Environmental Engineering Reference
In-Depth Information
Arctic Ocean Acidiication
When carbon dioxide dissolves in seawater, it forms carbonic acid (H
2
CO
3
), which dissoci-
ates, releasing hydrogen ions and bicarbonate:
CO
2
+ H
2
O
↔
H
2
CO
3
H
2
CO
3
↔
H
+
+ HCO
3
−
Although this process enables more carbon dioxide to dissolve into the sea from the at-
mosphere, it also sets in motion very important changes in seawater acid-base chemistry that
are collectively known as
ocean acidification
. This does not mean the ocean is becoming
acidic; it is becoming less alkaline.
The concentration ofhydrogen ions in a solution is measured according to the pHscale,
where
pH = −log [H
+
]
Therefore, the more hydrogen ions, the lower the pH. Notice that it is a log scale to the
base 10. Every unit decrease on the pH scale means that the hydrogen ion concentration has
increased tenfold. Solutions with a pH of less than seven are acidic, while those with a pH
greater than seven are alkaline.
Until recent times, global mean seawater had a pH of 8.16. However, we now have ex-
cess carbon dioxide dissolving into the ocean surface waters, which increases the concentra-
tion of hydrogen ions. This process is estimated to have led to an increase of global ocean
acidity of about pH 0.1 since the Industrial Revolution. That is the equivalent of an increase
in hydrogen ion concentration of 25%. It is actually the rate of change that is of prime con-
cern, given our knowledge of the amount of carbon dioxide known to be entering into the
atmosphere and “waiting” to enter the oceans. It is thought that the oceans may have nev-
er undergone such a rapid acidification in the past. The IPCC estimated in the AR5: “Earth
System Models project a global increase in ocean acidification for all RCP scenarios. The
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