Ocean acidification

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)^[1][2].

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Carbon cycle

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

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)^[4]. 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^[4][1][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.

Possible impacts

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 under-saturated, structures made of calcium carbonate are vulnerable to dissolution. Research has already found that corals^[6], coccolithophore algae^[7], foraminifera^[8], shellfish^[9] and pteropods^[1] 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].

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^[10].

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.

Gallery

See also

• Biological pump
• Carbon dioxide sinks
• Continental shelf pump
• Global Ocean Data Analysis Project
• Seawater pH
• Solubility pump

References

Further reading

• Cicerone, R.; J. Orr, P. Brewer et al. (2004). "The Ocean in a High CO₂ World". EOS, Transactions American Geophysical Union 85 (37): 351-353. doi:10.1029/2004EO370007. ISSN 0096-3941.

• Doney, S. C. (2006). "The Dangers of Ocean Acidification". Scientific American 294: 58-65. ISSN 0036-8733., (Article preview only).

• Feely, R. A.; Sabine, Christopher L.; Lee, Kitack; Berelson, Will; Kleypas, Joanie; Fabry, Victoria J.; Millero, Frank J. (2004). "Impact of Anthropogenic CO₂ on the CaCO₃ System in the Oceans". Science 305 (5682): 362-366. doi:10.1126/science.1097329. ISSN 0036-8075.

• Henderson, Caspar. "Ocean acidification: the other CO2 problem", NewScientist.com news service, 2006-08-05. 

• 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.

• 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.; Feely, Richard A.; Gruber, Nicolas; Key, Robert M.; Lee, Kitack; Bullister, John L. et al. (2004). "The Oceanic Sink for Anthropogenic CO₂". Science 305 (5682): 367-371. doi:10.1126/science.1097403. ISSN 0036-8075.

• Stone, R. (2007). "A World Without Corals?". Science 316 (5825): 678-681. doi:10.1126/science.316.5825.678. ISSN 0036-8075.

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