Figure 2.5 Estimated carbon dioxide concentration: since 1800 from air bubbles in an Antarctic ice core, early measurements from 1860 to 1960; observations at Mauna Loa, Hawaii, since 1957; and projected trends for this century.
Source: After Keeling, Callendar, Machta, Broecker and others.
Note: (A) and (B) indicate different scenarios of global fossil fuel use (IPCC, 2001).
form a fossil fuel. These transfers within the oceans and lithosphere involve very long timescales compared with exchanges involving the atmosphere.
As Figure 2.4 shows, the exchanges between the atmosphere and the other reservoirs are more or less balanced. Yet this balance is not an absolute one; between ad1750 and 2001 the concentration of atmospheric CO2 is estimated to have increased by 32 per cent, from 280 to 370 ppm (Figure 2.5). Half of this increase has taken place since the mid-1960s; currently, atmospheric CO2 levels are increasing by 1.5 ppmv per year. The primary net source is fossil fuel combustion, now accounting for 6.55 X 1012 kg C/year. Tropical deforestation and fires may contribute a further 2 X 1012 kg C/year; the figure is still uncertain. Fires destroy only above-ground biomass, and a large fraction of the carbon is stored as charcoal in the soil. The consumption of fossil fuels should actually have produced an increase almost twice as great as is observed. Uptake and dissolution in the oceans and the terrestrial biosphere account primarily for the difference.
Carbon dioxide has a significant impact on global temperature through its absorption and re-emission of radiation from the earth and atmosphere (see Chapter 3C). Calculations suggest that the increase from 320 ppm in the 1960s to 370 ppm (ad 2001) raised the mean surface air temperature by 0.5°C (in the absence of other factors).
Research on deep ice cores taken from Antarctica has allowed changes in past atmospheric composition to be calculated by extracting air bubbles trapped in the old ice. This shows large natural variations in CO2 concentration over the ice age cycles (Figure 2.6). These variations of up to 100 ppm were contemporaneous with temperature changes that are estimated to be about 10°C. These long-term variations in carbon dioxide and climate are discussed further in Chapter 13.
Methane (CH4) concentration (1750 ppbv) is more than double the pre-industrial level (750 ppbv). It increased by about 4 to 5 ppbv annually in the 1990s but this dropped to zero in 1999 to 2000 (Figure 2.7). Methane has an atmospheric lifetime of about nine years and is responsible for some 18 per cent of the greenhouse effect. Cattle populations have increased by 5 per cent per year over thirty years and paddy rice area by 7 per cent per year, although it is uncertain whether these account quantitatively for the annual increase of 120 ppbv in methane over the past decade. Table 2.4, showing the mean annual release and consumption, indicates the uncertainties in our knowledge of its sources and sinks.
Nitrous oxide (N2O), which is relatively inert, orig-
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