Geoscience Reference
In-Depth Information
by Steinacher
et al
. ( 2009 ) using another earth sys-
tem model found that Arctic climate-induced reduc-
tions in surface carbonate also exacerbated the
decline in carbonate from the invasion of anthropo-
genic CO
2
. Their analysis revealed that reductions
in [CO
3
2-
] from increased freshwater input (from
sea-ice melt, more precipitation, and less evapora-
tion) dominated increases from warming and
increased primary production. Furthermore, Arctic
warming led to reduced summer sea-ice cover and
thus greater invasion of anthropogenic CO
2
, which
reduced [CO
3
2-
] further. Overall, Steinacher
et al
.
(2009) estimated that the net effect of climate change
was to enhance the reduction of surface [CO
3
2-
] in
the Arctic by 34% by 2100.
More details concerning the magnitude of the
effects of climate change on ocean pH and CaCO
3
saturation states can be found in modelling studies
from Frölicher and Joos (2010) and those detailed by
Joos
et al
. in Chapter 14, with scenarios that go
beyond the end of this century, to 2500.
varies widely between these marginal seas, inl u-
ences the rate of acidii cation.
The acidii cation of the marginal seas surround-
ing Europe is just starting to be investigated, with
some initial studies in the Mediterranean Sea in
terms of uptake and storage of CO
2
( Touratier and
Goyet 2009 ; Louanchi
et al.
2009 ). No projections
have been made as to how the acidii cation rates of
these seas may differ during this century. Yet it has
been suggested that the relatively high
A
T
of the
Mediterranean Sea, which drives greater future
uptake of anthropogenic CO
2
relative to the open
ocean (Touratier and Goyet 2009), thereby implies a
greater future reduction in pH (Yilmaz
et al.
2008 ).
Let us examine this suggestion.
The basic equilibrium calculations made here do
show that the average surface pH
T
of the Black Sea
is substantially higher than that of the Baltic and
Mediterranean Seas. Indeed, differences in surface
pH
T
between these seas are largely explained by dif-
ferences in carbonate ion concentrations. However,
the projected absolute change in surface pH
T
over
the 21st century is very similar between the global-
ocean average and all these seas (Fig. 3.6). For
instance under the A2 scenario, the absolute change
in pH
T
over the 21st century for the Black Sea is
identical to that in the global ocean, for the
Mediterranean Sea it is 3% less, and for the Baltic it
is 9% more. Except for the Baltic, these regional dif-
ferences are smaller than 'seasonal' differences in
the absolute change for a given sea (i.e. the differ-
ence in absolute changes computed with summer
versus winter temperatures). Under winter condi-
tions, the absolute change in pH
T
is 5-7% greater
than under summer conditions.
For a greater understanding, let us consider these
changes in terms of ∂[H
+
]/∂
p
CO
2
, which was shown
in Section 3.6.1 to vary by only up to 6% with
increasing atmospheric CO
2
(278 to 788 ppmv) and
by about 15% across the full range of ocean temper-
atures. This marginal-sea comparison adds another
dimension to our understanding of ∂[H
+
]/∂
p
CO
2
.
That is, it does not vary substantially even across
the large range of
A
T
found in the global ocean,
Black Sea, and Mediterranean Sea. Conversely, in
the low-salinity, low-alkalinity Baltic Sea,
∂[H
+
]/∂
p
CO
2
is about 10% larger. This offset for the
Baltic Sea, although small, appears linked to its
3.6.7
Marginal seas and the coastal ocean
Besides the Arctic Ocean, global projections of
future ocean acidii cation have left out most mar-
ginal seas, including the Baltic, Mediterranean,
and Black Seas. As a i rst step, let us estimate how
21st-century acidii cation of these marginal seas
may differ from that of the global ocean by using
thermodynamic constants and assuming equilib-
rium between atmospheric and oceanic
p
CO
2
at
the chemical and hydrographic conditions typical
of each sea. Twenty-i rst century pH and satura-
tion states were computed for each sea by adopt-
ing typical local conditions for
A
T
and salinity at
winter and summer temperatures, i xing these
variables, and incrementing atmospheric CO
2
each
year following IPCC SRES scenarios B1 and A2
(Fig. 3.6). This simple equilibrium approach may
produce biased results in areas of deep-water for-
mation (e.g. the Gulf of Lions, Adriatic Sea) and in
the low-salinity, low-alkalinity waters of the Baltic
Sea, where [Ca
2+
] may not follow the open-ocean
proportionality to salinity and the role of carbon-
ate as the dominant base may be diminished.
Nonetheless, this new analysis clearly illustrates
the extent to which total alkalinity, which
