File:AYool GLODAP del pH.png

Ocean acidification is the name given to the ongoing decrease in the pH of the Earth's oceans, caused by their uptake of anthropogenic carbon dioxide from the atmosphere.[1] Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104 (a change of -0.075).[2][3]

Carbon cycleEdit

In the natural carbon cycle, the atmospheric concentration of carbon dioxide (CO2) represents a balance of fluxes between the oceans, terrestrial biosphere and the atmosphere. Human activities such as land-use changes, the combustion of fossil fuels, and the production of cement have led to a new flux of CO2 into the atmosphere. Some of this has remained in the atmosphere (where it is responsible for the rise in atmospheric concentrations), some is believed to have been taken up by terrestrial plants, and some has been absorbed by the oceans.

When CO2 dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species : dissolved free carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-). The ratio of these species depends on factors such as seawater temperature and alkalinity (see the article on the ocean's solubility pump for more detail).


Average surface ocean pH[2]
Time pH pH change Source
Pre-industrial (1700s) 8.179 0.000 analysed field[3]
Recent past (1990s) 8.104 -0.075 field[3]
2050 (2×CO2 = 560 ppm) 7.949 -0.230 model[2]
2100 (IS92a)[4] 7.824 -0.355 model[2]

Dissolving CO2 in seawater also increases the hydrogen ion (H+) concentration in the ocean, and thus decreases ocean pH. The use of the term "ocean acidification" to describe this process was introduced in Caldeira and Wickett (2003)[1].

Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly less than 0.1 units (on the logarithmic scale of pH), and it is estimated that it will drop by a further 0.3 - 0.5 units by 2100 as the ocean absorbs more anthropogenic CO2[1][2][5].

Note that, although the ocean is acidifying, its pH is still greater than 7 (that of neutral water), so the ocean could also be described as becoming less alkaline.

Although the largest changes are expected in the future[2], a report from NOAA scientists found large quantities of water undersaturated in aragonite are already upwelling close to the Pacific continental shelf area of North America[6]. Continental shelves play an important role in marine ecosystems since most marine organisms live or are spawned there, and though the study only dealt with the area from Vancouver to northern California, the authors suggest that other shelf areas may be experiencing similar effects[6]. Similarly, one of the first detailed datasets examining temporal variations in pH at a temperate coastal location found that acidification was occurring at a rate much higher than that previously predicted, with consequences for near-shore benthic ecosystems[7][8].

Possible impactsEdit

Although the natural absorption of CO2 by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO2, it is believed that the resulting decrease in pH will have negative consequences, primarily for oceanic calcifying organisms. These use the calcite or aragonite polymorphs of calcium carbonate to construct cell coverings or skeletons. Calcifiers span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs.

Under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ion is at supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. Research has already found that corals[9][10][11], coccolithophore algae[12][13][14][15], coralline algae[16], foraminifera[17], shellfish[18] and pteropods[2] experience reduced calcification or enhanced dissolution when exposed to elevated CO2. The Royal Society of London published a comprehensive overview of ocean acidification, and its potential consequences, in June 2005[5].

However, some studies have found different response to ocean acidification, with coccolithophore calcification and photosynthesis both increasing under elevated atmospheric pTemplate:CO2[19][20][21], an equal decline in primary production and calcification in response to elevated Template:Co2[22] or the direction of the response varying between species[23]. Recent work examining a sediment core from the North Atlantic found that while the species composition of coccolithophorids has remained unchanged for the industrial period 1780 to 2004, the calcification of coccoliths has increased by up to 40% during the same time[21].

While the full ecological consequences of these changes in calcification are still uncertain, it appears likely that many calcifying species will be adversely affected. There is also a suggestion that a decline in the coccolithophores may have secondary effects on climate change, by decreasing the earth's albedo via their effects on oceanic cloud cover[24]. Aside from calcification, organisms may suffer other adverse effects, either directly as reproductive or physiological effects (e.g. CO2-induced acidification of body fluids, known as hypercapnia), or indirectly through negative impacts on food resources[5]. However, as with calcification, as yet there is not a full understanding of these processes in marine organisms or ecosystems [25].

Leaving aside direct biological effects, it is expected that ocean acidification in the future will lead to a significant decrease in the burial of carbonate sediments for several centuries, and even the dissolution of existing carbonate sediments[26]. This will cause an elevation of ocean alkalinity, leading to the enhancement of the ocean as a reservoir for Template:Co2 with moderate (and potentially beneficial) implications for climate change as more Template:Co2 leaves the atmosphere for the ocean[27].


File:AYool GLODAP aCO2.png
File:AYool GLODAP invt aCO2.png
File:AYool GLODAP del co3.png

See alsoEdit


  1. 1.0 1.1 1.2 Caldeira, K.; Wickett, M.E. (2003). "Anthropogenic carbon and ocean pH". Nature 425 (6956): 365–365. doi:10.1038/425365a. ISSN 0028-0836, 
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Orr, James C.; Fabry, Victoria J.; Aumont, Olivier; Bopp, Laurent; Doney, Scott C.; Feely, Richard A. et al. (2005). "Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms". Nature 437 (7059): 681–686. doi:10.1038/nature04095. ISSN 0028-0836, 
  3. 3.0 3.1 3.2 Key, R.M.; Kozyr, A.; Sabine, C.L.; Lee, K.; Wanninkhof, R.; Bullister, J.; Feely, R.A.; Millero, F.; Mordy, C. and Peng, T.-H. (2004). "A global ocean carbon climatology: Results from GLODAP". Global Biogeochemical Cycles 18: GB4031. doi:10.1029/2004GB002247. ISSN 0886-6236. 
  4. Review of Past IPCC Emissions Scenarios, IPCC Special Report on Emissions Scenarios (ISBN 0521804930).
  5. 5.0 5.1 5.2 Raven, J. A. et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
  6. 6.0 6.1 Feely, Richard; Christopher L. Sabine, J. Martin Hernandez-Ayon, Debby Ianson, Burke Hales. (2008). "Evidence for Upwelling of Corrosive "Acidified" Seawater onto the Continental Shelf". Science 10. 
  7. Wootton, J.T.; Pfister, C.A. and Forester, J.D. (2008). "Dynamic patterns and ecological impacts of declining ocean pH in a high-resolution multi-year dataset". PNAS 105: 18848–18853. doi:10.1073/pnas.0810079105. 
  8. "Ocean Growing More Acidic Faster Than Once Thought; Increasing Acidity Threatens Sea Life". Science Daily (2008-11-26). Retrieved on 26 November 2008.
  9. Gattuso, J.-P.; Frankignoulle, M.; Bourge, I.; Romaine, S. and Buddemeier, R. W. (1998). "Effect of calcium carbonate saturation of seawater on coral calcification". Global and Planetary Change 18 (1-2): 37–46. doi:10.1016/S0921-8181(98)00035-6. ISSN 0921-8181, 
  10. Gattuso, J.-P.; Allemand, D.; Frankignoulle, M (1999). "Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: a review on interactions and control by carbonate chemistry". American Zoologist 39: 160–183, 
  11. Langdon, C; Atkinson, M. J. (2005). "Effect of elevated pCO2 on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment". J. Geophysical Res. 110 (C09S07): C09S07. doi:10.1029/2004JC002576. 
  12. Riebesell, Ulf; Zondervan, Ingrid; Rost, Björn; Tortell, Philippe D.; Zeebe, Richard E. and François M. M. Morel (2000). "Reduced calcification of marine plankton in response to increased atmospheric CO2" (abstract). Nature 407 (6802): 364–367. doi:10.1038/35030078. ISSN 0028-0836,  (Subscription required)
  13. Zondervan, I.; Zeebe, R.E., Rost, B. and Rieblesell, U. (2001). "Decreasing marine biogenic calcification: a negative feedback on rising atmospheric pCO2". Global Biogeochem. Cycles 15: 507–516. doi:10.1029/2000GB001321. 
  14. Zondervan, I.; Rost, B. and Rieblesell, U. (2002). "Effect of Template:Co2 concentration on the PIC/POC ratio in the coccolithophore Emiliania huxleyi grown under light limiting conditions and different day lengths". J. Exp. Mar. Biol. Ecol. 272: 55–70. doi:10.1016/S0022-0981(02)00037-0. 
  15. Delille, B.; Harlay, J., Zondervan, I., Jacquet, S., Chou, L., Wollast, R., Bellerby, R.G.J., Frankignoulle, M., Borges, A.V., Riebesell, U. and Gattuso, J.-P. (2005). "Response of primary production and calcification to changes of pTemplate:Co2 during experimental blooms of the coccolithophorid Emiliania huxleyi". Global Biogeochem. Cycles 19: GB2023. doi:10.1029/2004GB002318, 
  16. Kuffner, I.B.; Andersson, A.J., Jokiel, P.L., Rodgers, K.S. and Mackenzie, F.T. (2007). "Decreased abundance of crustose coralline algae due to ocean acidification". Nature Geoscience 1: 114–117. doi:10.1038/ngeo100. 
  17. Phillips, Graham; Chris Branagan (2007-09-13). "Ocean Acidification – The BIG global warming story", ABC TV Science: Catalyst, Australian Broadcasting Corporation. Retrieved on 18 September 2007. 
  18. Gazeau, F.; Quiblier, C.; Jansen, J. M.; Gattuso, J.-P.; Middelburg, J. J. and Heip, C. H. R. (2007). "Impact of elevated CO2 on shellfish calcification". Geophysical Research Letters 34: L07603. doi:10.1029/2006GL028554. ISSN 0094-8276, 
  19. Buitenhuis, E.T.; de Baar, H. J. W. and Veldhuis, M. J. W. (1999). "Photosynthesis and calcification by Emiliania huxleyi (Prymnesiophyceae) as a function of inorganic carbon species". J. Phycology 35: 949–959. doi:10.1046/j.1529-8817.1999.3550949.x. 
  20. Nimer, N.A.; Merrett, M.J. (1993). "Calcification rate in Emiliania huxleyi Lohmann in response to light, nitrate and availability of inorganic carbon". New Phytologist 123: 673–677. doi:10.1111/j.1469-8137.1993.tb03776.x. 
  21. 21.0 21.1 Iglesias-Rodriguez, M.D.; Halloran, P.R., Rickaby, R.E.M., Hall, I.R., Colmenero-Hidalgo, E., Gittins, J.R., Green, D.R.H., Tyrrell, T., Gibbs, S.J., von Dassow, P., Rehm, E., Armbrust, E.V. and Boessenkool, K.P. (2008). "Phytoplankton Calcification in a High-Template:CO2 World". Science 320: 336–340. doi:10.1126/science.1154122. PMID 18420926. 
  22. Sciandra, A.; Harlay, J., Lefevre, D. et al. (2003). "Response of coccolithophorid Emiliania huxleyi to elevated partial pressure of Template:CO2 under nitrogen limitation". Mar. Ecol. Prog. Ser. 261: 111–112. doi:10.3354/meps261111. 
  23. Langer, G.; Geisen, M., Baumann, K. H. et al. (2006). "Species-specific responses of calcifying algae to changing seawater carbonate chemistry". Geochem. Geophys. Geosyst. 7. doi:10.1029/2005GC001227. 
  24. Ruttiman, J. (2006). "Sick Seas". Nature 442 (7106): 978–980. doi:10.1038/442978a. ISSN 0028-0836,  (Subscription required)
  25. The Australian (2008). Swiss marine researcher moving in for the krill},,25197,24392216-27703,00.html. 
  26. Ridgwell, A.; Zondervan, I., Hargreaves, J.C., Bijma, J. and Lenton, T.M. (2007). "Assessing the potential long-term increase of oceanic fossil fuel Template:Co2 uptake due to Template:Co2-calcification feedback". Biogeosciences 4: 481–492. 
  27. Tyrrell, T. (2008). "Calcium carbonate cycling in future oceans and its influence on future climates". J. Plankton Res. 30: 141–156. doi:10.1093/plankt/fbm105. 

Further readingEdit

  • Jacobson, M. Z. (2005). "Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry". Journal of Geophysical Research - Atmospheres 110: D07302. doi:10.1029/2004JD005220. ISSN 0148-0227. 
  • Kolbert, E. (2006). The Darkening Sea: Carbon emissions and the ocean. The New Yorker magazine. 20 November 2006. (Article abstract only).

External linksEdit

Carbonate system calculatorsEdit

The following packages calculate the state of the carbonate system in seawater (including pH):