Atmospheric Carbon Dioxide Levels and Ocean Acidification

Reader Contribution by Richard Hilderman and Ph.D.

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.   

Scientific studies have suggested that coral will not be able to build calcium carbonate reef structures when the atmospheric level of carbon dioxide reaches 480 ppm.  At our current rate of carbon dioxide emissions it is estimated that we will reach this level in 2050.  As the world population continues to increase so will the rate of carbon dioxide emissions from the burning of fossil fuel.  This means we will most likely reach 480 ppm much earlier than 2050.   

A reduced ocean pH should also increase the concentrations of dissolved copper, zinc and other toxic metals.  How these factors will ultimately affect marine organisms’ health is not known.