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 dropped from approximately 8.25 to 8.14 (Jacobson, 2005).

Carbon cycle

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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.

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).

Acidification

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Dissolving CO₂ also increases the hydrogen ion (H⁺) concentration in the ocean, and thus reduces 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, ocean pH has dropped by approximately 0.1 units, 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 (Caldeira and Wickett, 2003; Orr et al., 2005).

Although this oceanic absorption will help ameliorate the climatic effects of anthropogenic emissions of CO₂, it is believed that it will have negative consequences 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 and 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, as 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 (Gattuso et al., 1998), coccolithophore algae (Riebesell et al., 2000) and pteropods (Orr et al., 2005) 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 (Raven, et al., 2005).

While the full ecological consequences of these changes in calcification are still uncertain, it appears likely that calcifying species will be adversely affected. Present evidence suggests that dramatic changes in the marine environment over the next 100-200 years can be avoided only with early and deep reductions in carbon dioxide emissions.

References

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• Caldeira, K., and Wickett, M.E. (2003). Anthropogenic carbon and ocean pH. Nature 425, 365-365.
• 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.
• 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.
• Orr, J. C. et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686.
• Raven, J. A. et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
• Riebesell, U. et al. (2000). Reduced calcification of marine plankton in response to increased atmospheric CO₂. Nature 407, 364-367 (Subscription required).

Further reading

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• 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.
• Sabine, C. L. et al. (2004). The Oceanic Sink for Anthropogenic CO₂. Science 305, 367-371.

See also

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• Biological pump
• Carbon dioxide sinks
• Solubility pump

External links

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• "The other CO2 problem"
• Announcement for Royal Society of London paper
• Orr et al. (2005) supplementary material
• Realclimate.org discussion
• "Coral Bones" - an investigation into the future of coral reefs
• Task Force on Marine Acidification in the Pacific
• Impacts of ocean acidification on coral reefs and other marine calcifiers - a guide for future research
• "Growing Acidity of Oceans May Kill Corals" - Washington Post

Biological oceanography | Chemical oceanography | Geochemistry | Oceanography