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combination of models and data, indicates that such
undersaturation will occur at 635 ppmv (in 2070
under IS92a). That 85 ppmv underestimate is sub-
stantial, but the difference in the predicted timing of
undersaturation is only 20 years, simply because
atmospheric CO
2
in the IS92a scenario continues to
rise sharply through to the end of this century. With
a more conservative scenario, the difference in tim-
ing would be larger. Despite their differences, both
approaches indicate that under the IS92a scenario,
Southern Ocean surface waters will become under-
saturated with respect to aragonite during this cen-
tury (see below).
The disequilibrium approach relies on one or
more ocean models. Model projections can be used
by themselves, or they can be improved by system-
atically correcting model results for their present-
day biases. The latter approach requires high-quality
data with adequate spatial coverage. For this pur-
pose, the discrete bottle data collected in the global
CO
2
survey during the Joint Global Ocean Flux
Study (JGOFS)/WOCE era in the 1990s (Wallace
2001) has served as the fundamental reference. Key
et al.
(2004) compiled these data, quality controlled
them, and then produced a near global, three-
dimensional gridded data product (GLODAP). Two
studies have exploited those gridded data to com-
pute a baseline reference for pH and CaCO
3
satura-
tion (Caldeira and Wickett 2005; Orr
et al.
2005 ) and
then used that reference to improve future model
predictions. To that GLODAP data reference, cen-
tred around 1994, they added model-simulated
changes in
C
T
relative to the same reference year.
Both studies assumed unchanged
A
T
, then recom-
puted saturation states and pH (on the seawater
scale for Caldeira and Wickett, and on the total scale
for Orr
et al.
).
Caldeira and Wickett (2005) derived the pre-
industrial state (by subtracting GLODAP's data-
based estimates of anthropogenic
C
T
from the
GLODAP i elds for modern
C
T
) and added to that
reference state the simulated changes in
C
T
from
one model (relative to the pre-industrial reference
year). They focused on zonal-mean changes during
this century under IPCC SRES scenarios and until
2500 under stabilization scenarios and logistic func-
tions (total releases of 1250 to 20 000 Pg C). Orr
et al.
(2005) also used the modern GLODAP data as the
reference, to which they separately added
C
T
per-
turbations from a group of 10 models, each of which
participated in phase 2 of the Ocean Carbon Cycle
Model Intercomparison Project (OCMIP) (Sarmiento
et al.
2000 ; Orr
et al.
2001 ; Dutay
et al.
2002 , 2004 ;
Doney
et al.
2004 ; Matsumoto
et al.
2004 ; Najjar
et al.
2007). Some related analyses of the Orr
et al.
( 2005 )
data are presented here for the i rst time, which for
simplicity will be referred to as the OCMIP study.
That study focused on regional variations during
the 21st century, providing results as the 10-model
median ± 2
for each of two IPCC scenarios, IS92a
and S650. The IS92a scenario reaches 788 ppmv in
2100, while the S650 scenario reaches 563 ppmv in
2100 and stabilizes at 650 ppmv before 2200.
Projected atmospheric CO
2
and surface-ocean pH
from these two scenarios resemble those from IPCC
SRES scenarios A2 and B1. Variations in the data-
model correction approach have been used to
account not only for increasing
C
T
, but also the
effects of climate change by correcting for model
biases in other relevant variables (
A
T
, temperature,
salinity, PO
4
3-
, SiO
2
) in climate-change simulations
( Orr
et al.
2005 ; Steinacher
et al.
2009 ).
σ
3.6.2 Future trends in open-ocean
surface chemistry
As anthropogenic
C
T
increases in the ocean, it
causes shifts in other carbonate system variables,
eroding the ocean's capacity to absorb more
anthropogenic CO
2
from the atmosphere. That
capacity, in terms of the average rate of increase in
surface-ocean
C
T
per unit change in atmospheric
CO
2
(i.e. ∂C
T
/∂
p
CO
2
in units of μmol kg
-1
ppmv
-1
),
had already decreased in 1994 to 72% of what it
was in the pre-industrial ocean (Fig. 3.2). If atmos-
pheric CO
2
were to reach 563 ppmv then 788 ppmv,
that capacity would drop to 40% then 26% of the
pre-industrial rate. These changes are well under-
stood (Sarmiento
et al.
1995), and for more than 30
years have been accounted for implicitly in ocean
models designed to project changes in air-sea CO
2
l uxes. This feedback of ocean acidii cation on
atmospheric CO
2
remains the largest by far,
although many much smaller feedbacks have been
identii ed (see Chapter 12). Recently, attention has
also focused on projecting associated shifts in other
