7.2.3 The global budget of natural and anthropogenic carbon dioxide
anthropogenic carbon dioxide
We now synthesize much of the knowledge outlined in previous sections on the
global budget of CO 2. Firstly, the relative sizes of the natural reservoirs are con-
sidered and then the natural flows between them, followed by how anthropogenic
CO 2 partitions between the boxes. Finally, likely future levels of atmospheric CO 2
are discussed in terms of possible scenarios of fossil fuel consumption.
Reservoir sizes
A simplified version of the carbon cycle is given in Fig. 7.9. By far the largest
reservoir is in marine sediments and sedimentary materials on land (20 000 000
GtC), mainly in the form of CaCO 3. However, most of this material is not in
contact with the atmosphere and cycles through the solid Earth on geological
timescales (see Section 4.1). It therefore plays only a minor role in the short-term
cycle of carbon considered here. The next largest reservoir is seawater (about
39 000 GtC), where the carbon is mainly in the dissolved form as HCO 3 - and
CO 32 -. However, the deeper parts of the oceans, which contain most of the carbon
(38 100 GtC), do not interact with the atmosphere at all rapidly, as discussed in
Global Change 251
10.0
1.0
0.1
1860 1880 1900 1920 1940 1960 1980 2000
Year
Fossil CO
production rate (GtC yr 2
–1
)
Fig. 7.6Global annual emissions of CO 2 from fossil-fuel combustion and cement
manufacture. After IPCC (1990). With permission of the Intergovernmental Panel on
Climate Change.
