Atmospheric Carbon Dioxide Levels and Ocean Acidification


Burning of fossil fuel has increased the atmospheric levels of carbon dioxide from a preindustrial revolution level of 285 ppm to 385 ppm in 2006 (Blog 1).  Average ocean pH has decreased since the industrial revolution.  The question becomes is there a connection between atmospheric levels of carbon dioxide and ocean pH?  Answering this question requires an understanding of how oceans maintain a stable pH.  

A buffer is a substance that prevents a sudden or large change in the acidity or alkalinity of a solution. The pH of oceans remains relatively constant due to the buffering action of carbon dioxide.  When carbon dioxide is dissolved in water it is converted to carbonic acid, bicarbonate and carbonate.  These four compounds are in equilibrium and prevent a sudden or large change in the pH of oceans.  The pH of oceans is inversely proportional to the concentration of carbon dioxide in the water.  In other words, the higher the carbon dioxide the lower the pH and the ocean becomes more acidic. 

The pH of oceans is subject to natural variations.  Ocean surface water for example has a pH of about 8.5 because less carbon dioxide is dissolved in surface water.  Primary producers live at or near the surface and utilize carbon dioxide through photosynthesis which lowers surface water levels of carbon dioxide (my posting entitled Global Warming and Ocean Deserts).  In addition, less carbon dioxide is dissolved in surface water because warm water contains less carbon dioxide than colder water.    

Middle depth ocean water (15,000 feet) and bottom ocean water contain high concentrations of dissolved carbon dioxide which lowers their pH to about 7.5 and 7.0, respectively.  The concentration of carbon dioxide is higher in deep ocean water because there is no photosynthesis to remove carbon dioxide from the water.  Animal respiration and scavenger decomposition also increase the level of carbon dioxide in deep water (my posting entitled Global Warming and Ocean Deserts). Since carbon dioxide solubility in ocean water increases with decreasing temperature and increasing pressure, this allows deeper water to store more carbon dioxide. 

The atmospheric and oceanic concentrations of carbon dioxide are in chemical equilibrium (my posting entitled Carbon Cycle). Clearly, the burning of fossil fuel has significantly increased the level of carbon dioxide in the atmosphere and the ocean triggering both global warming and ocean acidification.  

How severely will ocean acidification affect marine inhabitants and disrupt marine ocean systems?  Small changes in ocean pH will have profound effect on calcifying organisms such as shell fish and coral reefs.  Shell fish and coral reef skeletons are composed of calcium carbonate.  As the pH of the ocean decreases so does the amount of carbonate.  This lack of carbonate will inhibit the ability of shell fish and coral to build and maintain their shells and skeletons. Furthermore, the stability of calcium carbonate in shell fish and coral decrease as the pH decreases.  The shells of shell fish start dissolving when the pH drops below 7.5 which is the current pH of middle ocean water.  The continued burning of fossil fuel will cause an upward shift of the pH below 7.5 from middle ocean depth to a depth that could affect shell fish survival.   

Philip Haddad
12/1/2012 12:02:00 AM

Has any work been done in documenting the levels of carbonate and bicarbonate in ocean water over the past decades? It is apparent that pH has lowered during the industrial age, but is it not possible that this is being caused by "acid rain" also a product of combustion of fossil fuels, particularly coal. It would take only a slight increase of sulfate ion, for example, to account for the increased hydrogen ion to match the lowered pH. Are there records of sufficient accuracy over a long enough time period to answer this question? As to global warming the heat emitted from our energy use is more than sufficient to account for rising temperature without any contribution from increased infra red from solar sources. For example in 2008 energy use was 16 terrawatts. that is equivalent to 50x10E16 btus a year or 500 Mount Saint Helens's eruptions. That is enough to potentially raise the temperature by 0.17*F. Actual rise was one fourth that due to cooling by glacial melting and to removal of 5000 btus of energy for every pound of CO2 converted through photosynthesis.

Richard Hilderman, Ph.D.
12/28/2010 9:06:31 AM

H. Sanborn Thanks for your comments. The main purpose of my blog is to make climate change and global warming common knowledge.

Heather Berry Sanborn
12/26/2010 7:31:51 PM

Thank You for posting these articles. This needs to be common knowledge. The possibility of the collapse of ocean food chains would be devastating on so many levels.

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