The global budget of natural and anthropogenic carbon dioxide

3 The global budget of natural and anthropogenic carbon dioxide 

We now synthesize much of the knowledge outlined in previous sections on the global budget of C02. Firstly, the relative sizes of the natural reservoirs are considered and then the natural flows between them, followed by how anthropogenic C02 partitions between the boxes. Finally, likely future levels of atmospheric C02 are discussed in terms of possible scenarios of fossil fuel consumption. 

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 (20000000 GtC), mainly in the form of CaC03. 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 HC03 and C03 . 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 

Section 7.2.2. The reservoir of carbon in fossil fuels and mudrocks is also substantial and a major portion of the latter is thought to be recoverable and thus available for burning. The smallest reservoirs are the land biosphere (2000 GtC) and the atmosphere (749 GtC, equivalent to an atmospheric concentration of about 3 54 ppm). It is the small size of the latter which makes it sensitive to even small percentage changes in the other larger reservoirs, where these changes result in emissions to the atmosphere, as, for example, in the burning of fossil fuels. 

While examining Fig. 7.10, it is worth noting that the excursion in atmospheric C02 over this 420000-year period (about 110 ppm) is only marginally greater than that achieved by human activities over the last 200 years (90 ppm), as shown in Figs 7.1 and 7.2. A second point to note from Fig. 7.10 is the close 

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