Geoscience Reference
In-Depth Information
Chapter 3 ). Egleston
et al.
( 2010 ) have shown that
the absorption of CO
2
from the atmosphere will be
less efi cient and that [CO
2
], [H
+
], and saturation
states will be more sensitive to environmental
changes. For example, careful examination of the
buffer factor ∏
D
( Frankignoulle 1994 ) indicates that
the pH changes generated by CO
2
uptake will be
magnii ed as
C
T
increases (a 30% increase between
1766 and 2100; see also Chapter 3).
The deposition of compounds from the atmos-
phere can also alter the pH of surface waters. For
example, 'acid rain' resulting from the deposition of
sulphur and nitrogen compounds led to widespread
acidii cation of surface freshwaters in the early
1970s (e.g. Wright 2003) and to signii cant biotic
responses (Hendrey 1984). Alterations of the chem-
istry of the surface ocean due to anthropogenic
nitrogen and sulphur deposition represent only a
small fraction of the acidii cation resulting from the
uptake of anthropogenic CO
2
( Doney
et al.
2007 ).
However, the impacts can be much more substan-
tial in some areas of the coastal ocean, of the order
of 10-50% or more of the anthropogenic CO
2
-driven
changes near the major source regions and in mar-
ginal seas.
Future decrease of seawater pH can result either
from the passive uptake of atmospheric CO
2
as
described above or from purposeful dumping of
liquid CO
2
into the deep ocean (for CO
2
disposal).
Several studies have investigated the impact of the
very low pH (below 7) that such a dumping would
induce on various functions and organisms (e.g.
Bibby
et al.
2007 ; Widdicombe and Needham 2007 ).
Only the changes in the carbonate chemistry gener-
ated by the passive uptake of atmospheric CO
2
are
addressed in the present chapter.
The recent changes in the carbonate chemistry
inferred from modelling studies can actually be
measured. In the past decades, pH decreased by
0.0017 to 0.0024 units per year, depending on
location and whether the whole year or only win-
ter data are considered (Bates and Peters 2007;
Santana-Casiano
et al.
2007; Dore
et al.
2009 ;
Olafsson
et al.
2009 ; see Chapter 3 ). Another impor-
tant outcome of ocean acidii cation for calcifying
organisms and the global calcium carbonate cycle
Table 1.1
Average changes in the carbonate chemistry of surface seawater from 1766 to 2100 (Gattuso and Lavigne 2009). Total alkalin-
ity, CO
2
partial pressure ( p CO
2
), salinity, and temperature were i xed and used to derive all other parameters using the seacarb software
(Lavigne and Gattuso 2010) and the dissociation constant of carbonic acid of Lueker et al. (2000). It is assumed that the ocean and atmos-
phere are in equilibrium with respect to CO
2
. Values of temperature, salinity, total alkalinity, and total phosphate in 1766, 2007, and 2100
are from Plattner et al. ( 2001 ) prescribing historical CO
2
records and non-CO
2
radiative forcing from 1766 to 1990 and using the A2 IPCC
SRES emissions scenario (Nakic´enovic´ et al. 2000 ) thereafter. p CO
2
in 1766 and 2100 are from Plattner et al. (2001), while values for 2007
are from Keeling et al. (2008). The concentration of total silicate was calculated using the gridded data reported by Garcia et al. ( 2006 )
between 0 and 10 m and weighing the averages using the surface area of each grid cell. pH is expressed on the total scale.
Parameter
Unit
LGM
1766
2007
2100
o
C
Temperature
17.2
18.3
18.9
21.4
Salinity
36
34.9
34.9
34.7
10
-6
mol kg
-1
Total phosphate
0.66
0.66
0.63
0.55
10
-6
mol kg
-1
Total silicate
7.35
7.35
7.35
7.35
10
-6
mol kg
-1
Total alkalinity
2399
2326
2325
2310
CO
2
partial pressure (seawater)
μatm
180
267
384
793
10
-6
mol kg
-1
[CO
2
]
6.26
9.05
12.8
24.7
[HCO
3
-
]
10
-6
mol kg
-1
1660
1754
1865
2020
[CO
3
2-
]
10
-6
mol kg
-1
299
231
186
118
10
-6
mol kg
-1
Dissolved inorganic carbon
1966
1994
2064
2162
pH
T
8.33
8.2
8.07
7.79
[H
+
]
10
-9
mol kg
-1
4.589
6.379
8.600
16.13
Calcite saturation
—
7.1
5.5
4.5
2.8
Aragonite saturation
—
4.6
3.6
2.9
1.8
LGM, Last Glacial Maximum.
