Environmental Engineering Reference
In-Depth Information
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Figure 9.3
The structure of the marine ecosystem model that is applied at each ocean surface layer grid cell in the Canadian Climate
Centre AOGCM (Reproduced with permission from Arora, V.K., Boer, G.J., Christian, J.R.,
et al
. (2009). The effect of terrestrial
photosynthesis down regulation on the twentieth century carbon budget simulated with the CCCma Earth System Model.
Journal of
Climate
, 22, 6066-88.
American Meteorological Society).
(this description omits a sub-module for the build-up or
dissolution of carbonate-rich sediments on the sea floor,
which is relevant at times scales of 1000 years and longer).
The combination of photosynthesis in the surface layer
and downward settling of carbon and its release at depth is
referred to as the biological pump. By transferring carbon
to the deeper waters, a much lower CO
2
partial pressure
is maintained in the ocean surface layer, which in turn
produces a much lower atmospheric CO
2
concentration
than would otherwise be the case (namely, a preindustrial
concentration of about 280 ppmv rather than 450 ppmv
or higher in the absence of the biological pump).
Coupled AOGCMs with the carbon-cycle components
embedded in the ocean GCM can simulate the absorption
of anthropogenic CO
2
by the oceans in response to
increasing atmospheric CO
2
concentration, as well as the
impact of change in climate on the oceanic absorption or
release of CO
2
. The biological pump, although crucial to
the present-day distribution of carbon in the oceans and
to the pre-industrial atmospheric CO
2
concentration,
plays no role in the absorption of the additional CO
2
added to the atmosphere by humans unless its strength
changes for some reason. Rather, the oceanic absorption
of CO
2
occurs through the inorganic processes of air-sea
gas exchange and the generally slow downward mixing of
carbon-rich water. However, the biological pump is likely
to change as the climate itself changes, both due to changes
in ocean circulation altering the supply of nutrient to
the surface layer and due to changes in marine ecology
(and hence in the proportions and vitality of different
kinds of micro-organisms). These changes constitute a
climate-carbon cycle feedback. Another climate-carbon
cycle feedback arises through the decrease in the solubility
of CO
2
as the surface layer of the ocean warms, causing an
outgassing of CO
2
that is estimated to be about 10%of the
oceanic absorption of CO
2
that would otherwise occur
(i.e. an outgassing of about 0.2 Gt C yr
−
1
compared to a
background absorption of about 2 Gt C yr
−
1
at present)
(Goodwin and Lenton, 2009).
9.2.5 Modelsof atmosphericchemistryand
aerosols
Atmospheric chemistry is central to the distribution and
amount of ozone in the atmosphere. The dominant chem-
ical reactions and sensitivities are significantly different for
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