Ocean acidification

Assessment versions

Published (reviewed and quality assured)

• No published assessments

Rationale

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 concentrations because of emissions from human activities are now threatening this stability, as the CO₂ emitted is subsequently partially absorbed by the ocean. Currently, the ocean takes up about one quarter of global CO₂ emissions from human activities, e.g. the 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 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, these ions 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 CO_2, 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. Acidification will act differently across species and will have direct and indirect ecosystem effects. Decreasing carbonate ion concentrations reduce the rate of calcification of marine calcifying organisms, such as reef-building corals, shellfish and plankton (Doney et al., 2009). The process of biocalcification in cold water corals in the Mediterranean Sea has decreased by 50 % as a direct consequence of anthropogenic acidification (Maier et al., 2009). The pH also affects biological molecules and processes, e.g. enzyme activities and photosynthesis. The effects of acidification on phytoplankton species composition and primary production may be exacerbated by rising seawater temperatures. Primary producers are responsible for a significant part of global carbon fixation, thereby forming the basis of marine food webs. Thus, anthropogenic acidification could affect entire marine ecosystems. Changes in marine primary production will also have an impact on the global carbon cycle and the absorption of atmospheric CO₂ in the ocean.

The combined effects of elevated seawater temperatures, deoxygenation and acidification are expected to have negative effects on other marine organisms, as well. This will lead to changes in food webs and marine production, and will also cause economic loss, for example in aquaculture.

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.
• Bindoff et al., 2019, Changing Ocean, Marine Ecosystems, and Dependent Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]
• Doney et al., 2009, Ocean Acidification: The Other CO2 Problem Annual Review of Marine Science 1:1, 169-192.
• ICES, 2017, Report of the Workshop on Understanding the Impacts and Consequences of Ocean Acidification for Commercial Species and End-users (WKACIDUSE), 5–8 December 2016, ICES Headquarters, Copenhagen, Denmark. ICES CM 2016/SSGEPI

Indicator definition

This indicator illustrates the global mean average rate of ocean acidification, quantified by decreases in pH, which is a measure of acidity and is defined here 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 global change.

This indicator provides information on:

• - trends in ocean acidity measured at the Aloha station;
• - yearly mean surface sea water pH levels reported on a global scale, based on information from the Copernicus Marine Service (CMEMS), with the estimated long-term decreasing trend.

Units

• Acidity is measured as pH (dimensionless)

Policy context and targets

Context description

The 2008 Marine Strategy Framework Directive (MSFD; Directive 2008/56/EC) is the environmental pillar of the Integrated Maritime Policy and its main driver towards clean, healthy and productive European seas. The MSFD aims to protect and restore the marine environment and phase out pollution, leading to no significant impacts on or risks to marine biodiversity, human health and the legitimate use of marine resources. The MSFD requires the achievement of 'good environmental status' (GES) for EU marine waters by 2020. Acidification is addressed under MSFD Descriptor 7 (Hydrographic conditions).

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 to enable better-informed decision-making, which will be achieved by bridging the knowledge gap and further developing the European climate adaptation platform (Climate-ADAPT) as a ‘one-stop shop’ for adaptation information in Europe. Climate-ADAPT has been developed jointly by the EC and the European Environment Agency (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 vulnerable sectors 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.

Staff working document SWD(2013) 133 Climate change adaptation, coastal and marine issues was published alongside the EU Strategy, The paper provided an overview of the main impacts of climate change on coastal zones and marine issues, including environmental, economic and social systems aspects. The document also pointed out knowledge gaps and existing EU efforts to best adapt to the impacts of climate change on coastal zones and marine issues.

In November 2013, the European Parliament and the European Council adopted the Seventh 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. The planetary boundary framework identified nine processes that regulate the stability and resilience of the Earth system — ‘planetary life support systems’. The framework proposes precautionary quantitative planetary boundaries within which humanity can continue to develop and thrive, also referred to as a ‘safe operating space’. It suggests that crossing these boundaries increases the risk of generating large-scale abrupt or irreversible environmental changes that could shift the Earth system to states that are detrimental to or catastrophic for human development.

Ocean acidification is identified as one of the nine planetary boundaries.

The EC published an Evaluation of the EU Adaptation Strategy in November 2018. The evaluation package comprises a Report on the implementation of the EU Strategy on adaptation to climate change (COM(2018)738), the Evaluation of the EU Strategy on adaptation to climate change (SWD(2018)461), and the Adaptation preparedness scoreboard Country fiches (SWD(2018)460). The evaluation found that the EU Adaptation Strategy has been a reference point to prepare Europe for the climate impacts to come, at all levels.

The European Green Deal, communicated by the Commission on 11 December 2019, sets out a new growth strategy that aims to transform the EU into a fair and prosperous society, with a modern, resource-efficient and competitive economy, where there are no net emissions of greenhouse gases in 2050 and where economic growth is decoupled from resource use. It also aims to protect, conserve and enhance the Union's natural capital, and protect the health and well-being of citizens from environment-related risks and impacts. At the same time, this transition must be just and inclusive, leaving no one behind.

On 4 March 2020, the Commission proposed a European climate law to ensure a climate neutral European Union by 2050. The law is designed to be the basis for adaptable management, with a focus on the implementation of mitigation measures, monitoring of progress and improvement of management approaches if needed.

Acidification is also one of the topics addressed in the 2030 Agenda for Sustainable Development (https://www.un.org/sustainabledevelopment/development-agenda/). One of the targets under SDG 14 ('Conserve and sustainably use the oceans, seas and marine resources for sustainable development’), is SDG 14.3 (‘Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels’).

Targets

No binding targets have been specified. Under SDG 14.3, the target to minimise and address the impacts of ocean acidification by 2030 was formulated.

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.
• EU Biodiversity Strategy for 2030
  The European Commission has adopted the new  EU Biodiversity Strategy for 2030 and an associated Action Plan (annex)  - a comprehensive, ambitious, long-term plan for protecting nature and reversing the degradation of ecosystems. It aims to put Europe's biodiversity on a path to recovery by 2030 with benefits for people, the climate and the planet. It aims to build our societies’ resilience to future threats such as climate change impacts, forest fires, food insecurity or disease outbreaks, including by protecting wildlife and fighting illegal wildlife trade. A core part of the  European Green Deal , the Biodiversity Strategy will also support a green recovery following the COVID-19 pandemic.
• European Climate Law proposal
  aims to enshrine the 2050 climate-neutrality objective into EU law
• Evaluation of the EU Adaptation Strategy Package
  In November 2018, the EC published an evaluation of the EU Adaptation Strategy. The evaluation package comprises a Report on the implementation of the EU Strategy on adaptation to climate change (COM(2018)738), the Evaluation of the EU Strategy on adaptation to climate change (SWD(2018)461), and the Adaptation preparedness scoreboard Country fiches (SWD(2018)460). The evaluation found that the EU Adaptation Strategy has been a reference point to prepare Europe for the climate impacts to come, at all levels. It emphasized that EU policy must seek to create synergies between climate change adaptation, disaster risk reduction efforts and sustainable development to avoid future damage and provide for long-term economic and social welfare in Europe and in partner countries. The evaluation also suggests areas where more work needs to be done to prepare vulnerable regions and sectors.
• Marine Strategy Framework Directive 2008/56/EC
  Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive)
• Paris Agreement
  The Paris Agreement. Report of the Conference of the Parties on its twenty-first session, held in Paris from 30 November to 11 December 2015.
• Regulation (EU) 2018/1999
  Regulation (EU) 2018/1999 of the European Parliament and of the Council of 11 December 2018 on the Governance of the Energy Union and Climate Action
• Resolution adopted by the General Assembly on 25 September 2015. Transforming our world: the 2030 Agenda for Sustainable Development
  The United Nations General Assembly ad opted  the Resolution 70/1, Transforming our World: the 2030 Agenda for Sustainable Development on 25th September 2015. This document lays out the 17 Sustainable Development Goals , which aim to end poverty and hunger, protect human rights and human dignity, to protect the planet from degradation, and foster peace. 
• Sustainable Development Goal 14
  Aim: conserve and sustainably use the oceans, seas and marine resources
• The European Green Deal
  is roadmap for making the EU's economy sustainable; aims to turn climate and environmental challenges into opportunities across all policy areas

Methodology

Methodology for indicator calculation

• •  The time series are based on both direct pH measurement data from the Aloha station in the Hawaii Ocean Time-series as well as gap-filling calculations for this station (see Methodology references section below), and on a reconstruction of global yearly mean surface pH values by the Copernicus Marine Service (CMEMS).

  • A trend line has been added to the CMEMS data.

  • The Aloha time series is based on in situ measurements and calculation of pH from DIC concentrations and total alkalinity (Dore et al., 2009)

  • A time series of annual global mean surface sea water pH over the period 2001-2016, based on the CMEMS three-step methodology (Gehlen et al., 2019), 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 the Sustainable Development Goal Agenda (SDG 14). Global average ocean surface pH values derived from Copernicus Marine Service data are based on a reconstruction method using in situ and remote-sensing data, as well as empirical relationships. The indicator is available at annual resolution, and from the year 2001 onwards. The error on each yearly value is 0.003.

  •  The estimated global mean ocean surface pH is based on alkalinity values (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, based on CO2 system calculations (van Heuven et al., 2011). Data sets used for the analysis were sea surface salinity, temperature and height; mixed-layer depth and chlorophyll CMEMS products; and atmospheric CO2 from the Max Planck Institute for Biogeochemistry (www.bgc-jena.mpg.de) and pCO2 from the Surface Ocean CO₂ Atlas (SOCAT) database (Bakker et al. (2016, https://www.socat.info/), see Gehlen et al., 2019).

Methodology for gap filling

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

Methodology references

• Carter, B. R., Williams, N. L., Gray, A. R., & Feely, R. A., 2016 Locally interpolated alkalinity regression for global alkalinity estimation. Limnology and Oceanography: Methods, 14(4), 268-277, 2016.
• Carter, B. R., Feely, R. A., Williams, N. L., Dickson, A. G., Fong, M. B., & Takeshita, Y., 2018 Updated methods for global locally interpolated estimation of alkalinity, pH, and nitrate. Limnology and Oceanography: Methods, 16(2), 119-131.
• CMEMS web site Detailed information on the systems and products on carbon-ph-area-averaged.
• Dore et al. 2009: Physical and biogeochemical modulation of ocean acidification in the central North Pacific. Dore, J. E., Lukas, R., Sadler, D. W., Church, M. J. and Karl, D. M. (2009)  Proceedings of the National Academy of Sciences 106, 12235–12240. doi:10.1073/pnas.0906044106
• Gehlen, M., et al., 2019 'Global mean sea water pH', EU Copernicus Marine Service Information.
• The Sustainable Development Agenda, 2015 2030 Agenda for Sustainable Development and its 17 Sustainable Development  Goals,  Goal 13 is on climate action.
• van Heuven, S., D. Pierrot, J.W.B. Rae, E. Lewis, and D.W.R. Wallace, 2011 MATLAB Program Developed for CO2 System Calculations. ORNL/CDIAC-105b. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee.

Data specifications

EEA data references

• No datasets have been specified here.

External data references

• GLOBAL MEAN SEA WATER PH
• Hawaii Ocean Time-series (HOT)
• Coupled Model Intercomparison Project Phase 5 - CMIP5

Data sources in latest figures

Uncertainties

Methodology uncertainty

For CMEMS data: The total uncertainty of yearly mean surface sea water pH is 0.003 pH unit. It is evaluated from the contributions of (1) speciation uncertainty; (2) mapping uncertainty; (3) uncertainty due to spatial averaging; and (4) measurement uncertainty. See http://resources.marine.copernicus.eu/documents/QUID/CMEMS-OMI-QUID-GLO-HEALTH-carbon-ph-area-averaged.pdf

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 are still matters for research.

Rationale uncertainty

No uncertainty has been specified