Intergovernmental Panel on Climate Change (IPCC ... - ipcc-wg3

282 IPCC Special Report on Carbon dioxide Capture and Storage
so much CO 2
because of its large volume and because CO 2
dissolves in sea water to form various ionic species (Box 6.1).
The increase in atmospheric CO 2
over the past few centuries
has been driving CO 2
from the atmosphere into the oceans.
The oceans serve as an important sink of CO 2
emitted to the
atmosphere taking up on average about 7 GtCO 2
yr -1 (2 GtC yr -1 )
over the 20 years from 1980 to 2000 with ocean uptake over the
past 200 years estimated to be > 500 GtCO 2
(135 GtC) (Prentice
et al., 2001; Sabine et al., 2004). On average, the anthropogenic
CO 2
signal is detectable to about 1000 m depth; its near absence
in the deep ocean is due to the slow exchange between ocean
surface and deep –sea waters.
Ocean uptake of anthropogenic CO 2
has led to a perturbation
of the chemical environment primarily in ocean surface waters.
Increasing ocean CO 2
concentration leads to decreasing carbonate
ion concentration and increasing hydrogen ion activity (Box
6.1). The increase in atmospheric CO 2
from about 280 ppm in
1800 to 380 ppm in 2004 has caused an average decrease across
the surface of the oceans of about 0.1 pH units (∆pH ≈ –0.1)
from an initial average surface ocean pH of about 8.2. Further
increase in atmospheric CO 2
will result in a further change in
the chemistry of ocean surface waters that will eventually reach
the deep ocean (Figure 6.4). The anthropogenic perturbation of
ocean chemistry is greatest in the upper ocean where biological
activity is high.
Most carbon dioxide released to either the atmosphere or the
ocean will eventually reside in the ocean, as ocean chemistry
equilibrates with the atmosphere. Thus, stabilization of
atmospheric CO 2
concentration at levels above the natural level
of 280 ppm implies long-term addition of carbon dioxide to
the ocean. In equilibrium, the fraction of an increment of CO 2
released that will reside in the ocean depends on the atmospheric
CO 2
concentration (Table 6.1; Figure 6.3; Kheshgi et al., 2005;
Kheshgi, 2004a).
The capacity of the oceans to absorb CO 2
in equilibrium
with the atmosphere is a function of the chemistry of sea water.
The rate at which this capacity can be brought into play is a
function of the rate of ocean mixing. Over time scales of decades
to centuries, exchange of dissolved inorganic carbon between
ocean surface waters and the deep ocean is the primary barrier
limiting the rate of ocean uptake of increased atmospheric CO 2
.
Over many centuries (Kheshgi, 2004a), changes in dissolved
inorganic carbon will mix throughout the ocean volume with
the oceans containing most of the cumulative CO 2
emissions
to the atmosphere/ocean system (Table 6.1; Figure 6.3). Over
longer times (millennia), dissolution of CaCO 3
causes an even
greater fraction of released CO 2
(85–92%) to reside in the ocean
(Archer et al., 1997).
Both biological and physical processes lead to the observed
distribution of pH and its variability in the world ocean (Figure
6.6). As they transit from the Atlantic to Pacific Basins, deep
ocean waters accumulate about 10% more dissolved inorganic
carbon dioxide, primarily from the oxidation of sinking organic
matter (Figure 6.7).
6.2 Approaches to release of CO 2
into the ocean
6.2.1 Approaches to releasing CO 2
that has been
captured, compressed, and transported into the
ocean
Figure 6.4 Simulated ocean pH changes from CO 2
release to the
atmosphere. Modelled atmospheric CO 2
change and horizontally
averaged ∆pH driven by a CO 2
emissions scenario: historic atmospheric
CO 2
up to 2000, IS92a from 2000 to 2100, and logistic curve extending
beyond 2100 with 18,000 GtCO 2
(Moomaw et al., 2001) cumulative
emissions from 2000 onward (comparable to estimates of fossil-fuel
resources – predominantly coal; Caldeira and Wickett, 2003). Since
year 1800, the pH of the surface of the oceans has decreased about
0.1 pH units (from an initial average surface ocean pH of about 8.2)
and CO 3
2–
has decreased about 40 µmol kg –1 . There are a number of
pH scales used by ocean chemists and biologists to characterize the
hydrogen ion content of sea water, but ∆pH computed on different
scales varies little from scale to scale (Brewer et al., 1995).
6.2.1.1 Basic approach
The basic concept of intentional CO 2
storage in the ocean is to
take a stream of CO 2
that has been captured and compressed
(Chapter 3), and transport it (Chapter 4) to the deep ocean for
release at or above the sea floor. (Other ocean storage approaches
are discussed in Sections 6.2.2 and 6.2.3.) Once released, the
CO 2
would dissolve into the surrounding sea water, disperse
and become part of the ocean carbon cycle.
Marchetti (1977) first proposed injecting liquefied CO 2
into
the waters flowing over the Mediterranean sill into the middepth
North Atlantic, where the CO 2
would be isolated from
the atmosphere for centuries. This concept relies on the slow
exchange of deep ocean waters with the surface to isolate CO 2
from the atmosphere. The effectiveness of ocean storage will
depend on how long CO 2
remains isolated from the atmosphere.
Over the centuries and millennia, CO 2
released to the deep
ocean will mix throughout the oceans and affect atmospheric
CO 2
concentration. The object is to transfer the CO 2
to deep
waters because the degree of isolation from the atmosphere
generally increases with depth in the ocean. Proposed methods