Geoscience Reference
In-Depth Information
illustrates the modern penetration of anthropogenic
C T into the deep Atlantic Ocean and the resulting
changes in key carbonate system variables; it also
shows corresponding model projections for 2100
from the OCMIP study.
In 1994, the anthropogenic perturbation was
mostly coni ned to the upper 1000 m, except in the
North Atlantic. The modern surface perturbation in
C T averages about 50 μmol kg -1 . But by 2100 under
the IS92a scenario, that same level is projected to
penetrate generally beyond 1000 m, while twice
that level is reached in North Atlantic bottom
waters. Corresponding anthropogenic reductions
in pH T show patterns that are similar to those for
the anthropogenic C T increase, except that the pH T
perturbation appears to penetrate deeper and is
more intense in subsurface waters (typically at
about 200 m) rather than at the surface. The latter
i nding is consistent with the subsurface maximum
observed at HOT (see Section 3.4). The simulated
subsurface maximum must be due to different
chemical characteristics of the subsurface waters,
because the OCMIP models used here did not
account for climate change, which alters ocean cir-
culation. Indeed the spatial pattern of the simulated
changes in pH T is closely matched by the pattern of
modelled changes in the Revelle factor (not shown).
Changes in C T have also provoked the global-mean
depth of the ASH to shoal from its pre-industrial
level of 1090 m to 960 m in 1994. The global-average
ASH is projected to shoal to 280 m in 2100 under the
IS92a scenario.
There are large regional differences in projected
changes in saturation. For example, by 2100 under
IS92a, the ASH shoals from 180 m to the surface in
the subarctic Pacii c, from 1040 m to the surface in
the Southern Ocean, and from 2820 m to 110 m
in the North Atlantic north of 50°N. Although the
average calcite saturation horizon (CSH) in the
Southern Ocean remains below 2200 m, by 2100,
Weddell Sea surface waters become slightly under-
saturated even with respect to calcite. Under both
the IS92a and S650 scenarios, the OCMIP models
project that during this century there will be large
changes in surface and subsurface [CO 3 2- ] due to the
invasion of anthropogenic CO 2 . On longer timescales
(a few centuries), stabilization of atmospheric CO 2
at even the 450 ppmv target renders most of the
deep ocean volume corrosive to aragonite and cal-
cite (Caldeira and Wickett 2005). With a similar vol-
ume analysis but for the CSM1.4 earth system
model, Steinacher et al. (2009) found that under the
A2 scenario, waters with Ω a > 4 will disappear
within three decades, waters with Ω A > 3 will vanish
by 2070, and supersaturated waters (Ω a > 1) will
decrease from 42% during pre-industrial time to
25% by 2100 (see also Chapter 14).
These changes in saturation may affect arago-
nitic cold-water corals. Nearly all of these corals
now live in deep waters where Ω a > 1, but it is pro-
jected that by 2100 under the IS92a scenario, 70%
of them will be bathed in waters where Ω a < 1
( Guinotte et al. 2006). Whether or not acidii cation
will dramatically affect cold-water corals is an
active area of research (Maier et al. 2009 ). Other
deep-sea marine biota, which experience less vari-
ation in environmental conditions than surface
organisms, may also be affected by deep-ocean
increases in CO 2 as well as future reductions in
deep-sea oxygen, as climate change continues to
reduce ventilation of deep waters (Brewer and
Peltzer 2009 ).
3.6.6
Climate change and ocean acidii cation
In addition to the direct geochemical effect from the
CO 2 increase, ocean [CO 3 2- ] and related variables are
also altered by climate change. The OCMIP study
quantii ed the effects of climate change during this
century by analysing results from three atmos-
phere-ocean climate models, each of which included
an ocean carbon cycle module. All models illus-
trated how climate warming generally results in
increased surface-ocean [CO 3 2- ], but that increase
typically counteracts less than 10% of the decrease
from the increase in anthropogenic CO 2 ( Figs 3.3
and 3.4). Subsequent studies by McNeil and Matear
( 2006 ) and Cao et al . ( 2007 ) further coni rm that 21st-
century changes in [CO 3 2- ] due to warming are small
compared with chemical changes from invasion of
anthropogenic CO 2 .
But warming is not the only factor, particularly in
the Arctic Ocean. The i rst hint that the Arctic might
be different came from an earth system model that
projected future reductions in surface [CO 3 2- ] due to
climate change (Orr et al . 2005 ). An in-depth analysis
 
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