correlation between the CO 2 and the temperature records. This supports the
notion of CO 2 as an important greenhouse gas (Section 7.2.4), i.e. when CO 2 is
low the temperature is cool (as in glacial periods) and vice versa. Closer inspec-
tion of more detailed ice core records indicates that change in CO 2 is apparently
not the initiator of the temperature change, which probably arises from alter-
ations in the Earth’s orbit and/or changes in the amount of energy coming from
the sun. However, orbital or insolation changes cannot account for the magni-
tude of temperature changes recorded in ice cores, suggesting that CO 2 varia-
tions act to amplify the orbital and solar perturbations.
Anthropogenic fluxesThe primary human-induced flux to the atmosphere is that from fossil fuel
burning, cement production and so forth, and, as shown in Fig. 7.9, for the 1980s
its average value was 5.4±0.3 GtC yr-^1. Of this input, an amount equivalent to
3.3±0.1 GtC yr-^1 remains in the atmosphere and leads to the observed year-by-
year rise in CO 2 concentrations.
Change in land use arising from human activities leads to an addition to the
atmosphere of 1.7±1.1 GtC yr-^1. On the other hand, fertilization of terrestrial
plant growth, reforestation and regrowth are estimated to take up 1.9 GtC yr-^1 ,
giving a small net land sink of 0.2±0.7 GtC annum-^1.
In Table 7.1 these various flows of anthropogenic CO 2 are given as a budget.
Although the budget achieves balance this should not hide the considerable
uncertainties over several of the terms, particularly those for exchanges between
the land and the atmosphere. For example, the large error associated with the
terrestrial biosphere flux term implies that the land could be a small net source,
instead of the small net sink shown in Table 7.1, for anthropogenic CO 2.
Emissions and atmospheric carbon dioxide levels in the futureIn view of the ‘greenhouse’ properties of CO 2 (Section 7.2.4) and the fact that
atmospheric concentrations of the gas have risen substantially as a result of human
activities, considerable effort is currently being devoted to the task of predicting
what CO 2 levels will be in the atmosphere over the next century.
In this chapter we have identified the problems that exist in quantitatively
accounting for the CO 2 which enters the atmosphere from fossil fuel and other
Global Change 255Table 7.1Atmospheric sources and sinks of anthropogenic CO 2 for the 1980s. All units are
GtC yr-^1. Data from IPCC (2001).
Sources Sinks
Fossil fuel burning 5.4±0.3 Atmosphere 3.3±0.1
Oceans 1.9±0.6
Land 0.2±0.7
Total 5.4±0.3 Total 5.4±0.6