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Global Atmospheric Evolution: Impact of Anthropogenic Activities
the climate (Broccoli and Marrabe, 1987). The most recent versions of such
models have been run over a period covering a glacial cycle (Gallee et AI., 1991).
The second type of model calculates the evolution of the climate over a period of
about one million years, but without taking into account the general circulation.
Such models use the Milankovitch theory: it is the insolation variations, driven by
secular variations in the earth's orbital parameters (the astronomical signal), that
cause the climatic changes.
This theory was confirmed in the seventies by the correlation between the
spectrum of the marine sediment 8 18 0 record and that of the astronomical signal
(Hays et Al., 1976). The marine sediment 8 18 0 record represents the oxygen
isotopic variations of oceanic water. Knowing that in the evaporation process the
heavy oxygen-18 isotope remains preferentially in the liquid phase, we can
conclude that the formation of big ice sheets from precipitation during glaciation
periods leaves in 18 0 an enrichment in the oceanic water isotopic composition.
That is why the marine sediment 8 18 0 record is generally considered as a first
approximation to represent the global ice volume variation record.
Since at approximately the same time, the earth's orbital parameters were precisely
recalculated (Berger, 1976; Berger, 1978), models could then be developed
because of the availability of an input astronomical signal and an output
sedimentary signal. The principal classes of such models are: (i)those based on
rock and ice rheology and elasticity (Pollard, 1983), (ii) those based on the local
energy balance (Hyde et AI., 1990), (iii) those coupling both approaches (Ghil
and Le Trent, 1981) and those of a more mathematical nature (Saltzman and
Sutera, 1984).
3 PROPOSED MODELING
The model presented here is a very simple one. It deals only with mean values and
their evolution as a function of time. This simplification allows the introduction of
new parameters in the model, such as C02 feedback and deep ocean time constant,
and the use of a closed loop version of the model.
Doing a preliminary energy balance, one can see that the energy stored, for each
square meter of the planet, between a glacial and an interglacial period, is around
30 Giga joules for the ice crystallization latent heat, and 10 Giga joules per degree
Celsius in the oceans. These values have to be compared with the variations of
incident energy caused by secular variations of earth orbit that are of the order of
10 Giga joules for 1000 years. This energy balance shows the importance of such
considerations, the excess energy received during a few thousand years being
necessary to prepare the climatic system to be rocked from the glacial to the
interglacial state, and it will be the basis of the model presented in the next section.
The incident energy Ei is absorbed by the earth and the energy Er is emitted
according to the black body law. The difference between these two energies E is
used by the climatic system. This energy E modifies the surface temperature
(atmosphere, superficial layers of oceans and continents) and is modeled by an
integrator of capacity C3. This energy can then be stored in the two main energetic
reservoirs that are the ice sheets and the oceans (C1 and C2).
Global Atmospheric Evolution: Impact of Anthropogenic Activities
the climate (Broccoli and Marrabe, 1987). The most recent versions of such
models have been run over a period covering a glacial cycle (Gallee et AI., 1991).
The second type of model calculates the evolution of the climate over a period of
about one million years, but without taking into account the general circulation.
Such models use the Milankovitch theory: it is the insolation variations, driven by
secular variations in the earth's orbital parameters (the astronomical signal), that
cause the climatic changes.
This theory was confirmed in the seventies by the correlation between the
spectrum of the marine sediment 8 18 0 record and that of the astronomical signal
(Hays et Al., 1976). The marine sediment 8 18 0 record represents the oxygen
isotopic variations of oceanic water. Knowing that in the evaporation process the
heavy oxygen-18 isotope remains preferentially in the liquid phase, we can
conclude that the formation of big ice sheets from precipitation during glaciation
periods leaves in 18 0 an enrichment in the oceanic water isotopic composition.
That is why the marine sediment 8 18 0 record is generally considered as a first
approximation to represent the global ice volume variation record.
Since at approximately the same time, the earth's orbital parameters were precisely
recalculated (Berger, 1976; Berger, 1978), models could then be developed
because of the availability of an input astronomical signal and an output
sedimentary signal. The principal classes of such models are: (i)those based on
rock and ice rheology and elasticity (Pollard, 1983), (ii) those based on the local
energy balance (Hyde et AI., 1990), (iii) those coupling both approaches (Ghil
and Le Trent, 1981) and those of a more mathematical nature (Saltzman and
Sutera, 1984).
3 PROPOSED MODELING
The model presented here is a very simple one. It deals only with mean values and
their evolution as a function of time. This simplification allows the introduction of
new parameters in the model, such as C02 feedback and deep ocean time constant,
and the use of a closed loop version of the model.
Doing a preliminary energy balance, one can see that the energy stored, for each
square meter of the planet, between a glacial and an interglacial period, is around
30 Giga joules for the ice crystallization latent heat, and 10 Giga joules per degree
Celsius in the oceans. These values have to be compared with the variations of
incident energy caused by secular variations of earth orbit that are of the order of
10 Giga joules for 1000 years. This energy balance shows the importance of such
considerations, the excess energy received during a few thousand years being
necessary to prepare the climatic system to be rocked from the glacial to the
interglacial state, and it will be the basis of the model presented in the next section.
The incident energy Ei is absorbed by the earth and the energy Er is emitted
according to the black body law. The difference between these two energies E is
used by the climatic system. This energy E modifies the surface temperature
(atmosphere, superficial layers of oceans and continents) and is modeled by an
integrator of capacity C3. This energy can then be stored in the two main energetic
reservoirs that are the ice sheets and the oceans (C1 and C2).