Encyclopedia of Marine Geosciences

Living Edition
| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede


  • Ulf RiebesellEmail author
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DOI: https://doi.org/10.1007/978-94-007-6644-0_39-6


Carbonic Acid Ocean Acidification Coralline Alga Seawater Chemistry Preindustrial Level 
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Ocean acidification refers to the process of increasing seawater acidity by dissolving additional carbon dioxide (CO2) from the atmosphere.

As CO2 dissolves in seawater, it forms carbonic acid (H2CO3), which readily dissociates into bicarbonate (HCO3 ) and hydrogen (H+) ions. The hydrogen ion concentration determines the acidity of seawater, expressed by the pH scale. Part of the hydrogen ions released in this process is buffered by the seawater carbonate system by consuming carbonate ions (CO3 2−) and forming additional bicarbonate. As pH is defined as the negative logarithm of the hydrogen ion concentration, pH decreases as the acidity increases (Fig. 1).
Fig. 1

The process of ocean acidification: (1) atmospheric carbon dioxide (CO2) dissolving in seawater; (2) dissolved CO2 reacting with water to form carbonic acid (H2CO3); (3) carbonic acid dissociating to bicarbonate (HCO3 ) and hydrogen ion (H+); and (4) hydrogen ion reacting with carbonate (CO3 2−) to form bicarbonate. As a result of ocean acidification, the concentrations of CO2, HCO3 , and H+ increase, whereas the pH and CO3 2− concentrations decrease

The increase in atmospheric CO2 concentration since preindustrial times from 280 ppm (parts per million) to ca. 400 ppm has caused a decline in surface ocean pH by 0.12 units. This corresponds to an increase in seawater acidity (hydrogen ion concentration) of 30 %. If atmospheric CO2 concentrations continue to increase at present rates, surface ocean pH values will decline by an additional 0.35 units by the year 2100 (Wolf-Gladrow et al., 1999). This corresponds to more than a doubling in seawater acidity and a decline in carbonate ion concentrations by 45 % relative to preindustrial levels. These changes in seawater chemistry are expected to widely affect marine organisms and ecosystems (Raven et al., 2005). Most prominently, calcifying organisms such as corals, bivalves, sea urchins, coralline algae, and a variety of calcareous plankton species have shown to be sensitive to ocean acidification (Gattuso and Hansson, 2011). Equally important as the magnitude of ocean acidification is its rate of change, as this determines to what extent species have the ability to adapt. The current speed of CO2 increase in the atmosphere and the corresponding rate of ocean acidification are unprecedented in Earth’s history for at least 300 million years (Hönisch et al., 2012).


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© Springer Science+Business Media Dordrecht 2015

Authors and Affiliations

  1. 1.GEOMAR Helmholtz Centre for Ocean Research KielKielGermany