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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 2004 surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14.^[1]

Contents 1 Carbon cycle 2 Acidification 3 Gallery 4 References 5 Further reading 6 See also 7 External links

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Natural environment

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 (CO_{2 (aq)}), carbonic acid (H₂CO₃), bicarbonate (HCO₃^-) and carbonate (CO₃^2-). 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).

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).^[2] Since the industrial revolution began, ocean pH has dropped by approximately 0.1 units (on the logarithmic scale of pH), and it is estimated that it will drop by a further 0.3 - 0.4 units by 2100 as the ocean absorbs more anthropogenic CO₂.^[2][3] 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 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 some mollusks, especially pteropods. 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,^[4] coccolithophore algae^[5] and pteropods^[3] 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.^[6] 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 reducing the earth's albedo as well as oceanic cloud cover.^[7]

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.

Sea surface "present day" (1990s) anthropogenic CO2

Vertical inventory of "present day" (1990s) anthropogenic CO2

Change in surface CO32- ion from the 1700s to the 1990s

1. ^ Jacobson, M. Z. (2005). Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry. J. Geophys. Res. Atm. 110, D07302.
2. ^ ^{a b} Caldeira, K., and Wickett, M.E. (2003). Anthropogenic carbon and ocean pH. Nature 425, 365-365.
3. ^ ^{a b} Orr, J. C. et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686.
4. ^ Gattuso, J.-P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R. W. (1998). Effect of calcium carbonate saturation of seawater on coral calcification. Glob. Planet. Change 18, 37-46.
5. ^ Riebesell, U. et al. (2000). Reduced calcification of marine plankton in response to increased atmospheric CO₂. Nature 407, 364-367 (Subscription required).
6. ^ Raven, J. A. et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
7. ^ Ruttiman, J. (2006). Sick Seas. Nature 442, 978-980 (Subscription required).

• Cicerone, R., J. Orr, P. Brewer et al. (2004). The Ocean in a High CO₂ World. Eos. Transactions of the American Geophysical Union 85, 351-353.
• Doney, S. C. (2006). The Dangers of Ocean Acidification. Scientific American 294, 58-65 (Article preview only).
• Feely, R. A. et al. (2004). Impact of Anthropogenic CO₂ on the CaCO₃ System in the Oceans. (Abstract) Science 305, 362-366.
• Henderson, C. (2006) Ocean acidification: the other CO2 problem New Scientist 2563, 5 August 2006.
• 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.
• Kolbert, E. (2006). The Darkening Sea: Carbon emissions and the ocean. The New Yorker magazine. 20 November 2006.
• Sabine, C. L. et al. (2004). The Oceanic Sink for Anthropogenic CO₂. Science 305, 367-371.

• Biological pump
• Carbon dioxide sinks
• Continental shelf pump
• Solubility pump

• Announcement for Royal Society of London report
• Orr et al. (2005) supplementary material
• The Acid Ocean – the Other Problem with CO₂ Emission, David Archer, a RealClimate discussion
• Task Force on Ocean Acidification in the Pacific, PacificScience.org
• "Coral Bones" - an investigation into the future of coral reefs
• "Growing Acidity of Oceans May Kill Corals", Washington Post
• Ocean Acidification - a multimedia, interactive site from The World Ocean Observatory
• Dropping pH in the Oceans Causing a Rising Tide of Alarm by Tundi Agardy, The World Ocean Observatory
• "The other CO₂ problem", OpenDemocracy.net
• CO2SYS is a program for calculating the state of the carbonate system in seawater (including pH). CO2SYS is also available in a version for Microsoft Excel/VBA.