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and in 11% the tropics. The absolute change
∂[H
+
]/∂
p
CO
2
varies by even less (2% in the Southern
Ocean and 5% in the tropics). But both the absolute
and relative changes are higher in the Southern
Ocean than in the tropics. As atmospheric CO
2
increases from 278 to 788 ppmv, the Southern Ocean
to tropical regional ratio increases from 1.14 to 1.19
for ∂[H
+
]/∂
p
CO
2
, while it decreases from 1.13 to 1.09
for
H
-1
. These slight, opposite trends are explained
by the [H
+
]/
p
CO
2
ratio, which was identical in both
regions during pre-industrial times, but becomes
9% greater in the Southern Ocean when atmos-
pheric
p
CO
2
reaches 788 ppmv.
At any given time, one may attribute these small
regional differences in ∂[H
+
]/∂
p
CO
2
to differences
in temperature to the extent that the surface ocean
approximates a closed system, i.e. where
A
T
,
p
CO
2
,
and salinity can be considered roughly constant.
That is,
because as temperatures decline toward high lati-
tudes, the solubility of CO
2
increases and more CO
2
invades the ocean from the atmosphere (Sarmiento
and Gruber 2006). High
C
T
/
A
T
ratios are also found
in CO
2
-rich subsurface waters, which happen to be
cooler. When these waters upwell into high-latitude
regions such as the Southern Ocean, they further
reduce surface [CO
3
2-
]. These factors combine to
make a strong positive correlation between global-
scale, annual-mean surface maps of temperature and
modern [CO
3
2-
] (
R
2
= 0.92; slope of +5 μmol kg
-1
°C
-1
),
but the link with temperature is indirect. On shorter
space- and timescales, one would expect degraded
correlations of [CO
3
2-
] and
C
T
with temperature. For
instance, seasonal variations in
C
T
cannot keep up
with those of temperature, because air-sea CO
2
equilibration requires many months (see Box 3.1),
whereas heat transfer is much faster.
Increases in anthropogenic CO
2
have already
reduced modern, annual-mean surface [CO
3
2-
] by
more than 10%, relative to pre-industrial conditions
based on analysis of the GLODAP data combined
with estimates of anthropogenic
C
T
from data (Feely
et al.
2004 ; Sabine
et al.
2004 ) and models ( Orr
et al.
2005), assuming no change in
A
T
. In the OCMIP
study, when atmospheric CO
2
reached 788 ppmv in
2100 under the IS92a scenario, annual-mean surface
[CO
3
2-
] declined to levels of 149 ± 14 μmol kg
-1
in the
tropics and 55 ± 5 μmol kg
-1
in the Southern Ocean,
roughly half of pre-industrial values. The latter
level is 18% below the threshold where waters
become undersaturated with respect to aragonite
(~66 μmol kg
-1
). Thus, well before 2100, typical sur-
face waters of the Southern Ocean become corrosive
to aragonite throughout most of the year. Indeed,
Southern Ocean surface waters reach these corro-
sive conditions by 2100 under IPCC SRES scenarios
A1, A2, B1, and B2 as well as under any pathway
that stabilizes atmospheric CO
2
at 650 ppmv or
above (Caldeira and Wickett 2005). Under the same
SRES scenarios, the intermediate complexity model
from the University of Bern Physics Institute (PIUB)
shows similar results (Orr
et al.
2005 ). Additionally,
surface waters in the subarctic Pacii c become
slightly undersaturated by 2100 under the IS92a
scenario.
For comparison with [H
+
] and by analogy with
Eq. 3.7, let us evaluate the spatiotemporal variability
−
1
+
+
∂
[H ]
∂
[H ]
∂
p
CO
⎛
⎞
2
=
=
⎜
⎟
CO
∂
p
∂
T
∂
T
⎝
⎠
2
(3.9)
−
1
+
∂
[H ]
∂
ln
p
CO
1
CO
⎛
⎞
2
⎜
⎟
∂
T
∂
T
p
⎝
⎠
2
where
T
is temperature and ∂ln(
p
CO
2
)/∂
T
=
0.0423°C
-1
( Takahashi
et al.
1993). In the real ocean,
an open system, temperature still appears to act as
the dominant driver of the spatial variability of
∂[H
+
]/∂
p
CO
2
, which varies little with increasing
atmospheric CO
2
(shown above) and total alkalinity
(see Section 3.6.7). In any case, regional and tempo-
ral differences in ∂[H
+
]/∂
p
CO
2
are small, i.e. the
ocean remains on the l at part of the titration curve,
as the acid CO
2
is added and [H
+
] is buffered through
large reductions in [CO
3
2-
], our next focus.
Modern surface [CO
3
2-
] computed from the
GLODAP gridded data is naturally lower in colder
waters and higher in warmer waters (Caldeira and
Wickett 2005 ; Orr
et al.
2005). In the OCMIP study,
annual-mean [CO
3
2-
] averages varied from 105 μmol
kg
-1
for the Southern Ocean to 240 μmol kg
-1
for
tropical waters (Fig. 3.3). The polar and subpolar
oceans have naturally lower surface [CO
3
2-
] associ-
ated with their higher
C
T
/
A
T
ratios. Although varia-
tions in
A
T
are relatively small and generally follow
salinity, variations in surface
C
T
are much larger,
