O
OCEAN ACIDIFICATION
Ulf Riebesell
GEOMAR Helmholtz Centre for Ocean Research Kiel,
Kiel, Germany
Ocean acidification refers to the process of increasing seawater acidity by dissolving additional carbon dioxide
(CO 2 ) from the atmosphere.
As CO 2 dissolves in seawater, it forms carbonic acid
(H 2 CO 3 ), which readily dissociates into bicarbonate
(HCO 3
À ) 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 (CO 3
2À ) and forming additional bicarbonate. As pH is defined as the negative logarithm of the hydrogen ion concentration, pH decreases as
the acidity increases (Figure 1).
The increase in atmospheric CO 2 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 CO 2 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 CO 2 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).
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016
OCEAN ACIDIFICATION
Ulf Riebesell
GEOMAR Helmholtz Centre for Ocean Research Kiel,
Kiel, Germany
Ocean acidification refers to the process of increasing seawater acidity by dissolving additional carbon dioxide
(CO 2 ) from the atmosphere.
As CO 2 dissolves in seawater, it forms carbonic acid
(H 2 CO 3 ), which readily dissociates into bicarbonate
(HCO 3
À ) 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 (CO 3
2À ) and forming additional bicarbonate. As pH is defined as the negative logarithm of the hydrogen ion concentration, pH decreases as
the acidity increases (Figure 1).
The increase in atmospheric CO 2 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 CO 2 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 CO 2 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).
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016