Past trends

The annual mean atmospheric CO 2 concentration reached 397 ppm in 2014, which is 40% above the pre-industrial level (280 ppm); half of that increase has occurred in the last 30 years. Over the same time period ocean pH has been reduced from 8.2 to below 8.1, which corresponds to an increase of 26% 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 metre) are consistent with that calculated on the basis of increasing atmospheric CO 2 concentrations, assuming thermodynamic equilibrium between the surface ocean and the atmosphere [ii].

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

Projections

Average surface-water pH is projected to decline further to between 8.05 and 7.75 by the year 2100, depending on future CO 2 emissions (Figure 2). The largest projected decline represents more than a doubling in acidity [v].

Surface waters are projected to become seasonally corrosive to aragonite, which is a less stable form of calcium carbonate, in parts of the Arctic and in some coastal upwelling systems within a decade and in parts of the Southern Ocean within the next three decades in most scenarios. Aragonite undersaturation becomes widespread in these regions at atmospheric CO 2 levels of 500–600 ppm [vi]. Surface waters of the Baltic Sea will also become corrosive well before the end of the century. These changes affect many marine organisms and could alter marine ecosystems and fisheries. In the Black Sea and Mediterranean Sea there is no danger of surface waters becoming corrosive to calcium carbonate before 2100, but they will suffer sharp reductions in carbonate ion concentrations (Med Sea -37 %; Black Sea -45 %). These rapid chemical changes are an added pressure on marine calcifiers and ecosystems of the European seas that are already heavily suffering from other anthropogenic influences.

Without dramatic actions to curb CO 2 emissions, recovery from human-induced acidification will require thousands of years for the Earth system to re-establish roughly similar ocean chemical conditions [vii] and millions of years for coral reefs to return, based on palaeo-records of natural coral reef extinction events [viii].



[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, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 3, 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, no. 2 (2010): n/a – n/a, doi:10.1029/2009GL040999; Rhein et al., “Observations: Ocean.” [iii] 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): 12235–40., doi:10.1073/pnas.0906044106; J.E. Dore, “Hawaii Ocean Time-Series Surface CO2 System Data Product, 1988-2008.” (SOEST, University of Hawaii, Honolulu, HI., 2012), http://hahana.soest.hawaii.edu/hot/products/products.html. [iv] Rhein et al., “Observations: Ocean.” [v] F. Joos et al., “Impact of Climate Change Mitigation on Ocean Acidification Projections,” inOcean Acidification (Chapter 14) (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 (February 27, 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, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 6, http://www.climatechange2013.org/images/report/WG1AR5_Chapter06_FINAL.pdf; IGBP, IOC, 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. [vi] 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 (November 20, 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 (April 6, 2009): 515–33, doi:10.5194/bg-6-515-2009; Ciais et al., “Carbon and Other Biogeochemical Cycles.” [vii] David Archer, “Fate of Fossil Fuel CO₂ in Geologic Time,”Journal of Geophysical Research 110, no. C9 (2005), 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 (June 4, 2008): 283–97, doi:10.1007/s10584-008-9413-1. [viii] Orr et al., “Anthropogenic Ocean Acidification over the Twenty-First Century and Its Impact on Calcifying Organisms”; J. E. N Veron,A Reef in Time : The Great Barrier Reef from Beginning to End (Cambridge, Mass.: Belknap Press of Harvard University Press, 2008).

Supporting information Indicator definition Decline in ocean 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). Human activities are threatening this stability by adding large quantities of CO 2 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 CO 2 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 CO 2 to the ocean. As carbonate ion concentrations decline, so does the ocean’s capacity to take up anthropogenic CO 2 . Currently, the ocean takes up about one fourth of the global CO 2 emissions from combustion of fossil fuels, cement production and deforestation. Hence, the ocean serves mankind by moderating atmospheric CO 2 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 CO 2 level of 560 ppm due to the combined effects of acidification and warming. This CO 2 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 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 presented the EU Adaptation Strategy Package (http://ec.europa.eu/clima/policies/adaptation/what/documentation_en.htm). This package consists of the EU Strategy on adaptation to climate change /* COM/2013/0216 final */ and a number of supporting documents. One of the objectives of the EU Adaptation Strategy is Better informed decision-making, which should occur through Bridging the knowledge gap and Further developing Climate-ADAPT as the ‘one-stop shop’ for adaptation information in Europe. Further objectives include Promoting action by Member States and Climate-proofing EU action: promoting adaptation in key vulnerable sectors. Many EU Member States have already taken action, such as by adopting national adaptation strategies, and several have also prepared action plans on climate change adaptation. The European Commission and the European Environment Agency have developed the European Climate Adaptation Platform (Climate-ADAPT, http://climate-adapt.eea.europa.eu/) 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. Targets No targets have been specified. Related policy documents 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. 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 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. Further information on uncertainties is provided in Section 1.7 of the EEA report on Climate change, impacts, and vulnerability in Europe 2012 (http://www.eea.europa.eu/publications/climate-impacts-and-vulnerability-2012/) Rationale uncertainty No uncertainty has been specified Data sources Hawaii Ocean Time-series (HOT)

provided by University of Hawaii

provided by Coupled Model Intercomparison Project Phase 5 - CMIP5

provided by Lawrence Livermore National Laboratory (LLNL)