21.3: Carbon Dioxide, pH, and Ocean Acidification

Figure \(\PageIndex{1}\) Representative carbon dioxide profiles for the Pacific and Atlantic oceans (PW).

The anthropogenic addition of carbon dioxide into the atmosphere has an additional effect on ocean chemistry. About 30% of the carbon dioxide that is added to the atmosphere is absorbed by the oceans. This additional carbon dioxide affects the pH of the ocean. We previously learned the following:

When CO₂ gas dissolves in the ocean, it interacts with the water to produce a number of different compounds according to the reaction below:

CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃^– ↔ 2H⁺ + CO₃^2-

CO₂ reacts with water to produce carbonic acid (H₂CO₃), which then dissociates into bicarbonate (HCO₃^–) and hydrogen ions (H⁺). The bicarbonate ions can further dissociate into carbonate (CO₃^2-) and additional hydrogen ions (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\) The fate of dissolved carbon dioxide in the oceans. Most of the carbon ends up in the form of bicarbonate (PW).

Carbon dioxide and the other carbon compounds listed above play an important role in buffering the pH of the ocean. Currently, the average pH for the global ocean is about 8.1, meaning seawater is slightly basic. Because most of the inorganic carbon dissolved in the ocean exists in the form of bicarbonate, bicarbonate can respond to disturbances in pH by releasing or incorporating hydrogen ions into the various carbon compounds. If pH rises (low [H⁺]), bicarbonate may dissociate into carbonate, and release more H⁺ ions, thus lowering pH. Conversely, if pH gets too low (high [H⁺]), bicarbonate and carbonate may incorporate some of those H⁺ ions and produce bicarbonate, carbonic acid, or CO₂ to remove H⁺ ions and raise the pH. By shuttling H⁺ ions back and forth between the various compounds in this equation, the pH of the ocean is regulated and conditions remain favorable for life.

CO₂ and Ocean Acidification

In recent years there has been rising concern about the phenomenon of ocean acidification. As described in the processes above, the addition of CO₂ to seawater lowers the pH of the water. As anthropogenic sources of atmospheric CO₂ have increased since the Industrial Revolution, the oceans have been absorbing an increasing amount of CO₂, and researchers have documented a decline in ocean pH from about 8.2 to 8.1 in the last century. This may not appear to be much of a change, but remember that since pH is on a logarithmic scale, this decline represents a 30% increase in acidity. It should be noted that even at a pH of 8.1 the ocean is not actually acidic; the term “acidification” refers to the fact that the pH is becoming lower, i.e. the water is moving towards more acidic conditions.

Figure \(\PageIndex{3}\) presents data from observation stations in and around the Hawaiian Islands. As atmospheric levels of CO₂ have increased, the CO₂ content of the ocean water has also increased, leading to a reduction in seawater pH. Some models suggest that at the current rate of CO₂ addition to the atmosphere, by 2100 ocean pH may be further reduced to around 7.8, which would represent more than a 120% increase in ocean acidity since the Industrial Revolution.

Figure \(\PageIndex{3}\) Changes in atmospheric CO₂ (red), seawater CO₂ (green) and pH (blue) in the Hawaiian Islands (NOAA PMEL).

Why is this important? Declining pH can impact many biological systems. Of particular concern are organisms that secrete calcium carbonate shells or skeletons, such as corals, shellfish, and may planktonic organisms. At lower pH levels, calcium carbonate dissolves, eroding the shells and skeletons of these organisms (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{5}\) The results of an experiment placing the calcium carbonate shells of pterapods in seawater with a pH of 7.8, the projected ocean pH for the year 2100 under current rates of acidification. The top row shows the shells before the experiment, and the bottom row shows the dissolution of the shells after 45 days of exposure (NOAA).

Not only does a declining pH lead to increased rates of dissolution of calcium carbonate, it also diminishes the amount of free carbonate ions in the water. The relative proportions of the different carbon compounds in seawater is dependent on pH (Figure \(\PageIndex{6}\)). As pH declines, the amount of carbonate declines, so there is less available for organisms to incorporate into their shells and skeletons. So ocean acidification both dissolves existing shells and makes it harder for shell formation to occur.

Figure \(\PageIndex{6}\) Proportions of carbon compounds in the ocean at various pH levels. As the ocean pH declines, the proportion of carbonate ions also declines, reducing rates of shell formation (NOAA).

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Additional links for more information:

• NOAA Ocean Acidification Program website http://oceanacidification.noaa.gov/