The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 123

Because of the volcanic activity, all CO 2 ends up in the atmosphere of
Venus and as a result Venus experiences a very large greenhouse effect.
In this model, Venus resembles the earth in its very early existence. Mars
has a different problem. It is much smaller than the earth and because of
this the volcanic activity has stopped. As a result, Mars has no mecha-
nism for CO 2 to return to the atmosphere, and hence too little green-
house effect. Mars is thus too cold for liquid water.
Section 5
A box model for the global
carbon cycle
Let us now collect all our knowledge on the interactions within the carbon
reservoirs to discuss the global carbon cycle. This knowledge is sum-
marized in our system view ( Figure 3.5.1 ). We see that there is a nearly
perfect equilibrium between the uptake of CO 2 by the biosphere and the
release of CO 2 by the biosphere and by the decomposition of the ter-
restrial soils. Similarly for oceans, the uptake and release of CO 2 by the
surface is in equilibrium. The inorganic carbon cycle, which creates a fl ux
of only 0.15 Gt/year, is actually controlling the long-term CO 2 levels in the
atmosphere. This global carbon cycle is an accurate description of the
earth's carbon cycle until 1850.
Since 1850, humans started to use fossil fuels on a very large scale.
The current carbon cycle is shown in Figure 3.5.2. The main difference
is that by burning fossil fuels we add carbon from surface reservoirs to
the atmosphere at a current rate of 9 Gt/year. The total amount of carbon
that has been emitted since 1850 is about 500 Gt of carbon. If we look at
this number we see that the rate at which we emit is signifi cantly higher,
by a factor of 60, than the natural rate of emission from volcanos (which
is about 0.15 Gt/year). Anthropogenic emissions are depicted as the red
peak in Figure 3.1.3, and this perturbation is too large and occurs on a
time scale (100 years) too fast for natural process to remove the excess