Geoscience Reference
In-Depth Information
The oceans, therefore, have a leading role in climate regulation and
mitigation of global warming, and they have greatly contributed
toward slowing the increase in atmospheric CO
2
in the last 250 years.
Without the uptake of CO
2
by the oceans of 25 million tons per day
(thus 30% of anthropic CO
2
emissions since the Industrial
Revolution), the amount of CO
2
in the atmosphere would have already
reached 450 ppmv [LEQ 09, SAB 04]. If future CO
2
emissions
continue using a business-as-usual scenario, the concentration in CO
2
could reach 788 ppmv by 2100 [ORR 11].
5.1.1.
What is ocean acidification?
Ocean acidification corresponds to a decrease in the pH of the
ocean over long (10 or more years) periods of time, which is mainly
caused by the uptake of atmospheric CO
2
. It can also be caused by
other processes, generally minor on a global scale, such as the
atmospheric deposition of nitrogen and atmospheric sulfur [DON 07].
The delivery of nutrients and organic matter by rivers as well as
biogeochemical coastal processes can decrease or increase the
acidification caused by the absorption of atmospheric CO
2
[BOR 11,
CAI 11].
Ocean acidification, sometimes referred to as “the other CO
2
problem” due to its common origins with global warming, is an
expression that refers to a series of chemical modifications in seawater
elicited by the absorption of anthropogenic CO
2
by the oceans. The
dissolution of CO
2
in seawater leads to a disruption in ocean chemistry,
including a decrease of the pH and of the concentration in carbonate
ions (
CO
−
) (see the Appendix, section 5.8, and Table 5.1). The term
“ocean acidification”, coined by Broecker and Clark [BRO 01] and
popularized by Caldeira and Wickett [CAL 03], sometimes causes
misunderstandings with non-scientists. Oceans will not become acidic
(their pH will never be lower than 7), at least not in the foreseeable
future and not away from CO
2
sources of hydrothermal origin. The pH
is moving toward the acidic end on the pH scale. It has decreased by
0.1 units between 1750 and 1994 (8.2-8.1 on the total scale, Table 5.1
[ORR 11]), thus an increase in the concentration of protons, and
2
3
