European Environment Agency – Ocean acidification (CLIM 043) – Assessment published Nov 2012


  • Indicator definition
  • Key policy question: What is the trend in the acidity of ocean water?
  • Data sources
  • Justification for indicator selection
  • Policy context and targets
  • Methodology
  • Uncertainties
  • More information about this indicator
  • Contacts and ownership
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Key policy question: What is the trend in the acidity of ocean water?

Key messages

  • Surface-ocean pH has declined from 8.2 to 8.1 over the industrial era due to the growth of atmospheric CO2 concentrations. This decline corresponds to a 30 % change in oceanic acidity.
  • Observed reductions in surface-water pH are nearly identical across the global ocean and throughout Europe’s seas.
  • Ocean acidification in recent decades is occurring a hundred times faster than during past natural events over the last 55 million years.
  • Ocean acidification already reaches into the deep ocean, particularly in the high latitudes.
  • Average surface-water pH is projected to decline further to 7.7 or 7.8 by the year 2100, depending on future CO2 emissions. This decline represents a 100 to 150 % increase in acidity.
  • Ocean acidification may affect many marine organisms within the next 20 years and could alter marine ecosystems and fisheries.

Fig. 1: Decline in pH measured at the Aloha station as part of the Hawaii Ocean time-series

Note: Aloha station pH time series. Changes here are similar to those that are observed at a much shorter time scale in Europe.

Data source:

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Key assessment

Past trends

In December 2011, the atmospheric CO2 level reached 392 ppm, which is 40 % more than the pre-industrial concentration (280 ppm); half of that increase has occurred in the last 30 years. Ocean pH has been reduced from 8.2 to 8.1 over the industrial era, which corresponds to a 30 % increase in ocean acidity (defined here as the hydrogen ion concentration). This change has occurred at a rate that is about a hundred times faster than any change in acidity experienced during the last 55 million years. The current decline in pH is already measurable at the three ocean time series stations that are suitable to evaluate long-term trends, located offshore of Hawaii, Bermuda and the Canary Islands. Figure 1 shows the time series from Hawaii, which is the longest and best known one, and the changes here are similar to those that are observed at a much shorter time scale in Europe. The measured reductions in surface pH at those stations match exactly the values calculated on the basis of increasing atmospheric CO2 concentrations, assuming thermodynamic equilibrium between the surface ocean and the atmosphere [i].


Average surface-water pH is projected to decline further to 7.7 or 7.8 by the year 2100, depending on future CO2 emissions. This decline represents a 100 to 150 % increase in acidity. When atmospheric CO2 reaches 450 ppm, parts of the Southern Ocean will start becoming corrosive to calcium carbonate during winter [ii]. Ten per cent of the Arctic Ocean may become corrosive to calcium carbonate already by 2020 [iii], and surface waters of the Baltic Sea will still become corrosive well before the end of the century. 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 CO2 emissions, recovery from human-induced acidification will require thousands of years for the Earth system to re-establish roughly similar ocean chemical conditions [iv] and millions of years for coral reefs to return, based on palaeo-records of natural coral reef extinction events [v].

[i] N.R. Bates, „Ocean Carbon Time Series“ (IOCCP, 2005),; J. Magdalena Santana-Casiano et al., „The interannual variability of oceanic CO₂ parameters in the northeast Atlantic subtropical gyre at the ESTOC site“, Global Biogeochemical Cycles 21, Nr. 1 (März 8, 2007), doi:10.1029/2006GB002788; 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.

[ii] 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, Nr. 48 (November 20, 2008): 18860–18864, doi:10.1073/pnas.0806318105.

[iii] M. Steinacher et al., „Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model“, Biogeosciences 6, Nr. 4 (April 6, 2009): 515–533, doi:10.5194/bg-6-515-2009.

[iv] David Archer, „Fate of fossil fuel CO₂ in geologic time“, Journal of Geophysical Research 110, Nr. C9 (2005), doi:10.1029/2004JC002625; David Archer and Victor Brovkin, „The millennial atmospheric lifetime of anthropogenic CO₂“, Climatic Change 90, Nr. 3 (Juni 4, 2008): 283–297, doi:10.1007/s10584-008-9413-1; Toby Tyrrell, John G. Shepherd, and Stephanie Castle, „The long-term legacy of fossil fuels“, Tellus B 59, Nr. 4 (September 2007): 664–672, doi:10.1111/j.1600-0889.2007.00290.x.

[v] 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).

Data sources

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.

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More information about this indicator

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Contacts and ownership

EEA Contact Info

Trine Christiansen


EEA Management Plan

2012 2.0.1 (note: EEA internal system)


First draft created: 2012/11/15 09:58:35.671506 GMT+1
Publish date: 2012-11-20T15:32:15+01:00
Last modified: 2012/11/20 15:32:23.240450 GMT+1

European Environment Agency, 20 November 2012. Web site.

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