Geoscience Reference
In-Depth Information
Table 3.2
Annual-mean surface pH
T
(0-10 m) averaged
†
over the GLODAP domain
‡
Time (atmospheric CO
2
)
pH
T
pH
T
change
Pre-industrial
a
(278 ppmv)
8.18
-
1994
b
(360 ppmv)
8.10
-0.08
2050
c
(IS92a, 563 ppmv)
7.95
-0.23
2100
c
(IS92a, 788 ppmv)
7.82
-0.36
†
Averages given as area-weighted means of pH
T
. Identical results are found when pH
T
is i rst converted to [H
+
], then aver-
aged and reconverted back to pH
T
.
‡
The near-global GLODAP domain excludes the Arctic Ocean, Indonesian seas, and most other marginal seas.
a
Pre-industrial pH
T
was recomputed from the GLODAP data after subtracting data-based estimates for anthropogenic
C
T
( Sabine et al. , 2004 ; Key et al. , 2004 ).
b
The 1994 average is based on the GLODAP data.
c
The estimates at 563 ppmv and 788 ppmv (nominal years 2050 and 2100 under IS92a) are the medians of the models
from the OCMIP study.
carbonate system variables, including pH and
[CO
3
2-
], and how they vary regionally as atmos-
pheric CO
2
continues to increase.
Table 3.2 shows the annual-mean pH
T
for the
modern ocean based on the 1994 GLODAP data, as
well as pre-industrial estimates and future projec-
tions. The average change in surface-ocean pH
T
,
relative to the pre-industrial state, has reached
already about -0.1. That change could nearly quad-
ruple by the end of the century under the IS92a or
A2 scenarios. To what extent do these changes in
surface-ocean pH differ regionally, given the large
regional variability for uptake and storage of
anthropogenic CO
2
( Sarmiento
et al.
1992 ; Orr
et al.
2001 ; Sabine
et al.
2004 )?
Reductions in zonal, annual-mean pH
T
relative to
the pre-industrial distribution vary from 0.33 in the
tropics to 0.39 in the Southern Ocean for 2100 under
the IS92a scenario (Fig. 3.3). Although pre-industrial
[H
+
] in the Southern Ocean is on average 13% lower
(pH
T
= 8.20) than in the tropics (pH
T
= 8.14), the
greater anthropogenic [H
+
] increase in the Southern
Ocean causes both regions to have the same aver-
age pH
T
of 7.81 when atmospheric CO
2
reaches 788
ppmv. The anthropogenic increase in surface [H
+
] is
smaller in the tropics, because carbonate-rich sur-
face waters provide greater chemical capacity to
take up anthropogenic CO
2
and buffer those changes
(they have a lower Revelle factor). Greater carbon-
ate concentrations mean that more carbonate is
available to be consumed, neutralizing more of the
incoming excess CO
2
and producing less H
+
.
Relative to the Southern Ocean, the tropics have an
average anthropogenic
C
T
increase that is 40%
greater, whereas the anthropogenic increase in trop-
ical [H
+
] is 12% less. This greater tropical buffer-
ing necessitates 72% greater [CO
3
2-
] consumption
(-120 μmol kg
-1
) relative to the Southern Ocean
(-70 μmol kg
-1
).
To better understand regional differences in pH,
let us separate the rates at which [H
+
] changes with
respect to changes in
p
CO
2
and
C
T
:
+
+
CO
∂
[H ]
∂
[H ]
∂
p
2
.
=
(3.6)
CO
∂
Cp
∂
∂
C
T
2
T
We may expect that spatiotemporal variability in
∂[H
+
]/∂
C
T
is driven largely by the ∂pCO
2
/∂
C
T
term,
which was shown above to vary greatly. But our
goal here is to quantify how [H
+
] is directly affected
by the increase in
p
CO
2
. To what degree does the
less familiar term ∂[H
+
]/∂
p
CO
2
vary, and what
drives that variability? Analogous to the familiar
Revelle factor, one can frame these questions in
terms of the 'relative change' of
p
CO
2
to that of [H
+
].
Omta
et al.
(2010) denote that ratio as
H
and con-
sider it to be constant. Here its variability is quanti-
i ed to explain the regional differences in
anthropogenic pH changes mentioned above.
Because the increase in
p
CO
2
is the driver, let us
focus on the inverse of
H
:
+
+
+
g
b
∂
[H ]/[H ]
∂
ln[H ]
1
(3.7)
−
C
H
=
=
=
.
T
∂
pCO /pCO
∂
ln pCO
2
2
2
C
T
