Indicator Assessment

Ocean acidification

Indicator Assessment

Prod-ID: IND-349-en

Also known as: CLIM 043

Published 20 Dec 2016 Last modified 11 May 2021

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Topics: Climate change adaptation Water and marine environment

This page was archived on 15 Nov 2019 with reason: Other (New version data-and-maps/indicators/ocean-acidification-2/assessment was published)

Ocean surface pH has declined from 8.2 to below 8.1 over the industrial era as a result of the increase in atmospheric CO2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %.
Ocean acidification in recent decades has been occurring 100 times faster than during past natural events over the last 55 million years.
Observed reductions in surface water pH are nearly identical across the global ocean and throughout continental European seas, except for variations near the coast. The pH reduction in the northernmost European seas, i.e. the Norwegian Sea and the Greenland Sea, is larger than the global average.
Ocean acidification already reaches into the deep ocean, particularly at the 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 21st century, depending on future CO2 emissions levels. The largest projected decline represents more than a doubling in acidity.
Ocean acidification is affecting marine organisms and this could alter marine ecosystems.

Decline in ocean pH measured at the Aloha station

Chart

Data sources:

Station ALOHA Surface Ocean Carbon Dioxide provided by The Laboratory for Microbial Oceanography (Hawaii)

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Projected change in global ocean surface pH

Chart

Data sources:

CMIP5 climate projections (dataset URL does not lead direct to data) provided by CMIP coupled model intercomparison project

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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 time period, ocean pH has been reduced from 8.2 to below 8.1, which corresponds to an increase of about 30 % in ocean acidity (defined here as the hydrogen ion concentration). This change has occurred at rates ranging between –0.0014 and –0.0024 per year, which is about a hundred times faster than any change in acidity experienced during the last 55 million years [i]. 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 [ii]. 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 [iii].

Figure 1 shows the decline in ocean surface pH over the period 1988–2014 from a station offshore of Hawaii, for which the longest time series is available [iv]. The changes observed at two other ocean stations suitable for evaluating long-term trends (offshore of the Canary Islands and Bermuda) are very similar [v].

Projections

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

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 under-saturation will become widespread in these regions at atmospheric CO₂ levels of 500–600 ppm [viii]. The waters of the Baltic Sea will also become more acidic before the end of the century [ix]. Such changes affect many marine organisms and could alter marine ecosystems and fisheries. These rapid chemical changes are an added pressure on marine calcifiers and 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 [x] and millions of years for coral reefs to return, based on palaeo-records of natural coral reef extinction events [xi].

[i] M. Rhein et al., ‘Observations: Ocean’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge; New York: Cambridge University Press, 2013), 255–316, http://www.climatechange2013.org/images/report/WG1AR5_Chapter03_FINAL.pdf.

[ii] Robert H. Byrne et al., ‘Direct Observations of Basin-Wide Acidification of the North Pacific Ocean’,Geophysical Research Letters 37 (2010): L02601, doi:10.1029/2009GL040999; Rhein et al., ‘Observations: Ocean’.

[iii] Skjelvan Ingunn et al., ‘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’ (Bergen: Uni Research, Havforskningsinstituttet og Universitetet i Bergen, 2014), http://www.miljodirektoratet.no/Documents/publikasjoner/M244/M244.pdf.

[iv] J.E. Dore et al., ‘Physical and Biogeochemical Modulation of Ocean Acidification in the Central North Pacific’,Proceedings of the National Academy of Sciences 106 (2009): 12235–12240, doi:10.1073/pnas.0906044106; J.E. Dore, ‘Hawaii Ocean Time-Series Surface CO2 System Data Product, 1988-2008’ (Honolulu: SOEST, University of Hawaii, 2012), http://hahana.soest.hawaii.edu/hot/products/products.html.

[v] Rhein et al., ‘Observations: Ocean’.

[vi] HELCOM, ‘Climate Change in the Baltic Sea Area: HELCOM Thematic Assessment in 2013’, Baltic Sea Environment Proceedings (Helsinki: Helsinki Commission, 2013).

[vii] F. Joos et al., ‘Impact of Climate Change Mitigation on Ocean Acidification Projections’, inOcean Acidification (Oxford: Oxford University Press, 2011), 272–90, http://www.princeton.edu/aos/people/research_staff/frolicher/publications/joos_book11.pdf; L. Bopp et al., ‘Multiple Stressors of Ocean Ecosystems in the 21st Century: Projections with CMIP5 Models’,Biogeosciences Discussions 10, no. 2 (27 February 2013): 3627–76, doi:10.5194/bgd-10-3627-2013; P. Ciais et al., ‘Carbon and Other Biogeochemical Cycles’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge; New York: Cambridge University Press, 2013), 465–570, http://www.climatechange2013.org/images/report/WG1AR5_Chapter06_FINAL.pdf; IGBP, IOC, and SCOR, ‘Ocean Acidification Summary for Policymakers — Third Symposium on the Ocean in a High-CO2 World’ (Stockholm: International Geosphere-Biosphere Programme, 2013), http://www.igbp.net/download/18.30566fc6142425d6c91140a/1384420272253/OA_spm2-FULL-lorez.pdf.

[viii] B. I. McNeil and R. J. Matear, ‘Southern Ocean Acidification: A Tipping Point at 450-Ppm Atmospheric CO2’,Proceedings of the National Academy of Sciences 105, no. 48 (20 November 2008): 18860–64, doi:10.1073/pnas.0806318105; M. Steinacher et al., ‘Imminent Ocean Acidification in the Arctic Projected with the NCAR Global Coupled Carbon Cycle-Climate Model’,Biogeosciences 6, no. 4 (6 April 2009): 515–33, doi:10.5194/bg-6-515-2009; Ciais et al., ‘Carbon and Other Biogeochemical Cycles’.

[ix] HELCOM, ‘Climate Change in the Baltic Sea Area: HELCOM Thematic Assessment in 2013’.

[x] David Archer, ‘Fate of Fossil Fuel CO₂ in Geologic Time’,Journal of Geophysical Research 110 (2005): C09S05, doi:10.1029/2004JC002625; Toby Tyrrell, John G. Shepherd, and Stephanie Castle, ‘The Long-Term Legacy of Fossil Fuels’,Tellus B 59, no. 4 (September 2007): 664–72, doi:10.1111/j.1600-0889.2007.00290.x; David Archer and Victor Brovkin, ‘The Millennial Atmospheric Lifetime of Anthropogenic CO₂’,Climatic Change 90 (4 June 2008): 283–97, doi:10.1007/s10584-008-9413-1.

[xi] James C. Orr et al., ‘Anthropogenic Ocean Acidification over the Twenty-First Century and Its Impact on Calcifying Organisms’,Nature 437, no. 7059 (2005): 681–86, doi:10.1038/nature04095; J. E. N Veron,A Reef in Time: The Great Barrier Reef from Beginning to End (Cambridge, MA: Belknap Press of Harvard University Press, 2008).

Supporting information

Indicator definition

Decline in ocean acidity measured at the Aloha station
Projected change in global ocean surface acidity

Units

Acidity (pH)

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 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.

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 7^th EU Environment Action Programme (7^th EAP) to 2020, ‘Living well, within the limits of our planet’. The 7^th 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 7^th EAP refer to climate change adaptation.

Targets

No targets have been specified.

Methodology

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

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

Uncertainties

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.