Ocean acidification

Indicator Assessment
Prod-ID: IND-349-en
Also known as: CLIM 043
Created 05 Apr 2019 Published 15 Nov 2019 Last modified 15 Nov 2019
6 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 CO 2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %. In recent decades, ocean acidification has been occurring 100 times faster than during natural events over the past 55 million years. Observed reductions in surface water pH are nearly identical across the Global Ocean and throughout European seas, except for variations near coasts. The reduction in pH in the northernmost European seas, i.e. the Norwegian Sea and the Greenland Sea, is larger than the global average. Ocean acidification has already affected the deep ocean, particularly at high latitudes. Models consistently project further ocean acidification worldwide. Ocean surface pH is projected to decrease to values between 8.05 and 7.75 by the end of the 21st century, depending on future CO 2 emission levels. The largest projected decline represents more than a doubling in acidity. Ocean acidification is affecting marine organisms and this could alter marine ecosystems.

Key messages

  • 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 %.
  • In recent decades, ocean acidification has been occurring 100 times faster than during natural events over the past 55 million years.
  • Observed reductions in surface water pH are nearly identical across the Global Ocean and throughout European seas, except for variations near coasts. The reduction in pH in the northernmost European seas, i.e. the Norwegian Sea and the Greenland Sea, is larger than the global average.
  • Ocean acidification has already affected the deep ocean, particularly at high latitudes.
  • Models consistently project further ocean acidification worldwide. Ocean surface pH is projected to decrease to values between 8.05 and 7.75 by the end of the 21st century, depending on future CO2 emission levels. The largest projected decline represents more than a doubling in acidity.
  • Ocean acidification is affecting marine organisms and this could alter marine ecosystems.

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

Decline in ocean pH measured at the Aloha station

Chart
Data sources: Explore chart interactively
Table
Data sources: Explore chart interactively

Yearly mean surface sea water pH reported on a global scale

Chart
Data sources: Explore chart interactively
Table
Data sources: Explore chart interactively

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 since the 1980s. Over the same period, ocean pH reduced from 8.2 to below 8.1, which corresponds to an approximately 30 % increase in ocean acidity (defined here as the hydrogen ion concentration). This decrease in pH occurred at rates ranging between 0.0014 and 0.0024 pH units per year, which is about 100 times faster than any change in acidity experienced during the past 55 million years (Rhein et al., 2013). The measured reduction in surface pH in the surface mixed layer (depths to 100 m) is consistent with that calculated on the basis of increasing atmospheric CO2 concentrations, assuming thermodynamic equilibrium between the ocean surface and the atmosphere (Byrne et al., 2010). The northernmost seas, i.e. the Norwegian Sea and the Greenland Sea, have experienced surface water pH reductions of 0.13 and 0.07, respectively, since the 1980s, both of which are larger than the global average (Ingunn et al., 2014).

Fig. 1 shows the decline in ocean surface pH over the period 1988-2014 from a station offshore of Hawaii (the Aloha station), for which the longest time series is available (Dore et al., 2012). The changes observed at two other ocean stations suitable for evaluating long-term trends (offshore of the Canary Islands and Bermuda) are very similar (Rhein et al., 2013).

The global average of surface ocean pH from the Copernicus Marine Environment Monitoring Service (CMEMS) is also used for the indicator. The indicator is available at annual resolution, and from the year 2001 onwards (Fig. 2). The global mean surface sea water pH estimated by CMEMS shows a trend closely following the in situ measurements in Fig. 1. According to the estimated global mean surface sea water pH (Fig. 2), there has been a decrease in pH since 2001 ranging between 0.0017 and 0.0002 pH units per year, with an error on each yearly value of 0.003 (Gehlen et al., 2019).


Projections

Average surface water pH is projected to decline further to between 8.05 and 7.75 by 2100, depending on future CO2 emissions (Fig. 2). Similar declines are also expected for enclosed, coastal seas such as the Baltic Sea (Helcom, 2013). The largest projected decline represents more than a doubling in acidity (Joos et al., 2011).

Surface waters are projected to become seasonally corrosive to aragonite in parts of the Arctic within a decade and in parts of the Southern Ocean within the next three decades in most scenarios. Aragonite is a less stable form of calcium carbonate and undersaturation will become widespread in these regions at atmospheric CO2 levels of 500-600 ppm (McNeil and Matear, 2008). The waters of the Baltic Sea will also become more acidic before the end of the century (Helcom, 2013). Such changes will affect many marine organisms and could alter marine ecosystems and fisheries. These rapid chemical changes are an added pressure on marine calcifiers and the ecosystems of Europe’s seas.

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

  • Archer, D. and Brovkin, V., 2008, ‘The millennial atmospheric lifetime of anthropogenic CO2’, Climatic Change 90, pp. 283-297, doi:10.1007/s10584-008-9413-1.
  • 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 America 106, pp. 12235-12240, doi:10.1073/pnas.0906044106.
  • Dore, J. E., 2012, 'Hawaii ocean time-series surface CO2 system data product, 1988-2008’ (http://hahana.soest.hawaii.edu/hot/products/products.html).
  • Gehlen, M., et al., 2019, 'Global mean sea water pH', EU Copernicus Marine Service Information (http://marine.copernicus.eu/).
  • Helcom, 2013, ‘Climate change in the Baltic Sea area: Helcom thematic assessment in 2013’, Baltic Sea Environment Proceedings 137.
  • Ingunn, S., et al., 2014, ‘Havforsuring Og Opptak Av Antropogent Karbon I de Nordiske Hav, 1981-2013 [Ocean Acidification and Uptake of Anthropogenic Carbon in the Nordic Seas, 1981-2013]’, Uni Research, Havforskningsinstituttet og Universitetet i Bergen, Bergen (http://www.miljodirektoratet.no/Documents/publikasjoner/M244/M244.pdf).
  • Joos, F., et al., 2011, ‘Impact of climate change mitigation on ocean acidification projections’, in: Ocean acidification, Oxford University Press, Oxford, pp. 272-290 (http://www.princeton.edu/aos/people/research_staff/frolicher/publications/joos_book11.pdf).  
  • McNeil, B. I. and Matear, R. J., 2008, ‘Southern ocean acidification: a tipping point at 450-ppm atmospheric CO2’,Proceedings of the National Academy of Sciences of the United States of America 105, pp. 18860-18864, doi:10.1073/pnas.0806318105.
  • Orr, J. C., et al., 2005, ‘Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms’, Nature 437, pp. 681-686, doi:10.1038/nature04095.
  • Rhein, M., et al., 2013, ‘Observations: Ocean’, in: Climate change 2013: The physical science basisContribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Stocker, T. F., et al. (eds), Cambridge University Press, Cambridge, UK, and New York, NY, pp. 255-316 (http://www.climatechange2013.org/images/report/WG1AR5_Chapter03_FINAL.pdf).

Indicator specification and metadata

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 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;
  • the projected change in global ocean surface acidity;
  • yearly mean surface sea water pH levels reported on a global scale based on information from CMEMS.

 

Units

  • Acidity is measured as 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 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 beside 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 and social systems aspects. The document also pointed out knowledge gaps and existing efforts of the European Union 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.

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.

Acidification is also one of the topics addressed in the 2030 Agenda for Sustainable Development (https://www.un.org/sustainabledevelopment/development-agenda/).

Targets

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.
  • 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.
  • 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. 

Methodology

Methodology for indicator calculation

  • The time series is based on both direct pH measurement data from the Aloha station as well as gap-filling calculations (see Methodology references section below).
  • A trend line has been added.
  • 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), is 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 (SD Goal  14). Global average surface ocean 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 surface sea water 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).
  • Uncertainties calculation: The uncertainty on the yearly values (notes under Figure 2) is a combination of different issues, i.e. it is evaluated from the contributions of (1) speciation uncertainty, (2) mapping uncertainty, (3) uncertainty due to spatial averaging and (4) measurement uncertainty. This uncertainty in annual measurements still allows the estimation of a multi-anual trend

Methodology for gap filling

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

Methodology references

Uncertainties

Methodology uncertainty

CMEMS three-step methodology.

Error on each yearly value: 0.003

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

Data sources

Metadata

Topics:

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

Dates

Frequency of updates

Updates are scheduled once per year

EEA Contact Info

Monika Peterlin
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