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. Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104 (a change of -0.075).
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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.
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). 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. 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 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 aragonitepolymorphs 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 under-saturated, structures made of calcium carbonate are vulnerable to dissolution. Research has already found that corals, coccolithophore algae, foraminifera, shellfish and pteropods 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.
While the full ecological consequences of these changes in calcification are still uncertain, it appears likely that calcifying species will be adversely affected. There is also some evidence that the effect of acidification on coccolithophores (among the most abundant phytoplankton in the ocean) in particular may eventually exacerbate climate change, by decreasing the earth's albedo via their effects on oceanic cloud cover.
Aside from calcification (and specifically calcifiers), 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. However, as with calcification, as yet there is not a full understanding of these processes in marine organisms or ecosystems.
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^ abc 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 Cycles18: GB4031. doi:10.1029/2004GB002247. ISSN 0886-6236.
^Review of Past IPCC Emissions Scenarios, IPCC Special Report on Emissions Scenarios (ISBN 0521804930).
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^ ab Raven, J. A. et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
^ 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 Change18 (1-2): 37-46. doi:10.1016/S0921-8181(98)00035-6. ISSN 0921-8181.
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Cicerone, R.; J. Orr, P. Brewer et al. (2004). "The Ocean in a High CO2 World". EOS, Transactions American Geophysical Union85 (37): 351-353. doi:10.1029/2004EO370007. ISSN 0096-3941.
Doney, S. C. (2006). "The Dangers of Ocean Acidification". Scientific American294: 58-65. ISSN 0036-8733., (Article preview only).
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Kleypas, J.A., R.A. Feely, V.J. Fabry, C. Langdon, C.L. Sabine, and L.L. Robbins. (2006). Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Further Research, report of a workshop held 18-20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA and the U.S. Geological Survey, 88pp.
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Carbonate system calculators
The following packages calculate the state of the carbonate system in seawater (including pH):
CO2SYS, a stand-alone executable (also available in a version for Microsoft Excel/VBA)
seacarb, a R package for Windows, Mac OS X and Linux (also available here)
CSYS, a Matlab script
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