The atmospheric record

The best place to start our examination is in the atmosphere, where the observational record is most complete and historical changes are best documented. Figure 7.1 shows values of the atmospheric concentration of CO2 measured at Mauna Loa (Hawaii) and the South Pole from the late 1950s until the end of the 20th century. It indicates that over this period there was a clear increase in atmospheric CO2 and that this was a worldwide phenomenon. The rate of increase varied somewhat from year to year, generally ranging from 1 to 2 ppm or about 0.4% per year.

The data in Fig. 7.1 cover the period during which reliable analytical techniques have been in use. In order to extend the record further into the past, resort is made to measurements obtained using cruder techniques (by modern-day standards) in the latter part of the 19th century and the first half of the 20 th century. These early data are shown in Fig. 7.2, together with the most complete recent dataset, which is from Mauna Loa. A more reliable way of extending the record backwards has been through the extraction and analysis of bubbles of air trapped in ice cores collected from the polar ice-caps. The principle of this method is that the trapped air bubbles record the atmospheric composition at the time the ice formed. By dating the various layers in the cores from which the air bubbles have

Fig. 7.1 Direct measurements of atmospheric CO2 and O2 concentrations for northern (Mauna Loa, Barrow) and southern (South Pole, Cape Grim) hemispheres, showing the changing concentrations and seasonal cycles. After IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.

• Compilation of early observations 1—$—1 Concentrations measured in ice cores

Mauna Loa

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Fig. 7.2 Atmospheric and ice core measurements of CO2 in air. After Crane and Liss (1985), reprinted with permission of New Scientist.

been extracted, the time history of the composition of the atmosphere can be established. The results from measurements made using this approach on an ice core from western Antarctica are also shown in Fig. 7.2. It appears that in the mid-18th century, before major industrialization (and agricultural development)

had taken place, the atmosphere contained close to 280ppm CO2, and more recent data from ice cores indicate that the 280 ppm level extends back to at least 900 AD. Since about 1850, the CO2 concentration has increased nearly exponentially, due to humans burning fossil fuels and developing land for agricultural use. In 2001 the level was close to 371 ppm, indicating an increase of almost 33% over the pre-industrial concentration.

Detailed examination of the Mauna Loa data, where measurements are available on a monthly basis (Fig. 7.1), shows a large and regular seasonal pattern of concentration change. Similar seasonalities are found at other sites, although the amplitude of the variation changes with latitude and between hemispheres; note the much smaller seasonal amplitude for the South Pole record in Fig. 7.1. These seasonal effects will be discussed further in connection with biological cycling of CO2, as will the accompanying changes in atmospheric oxygen shown in the inset to Fig. 7.1 (Section 7.2.2).

Although the trend of atmospheric CO2 concentrations is clearly upward in Fig. 7.1, the increase is only about half of what would be expected if all the CO2 from fossil fuel burning since 1958 had remained in the atmosphere, as shown in Fig. 7.8. This indicates that the half that does not appear in the atmospheric record must have been taken up by some other environmental reservoir. This is a simplistic deduction since it assumes that the other reservoirs have themselves not changed in size and that they have not had net exchange with the atmosphere during the relevant period. Despite these simplifications, the calculation forces us to examine the other reservoirs and so stresses the importance of looking at the system as an entity, rather than as disconnected environmental compartments.

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