Geoscience Reference
In-Depth Information
of [CO
3
2-
] in terms of the 'inverse saturation factor'
(
S
-1
), i.e. the ratio of the relative change of [CO
3
2-
] to
that of
p
CO
2
:
the increase in
C
T
, as required by the overall reduc-
tion in [CO
3
2-
] and the dei nition of total inorganic
carbon (see also Eq. 3.3). Relative to average changes
in tropical surface waters, changes in [CO
2
] in the
Southern Ocean are 2.4 times larger due to enhanced
CO
2
solubility (
K
0
), which increases
C
T
and drives
[CO
3
2-
] downward. Polar and subpolar regions also
have lower buffer capacities (higher Revelle factors)
associated with lower [CO
3
2-
]. In the high latitudes,
although absolute changes in [CO
3
2-
] are smallest,
changes relative to the pre-industrial level are the
largest.
Let us now consider all these changes in terms of
the most common denominator,
C
T
. At 1994 condi-
tions, for every μmol kg
-1
increase in tropical
C
T
, 0.65
μmol kg
-1
of [CO
3
2-
] is consumed while 1.60 μmol kg
-1
of [ HCO
3
-
] is produced. These numbers deviate from
the stoichiometric coefi cients in Eq. 3.1. Simul-
taneously, there is only a small increase in [CO
2
] of
0.044 μmol kg
-1
and a minute increase in [H
+
] of 0.03
× 10
-3
μmol kg
-1
(pH
T
declines by 0.0013 pH units),
illustrating the remarkable effectiveness of the seawa-
ter buffering system. At 788 ppmv, consumption of
[CO
3
2-
] and production of [HCO
3
-
] are at 94% of the
1994 values, while changes in [CO
2
], [H
+
], and pH
T
are about 2.5 times higher. In the Southern Ocean in
1994, consumption of [CO
3
2-
] and production of
[HCO
3
-
] are 88% of tropical values in the same year,
but the increase in [CO
2
] is threefold greater and 1.5
times more [H
+
] is produced . By 788 ppmv, Southern
Ocean consumption of [CO
3
2-
] and production of
[HCO
3
-
] decline to about 72% of 1994 values, while
changes in [CO
2
], [H
+
], and pH
T
are 2.7 times greater.
These assessments illustrate the fundamental
nature of the GLODAP data for recent evaluations
of how ocean acidii cation is affecting carbonate
chemistry and pH. Yet gaps remain. GLODAP is
based on 'one-time' survey data that do not cover
some key areas, including the Arctic Ocean, mar-
ginal seas, and coastal zones. Nor does it account
for seasonal variations. These concerns are
addressed below.
2
−
2
−
ln[CO
2
−
]
γ
∂
[CO
]/[CO
]
∂
C
S
−
1
=
3
3
=
3
=
(3.10)
T
∂
p
CO /
p
CO
∂
ln
p
CO
ω
2
2
2
C
T
γ
C
T
where
is
dei ned
above
and
is another buffer factor
derived by Egleston
et al.
(2010). It follows that the
corresponding absolute change with respect to
p
CO
2
is
ω=∂
(ln[CO]/ )
2
−
∂
C
−
1
C
3
T
T
∂
[CO
2
−
]
γ
[CO
2
−
]
3
=
DIC
3
(3.11)
∂
p
CO
ω
DIC
p
CO
2
2
As atmospheric CO
2
increases from 278 to 788 ppmv,
the relative rate of change
S
-1
increases by 32% in the
tropics and 17% in the Southern Ocean, while it
remains 38% to 22% higher in the latter region rela-
tive to the former (Fig. 3.2). In contrast, the absolute
change
2
3
−
declines sharply with
increasing atmospheric CO
2
. Relative to pre-indus-
trial values of
∂
[CO
]/
∂
p
CO
2
2
3
−
, those at 788 ppmv
are more than three times lower in the tropics and
i ve times lower in the Southern Ocean. Thus tem-
poral changes in
∂
[CO
]/
∂
p
CO
2
2
3
−
are much larger
than those for ∂[H
+
]/∂
p
CO
2
, which was shown above
to decline by at most 6% during the same increase in
atmospheric CO
2
. Regional differences are also about
three times larger for
∂
[CO
]/
∂
p
CO
2
2
3
−
. By far, the
dominant factor controlling spatiotemporal variabil-
ity in the absolute change
∂
[CO
]/
∂
p
CO
2
2
3
−
∂
[CO
]/
∂
p
CO
is the
2
p
−
ratio, which is reduced in all regions
by about i vefold as atmospheric CO
2
increases from
278 to 788 ppmv, with tropical ratios about twice
those in the Southern Ocean.
Changes in [CO
3
2-
] are also closely tied to changes
in other carbonate chemistry variables (Fig. 3.4).
The increase in anthropogenic
C
T
is largest in the
warm tropical waters, where lower
C
T
/
A
T
ratios
render these waters more chemically suitable to
taking up anthropogenic CO
2
. This high chemical
capacity for taking up anthropogenic CO
2
is linked
to its high [CO
3
2-
] (Fig. 3.3) and thus high buffer
capacity and low Revelle factor (Fig. 3.4).
Everywhere, the increase in [HCO
3
-
] is larger than
2
3
[CO
]/ CO
2
3.6.3
Future trends in the Arctic Ocean
Recent studies project that undersaturation in the
Arctic will occur sooner and be more intense than in
the Southern Ocean. A tenth of Arctic surface waters
