What is ocean acidification?

Changes in Ocean Chemistry

Increased levels of atmospheric CO₂ is having two serious impacts on the earth, the first is the well known phenomenon of global warming and the second is the subject of this blog- ocean acidification.  For over 650 000 years prior to the industrial revolution the concentration of CO₂ in the atmosphere was somewhere between 180 and 300ppmv (Siegenthaler et al. 2005).  Due to an increase in anthropogenic emissions, mainly through fossil fuel burning, the atmospheric carbon dioxide levels are now between 380 and 390ppmv resulting in a rate of change that is almost 100 times faster than anything the earth has experienced over the last 650kyr (IPCC, 2007).

Over the last 200 years the oceans have absorbed approximately 1/3 (11bn tonnes) of all the carbon dioxide that has been released into the atmosphere (Sabine et al. 2004). Dissolved organic carbon occurs in the ocean in 3 different forms- bicarbonate ions, aqueous carbon dioxide (including carbonic acid) and carbonate ions (Fabry et al. 2008).  The current pH of the ocean is 8.2 and 88% of the carbon occurs as bicarbonate ions.  Carbon dioxide dissolves in the sea water and forms carbonic acid which quickly dissociates to form H⁺ and HCO₃^- . The H⁺ may react with CO₃^2- to form bicarbonate.   This means that by adding CO₂ to the ocean we are increasing the amount of H₂CO₃, H⁺ and HCO₃^- and as an increase in the concentration of H⁺ ions causes pH to fall we are making the oceans more and more acidic (Ibid.).     Since the pre industrial times the ocean pH has fallen by 0.1, which has caused a 30% increase in acidity (Boyd, 2011).  Over the next century it is believed that the pH will fall by approximately 0.5 units, making the oceans more acidic than they have been over the past 30 million years (Calderia and Wickett, 2003). If the change in CO₂ is gradual a buffer system operates and so, interactions with carbonate minerals increase which helps to reduce the sensitivity of the ocean and stabilises the pH (Caldeira and Wickett, 2003).  If the change in CO₂ occurs over a short timescale (less than 10⁴ years) the oceans don’t have time to shift their equilibrium and so, the pH is altered.    The change in pH is most prominent towards the poles (i.e the North Atlantic and Southern Ocean) as more carbon dioxide is taken up at colder temperatures.

References

Boyd, P. (2011). ‘Beyond ocean acidification’, Nature Geoscience, 4: 273-274.

Calderia, K. and Wickett, M. (2003). ‘Anthropogenic carbon and ocean pH’, Nature, 425: 365.

Fabry, V. Seibel, B. Feely, R. Orr, J. (2008). ‘Impacts of ocean acidification on marine fauna and ecosystem processes’, Journal of marine science, 65, 3: 414-432

Sabine, C. Feely, R. Gruber, N. Key, R. Lee, K. Bullister, L. Wanninkhof, R. Wong, C. Wallace, D. Tilbrook, B. Millero, F. (2004). ‘The oceanic sink for anthropogenic CO₂’, Science, 305, 5682: 367-371.

Siegenthaler, U. Stocker, T. Monnin, E. Luethi, D. Schwander, J. Stauffer, B. Raynaud, D. (2005). ‘Stable carbon cycle-climate relationship during the late Pleistocene’, Science, 310: 1313– 1317.