Biology Reference
In-Depth Information
Table 19.1
Average changes to ocean chemistry and pH based on ocean carbon models and surface-
ocean measurements (modified from The Royal Society
2005
)
Preindustrial Today
2
preindustrial 3
preindustrial 4
preindustrial
CO
2atm
(ppm)
280
380
560
840
1,120
mol kg
1
)
H
2
CO
3
(
m
9
13
19
28
38
mol kg
1
) 1,768
HCO
3
¯(
m
1,867
1,976
2,070
2,123
CO
3
2
¯(
mol kg
1
)
m
225
185
141
103
81
Total DIC
(
2,003
2,065
2,136
2,201
2,242
mol kg
1
)
m
Mean surface pH
8.18
8.07
7.92
7.77
7.65
Ω
calcite
5.3
4.4
3.3
2.4
1.9
Ω
aragonite
3.4
2.8
2.1
1.6
1.2
et al.
2009
), and the seawater carbonate chemistry is governed by a series of
chemical reactions that are in equilibrium (see also Chap.
2
by G´mez and
Huovinen and Chap.
4
by Gordillo):
CO
2
ð
atoms
Þ
$
CO
2
ð
aq
Þ
þ
H
2
O
$
H
2
CO
3
$
H
þ
þ
HCO
3
$
2H
þ
þ
CO
2
3
CO
2
reacts with H
2
O causing the equilibrium to shift such that the concentration
of the bicarbonate ion (HCO
3
¯) and carbonic acid (H
2
CO
3
) increases, while that of
carbonate ions (CO
3
2
¯) declines. The increase in hydrogen ion concentration lowers
pH because pH
-log
10
[H
+
]. The mean surface ocean pH has already decreased
by 0.1 unit (equivalent to a 30% increase in H
+
) from ~8.18 at the beginning of the
industrial revolution. For surface seawater with pH of ~8.07, approximately 91% of
the inorganic carbon is HCO
3
¯, 8% CO
3
2
¯, and only 1% CO
2(aq)
. By 2100,
concentrations of CO
2(aq)
and HCO
3
¯ are predicted to increase by 192% and 14%,
respectively, and CO
3
2
¯ decrease by 56%, with a concomitant decline in pH to 7.65
(The Royal Society
2005
). Aside from the decline in carbonate concentration, a
reduction in the saturation state of aragonite and calcite is also predicted
(Table
19.1
). As the seawater saturation state for a mineral measures its thermody-
namic potential to form or to dissolve, a reduction will make calcified structures
more vulnerable to erosion.
Carbonate ions play an important role in the formation of calcareous “skeletons”
for a large number of phototrophic marine organisms including corals, phytoplank-
ton, and macroalgae. If mitigation of emissions does not occur, CO
2
concentrations
could reach
¼
800 ppm by 2100 that could result in a decrease of ~0.4 pH units.
Under a worst-case scenario of continued and unabated usage of known fossil fuel
reserves, modeling indicates that average surface ocean pH could fall by a maxi-
mum of 0.77 pH units by 2300 (Caldeira and Wickett
2003
). Marine organisms
would then experience the lowest pH level for ~20 million years and the most rapid
rate of decline in pH ever (Turley
2008
). While the OA event experienced 55
million years ago during the Paleocene-Eocene thermal maximum probably
occurred over several thousand years, the present-day anthropogenically induced
event is happening over decades to centuries (Zachos et al.
2005
).
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