Climate and Climate-System Modelling - Environmental Modelling: Finding Simplicity in Complexity - page 150

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Figure 9.2 The structure of the terrestrial biosphere model that is applied at each land grid square 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).

and photosynthetic demand. Fluxes of moisture and CO 2
between the land and atmosphere are thereby coupled in
the latest models. In this way, the terrestrial biosphere
component of the global carbon cycle has been incorpo-
rated into the latest AOGCMs. This coupling permits the
simulation of carbon fluxes to or from the atmosphere in
response to changes in the distribution of biomes (includ-
ing potential collapse of the Amazon rainforest) and due
to changes in the carbon balance of individual biomes due
to the direct physiological effects of higher atmospheric
CO 2 on photosynthesis and due to changes in climate.
Thus, some of the potential slow positive climate-carbon
cycle feedbacks can be included.

AOGCM computes ocean currents and turbulent mixing
processes that determine, in part, the distribution of
temperature and salinity in the ocean. The OGCM can
also be used to compute the distribution of dissolved
carbon, alkalinity and nutrients in the ocean. Marine
micro-organisms are critical to the distribution of carbon,
alkalinity and nutrients in the oceans, as photosynthesis
in the surface layer leads to the incorporation of dissolved
carbon into organic tissue, some of which settles to the
deep ocean, where it is released through respiration. The
rate of photosynthesis depends in part on the supply
of nutrients in the surface layer, which in turn depends
on the intensity of removal in sinking organic material
and the upwelling of nutrient-rich deep water. Some
micro-organisms build skeletal material out of calcium
carbonate (CaCO 3 ), which also settles to the deep ocean,
carrying carbon and alkalinitywith it. Figure 9.3 illustrates
a typical marine biosphere model as used in AOGCMs.
By embedding a model of the marine ecosystem in each
surface grid cell of an ocean GCM, and combining this
with calculations of the distribution of carbon, alkalinity
and nutrients in the ocean and of the partial pressure of
CO 2 in the surface water, much of the oceanic component
of the carbon cycle can be included in an AOGCM

9.2.4 Addingtheoceaniccomponentof the
carboncycle toAOGCMs

The exchange of CO 2 between the atmosphere and ocean
depends on the difference between the partial pressure of
CO 2 in the atmosphere and in the surface water that is in
contact with the atmosphere. The latter in turn depends
on the temperature, salinity, alkalinity and concentration
of total dissolved carbon (CO 2 +

CO 2 3 )in
the surface water. The ocean GCM component of an

HCO 3 − +