The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 102

Section 1
Introduction
In Chapter 2 we have seen that the CO 2 in the atmosphere plays an
important role in regulating the temperature of the earth. It is therefore
important to understand the biological, geological, and chemical
mechanisms that control CO 2 levels in the atmosphere. In this chapter,
we discuss these mechanisms.
Figure 3.1.1 illustrates the various mechanisms by which carbon is
exchanged between different regions of the earth [3.1]. Photosynthesis
takes CO 2 from the atmosphere and converts the carbon into biomass.
The inverse process, the decomposition of biomass stored in the soils,
converts the carbon of the biomass into CO 2 which is subsequently
released into the atmosphere. If we look carefully at the fi gure, we see
many different mechanisms that transfer carbon from one chemical state
to another or transfer carbon from one “reservoir” to another. If we would
like to understand how CO 2 concentrations in the atmosphere are regu-
lated, we have to elucidate the interactions between the different carbon
reservoirs. For example, an increased atmospheric CO 2 concentration
will stimulate biosynthesis and hence increase the amount of biomass.
Similarly, an increase in atmospheric CO 2 will increase the uptake of CO 2
in the oceans. Quantifi cation of this carbon cycle should answer ques-
tions such as: how much of the anthropogenic CO 2 that is added to the
atmosphere will end up in biomass or in the oceans?
To answer this and other questions we will look at the earth from a
systems point of view. The idea of system analysis is to describe a cell,
organism, or a chemical factory as a system of mathematical objects that
interact with each other. This sounds abstract, but we will see that this
approach gives us interesting insights. In the case of the carbon cycle,
the fi rst step is to quantify the different reservoirs of carbon.
In Table 3.1.1 the amount of carbon in the various reservoirs is sum-
marized [3.2] and in Figure 3.1.2 these numbers are presented in a sys-
tem representation [3.3]. The difference between Table 3.1.1 and
Figure 3.1.1 is that in the latter, we did not consider the reservoir of
carbon that exists more than 20 km below the surface of the earth. If we
look at the numbers, we can already draw some important conclusions.
It is apparent that most of the carbon on earth is in the form of carbonate