Ocean acidification

From Freepedia

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.

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. Land-use changes, the combustion of fossil fuels, and the production of cement have led to a flux of CO₂ to the atmosphere. Some of this has remained in the atmosphere, 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 (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 also increases the hydrogen ion (H⁺) concentration in the ocean, and thus reduces ocean pH. 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 (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 ecological consequences of these changes in calcification are still uncertain, it appears likely that calcifying species will be adversely affected.

References

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.

Orr, J. C. et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686.