What is ocean acidification?

The consequences of man's use of fossil fuels (coal, oil and natural gas) in terms of global warming has not escaped anyones attention. Ocean acidification is another, and much less known, result of the approximately 79 million tons of carbon dioxide (CO2) released into the atmosphere every day, not only as a result of fossilfuel burning but also of deforestation and production of cement (7). Since the beginning of the industrial revolution, about one third ofthe CO2 released in the atmosphere by anthropogenic (human-caused) activities has been absorbed by the world’s oceans,which play a key role in moderating climate change (5). Without this capacity of the oceans, the CO2 content in the atmosphere would have been much higher and global warming and its consequences more dramatic. The impacts of ocean acidification on marine ecosystems are still poorly known but one of the most likely consequences is the slower growth of organisms forming calcareous skeletons or shells, suchas corals and mollusks.

 

The carbon cycle

Inorder to understand ocean acidification and its possible impacts, one needs to understand the behaviour of carbon in nature. Carbon, as other elements, is circulating in different chemical forms and between different parts of the Earth system (atmosphere, biosphere and the oceans). These fluxes of carbon in inorganic (e.g. CO2) and organic forms (sugar and more complex carbohydrates in the biosphere) constitute the carbon cycle. In a very short time span, human activities use an old reservoir of carbon (fossil fuels) which took millions of years to accumulate, thus creating a new and massive flux of CO2 into the atmosphere. The oceans can mitigate this additional carbon dioxide flux and thus help moderate global warming but this is not without consequences.


The world's oceans play a fundamental role in the exchange of CO2 with the atmosphere and constitute an important sink for atmospheric CO2. Once dissolved in sea water, carbon dioxide is subject to two possible fates. It can either be used by photosynthesis or other physiological processes, or remain free in its differentdissolved forms in the water. The latter leads to ocean acidification.

The chemical process of ocean acidification

There is a constant exchange between the upper layers of the oceans and the atmosphere. Nature strives towards equilibrium, and thus for the ocean and the atmosphere to contain equal concentrations of CO2. Carbon dioxide in the atmosphere therefore dissolves in the surfacewaters of the oceans in order to establish a concentration inequilibrium with that of the atmosphere. As CO2 dissolves in the ocean it generates dramatic changes in sea water chemistry. CO2 reacts with water molecules (H2O) and forms the weak acid H2CO3 (carbonic acid). Most of this acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in H+ ions reduces pH (measure of acidity) and the oceans acidify, that is they become more acidic or rather less alkaline since although the ocean is acidifying, its pH is still greater than 7 (that of water with a neutral pH). The average pH of today's surface waters is 8.1, which is approximately 0.1 pH units less than the estimated pre-industrial value 200 years ago (2,3).

 

Projections of future changes

Modeling demonstrates that if CO2 continues to be released on current trends, ocean average pH will reach 7.8 by the end of this century, corresponding to 0.5 units below the pre-industrial level, a pH level that has not been experienced for several millions of years (1). A change of 0.5 units might not sound as a very big change, but the pH scale is logaritmic meaning that such achange is equivalent to a three fold increase in H+ concentration. All this is happening at a speed 100 times greater than has ever been observed during the geological past. Several marine species, communities and ecosystems might not have the time to acclimate or adapt to these fast changes in ocean chemistry.


Possible consequences on marine organisms

The dissolution of carbon dioxide in sea water not only provokes an increase in hydrogen ions and thus a decline in pH, but also a decreasein a very important form of inorganic carbon: the carbonate ion (CO32-). Numerous marine organisms such as corals, mollusks, crustaceans and seaurchins rely on carbonate ions to form their calcareous shells or skeletons in a process known as calcification. The concentration of carbonate ions in the ocean largely determines whether there is dissolution or precipitation of aragonite and calcite, the two natural polymorphs of calcium carbonate (CaCO3), secreted in the form of shells or skeletons by these organisms. Today, surface waters are super saturated with respect to aragonite and calcite, meaning that carbonate ions are abundant. This super saturation is essential, not only for calcifying organisms to produce their skeletons or shells, but also to keep these structures intact. Existing shells and skeletons might dissolve if pH reach lower values, and the oceans turn corrosive for these organisms. Consequently, the results ofthe decrease in carbonate ions might be catastrophic for calcifying organisms which play an important role in the food chain and form diverse habitats helping the maintenance of biodiversity.


The magnitude of ocean acidification can be predicted with a high level of confidence since the ocean chemistry is well known. But the impacts of the acidification on marine organisms and their ecosystems is much less predictable. Not only calcifying organisms are potentially affected by ocean acidification. Other main physiological processes such as reproduction, growth and photosynthesis are susceptible to be impacted, possibly resulting in an important loss in marine biodiversity. But it is also possible that some species, like seagrasses that uses CO2 for photosynthesis, are positively influenced by ocean acidification. Ocean acidification research is still in its infancy and more studies are required to answer the numerous questions related to its biological and biogeochemical consequences.



References:

1) Caldeira, K., Wickett, M.E., 2003. Anthropogenic carbon and ocean pH. Nature 425 (6956): 365–365.
2) 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 Cycles 18
3) Orr J. C., Fabry V. J., Aumont O., Bopp L., Doney S. C., Feely R. A. et al. 2005. "Anthropogenic ocean acidification over the twenty-first century and its impact oncalcifying organisms". Nature 437 (7059): 681–686.
4) Raven, J. A. et al. 2005. Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
5) Sabine C. L. et al., 2004. The oceanic sink for anthropogenic CO2. Science 305:367-371.
6) Martin S. et al. 2008. Ocean acidification and its consequences. French ESSP Newsletter 21: 5-16.
7) IPCC 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Summary for Policymakers.

 

For more information on ocean acidification, carbonate chemistry and the carbon cycle, see the key documents and web resources.


Glossary

Acclimate - To accustom or become accustomed to a new environment or situation.

Aragonite - An orthorhombic (system of crystallization characterized by three unequal axes at right angles to each other) mineral form of crystalline calcium carbonate, dimorphous with calcite

Biosphere – The living organisms and their environment

Calcite - A common crystalline form of natural calcium carbonate, CaCO3, that is the basic constituent of limestone, marble, and chalk. Also called calcspar.

Inorganic - Involving neither organic life nor the products of organic life

Ocean acidification – The process by which carbon dioxide dissolves in seawater, giving rise to a decrease in pH and other changes in ocean carbonate chemistry

Organic - Of, relating to, or derived from living organisms

pH – Measure of acidity (pH= -log[H+])

Photosynthesis - The process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct.

Phytoplankton - Minute, free-floating aquatic plants (algae, protists, and cyanobacteria).

Polymorph – Chemistry: A specific crystalline form of a compound that can crystallize in different forms.

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