process is referred to as ocean acidification because sea water pH is declining, even though ocean surface waters will remain alkaline.

When CO2 is absorbed by the ocean, it reacts with water, producing carbonic acid. The role of the carbonate ion is special because it acts as a buffer, helping to limit the decline in ocean pH; however, it is being used up as we add more and more anthropogenic CO2 to the ocean. As carbonate ion concentrations decline, so does the ocean’s capacity to take up anthropogenic CO2. Currently, the ocean takes up about one fourth of the global CO2 emissions from combustion of fossil fuels, cement production and deforestation. Hence, the ocean serves mankind by moderating atmospheric CO2 and thus climate change, but at a cost, namely changes in its fundamental chemistry.

It has been shown that corals, mussels, oysters and other marine calcifiers have a more difficult time constructing their calcareous shell or skeletal material as the concentration of carbonate ions decreases. Most, but not all, marine calcifying organisms exhibit the same difficulty. Furthermore, pH is a measure which affects not only inorganic chemistry but also many biological molecules and processes, including enzyme activities, calcification and photosynthesis. Thus, anthropogenic reductions in sea water pH could affect entire marine ecosystems. A comprehensive recent study suggests that all coral reefs will cease to grow and start to dissolve at an atmospheric CO2 level of 560 ppm due to the combined effects of acidification and warming. This CO2 concentration would be attained by 2050 under high business-as-usual emissions scenarios. Other organisms and ecosystems are likely to have different thresholds.

Scientific references:

Indicator definition

  • Decline in ocean acidity


  • acidity (pH)

Policy context and targets

Context description

In April 2009 the European Commission presented a White Paper on the framework for adaptation policies and measures to reduce the European Union’s vulnerability to the impacts of climate change. The White Paper stresses the need to improve the knowledge base and to mainstream adaptation into existing and new EU policies. The European Commission will be publishing an EU Adaptation Strategy in 2013. A number of Member States have already taken action, and several have prepared national adaptation plans.

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

Key policy question

What is the trend in the acidity of ocean 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 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.

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

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

Trine ChristiansenOwnership

European Environment Agency (EEA)


Indicator code

CLIM 043


Version id: 0
First draft created: 2012/11/15 09:49:22.138407 GMT+1
Publish date: 2012/11/20 15:25:29.242020 GMT+1
Last modified: 2012/11/20 15:30:36.530986 GMT+1
Primary theme:
Climate change


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

Latest figures

Decline in pH measured at the Aloha station as part of the Hawaii Ocean time-series

Decline in pH measured at the Aloha station as part of the Hawaii Ocean time-series

Read more

European Environment Agency, 20 November 2012. Web site.

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