36 The greenhousegases
This process, whose contribution to the carbon cycle is known as the
biological pump(see box), was important in determining the changes
of carbon dioxide concentration in both the atmosphere and the ocean
during the ice ages (see Chapter 4).
Computer models – which calculate solutions for the mathemati-
cal equations describing a given physical situation, in order to predict
its behaviour (see Chapter 5) – have been set up to describe in detail
the exchanges of carbon between the atmosphere and different parts of
the ocean. To test the validity of these models, they have also been ap-
plied to the dispersal in the ocean of the carbon isotope^14 C that entered
the ocean after the nuclear tests of the 1950s; the models simulate this
dispersal quite well. From the model results, it is estimated that about
2Gt(±0.8 Gt) of the carbon dioxide added to the atmosphere each year
ends up in the oceans (Table 3.1 and Figure 3.3). Observations of the
relative distribution of the other isotopes of carbon in the atmosphere
and in the oceans also confirm this estimate (see Box below).
Further information regarding the broad partitioning of added
atmospheric carbon dioxide between the atmosphere, the oceans and
the land biota as presented in Table 3.1 comes from comparing the
trends in atmospheric carbon dioxide concentration with the trends in
very accurate measurements of the atmospheric oxygen/nitrogen ratio.^5
This possibility arises because the relation between the exchanges of
carbon dioxide and oxygen with the atmosphere over land is different to
that over the ocean. On land, living organisms through photosynthesis
take in carbon dioxide from the atmosphere and build up carbohydrates,
returning the oxygen to the atmosphere. In the process of respiration
they also take in oxygen from the atmosphere and convert it to
carbon dioxide. In the ocean, by contrast, carbon dioxide taken from
the atmosphere is dissolved, both the carbon and the oxygen in the
molecules being removed. How such measurements can be interpreted
for the period 1990–4 is shown in Figure 3.4. These data are consistent
with budget for the 1990s shown in Table 3.1.
The global land–atmosphere flux in Table 3.1 represents the balance
of a net flux due to land-use changes which has generally been posi-
tive or a source of carbon to the atmosphere (Figure 3.3) and a residual
component that is, by inference, a negative flux or carbon sink. The esti-
mates of land-use changes (Table 3.1) are dominated by deforestation in
tropical regions although some uptake of carbon has occurred through
forest regrowth in temperate regions of the northern hemisphere and
other changes in land management. The main processes that contribute
to the residual carbon sink are believed to be the carbon dioxide ‘fer-
tilisation’ effect (increased carbon dioxide in the atmosphere leads to
increased growth in some plants – see box in Chapter 7 on page 166),