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Dissolved organic carbon (<700)

Surface sediment (150)

FIGURE 14-11 Summary of global carbon cycle. Amount (in gigatons of C = 109 metric tons = 1015 g of C). Reservoirs are shown in parentheses, and fluxes (gigatons of C per year) are indicated by arrows. Note that the time scales associated with the various processes vary (adapted from IPCC, 1996).

slightly more than half of the C02 that has been emitted to date remains in the atmosphere. (A detailed analysis of the data in Fig. 14.12 shows a slight anomaly in the 1980s in that the C02 concentrations were higher than expected based on industrial emissions; Keeling and co-workers (1995) suggest this is due to changes in terrestrial and ocean sinks associated with changes in global temperatures.) Similar increases in C02 have been documented at locations around the world, including the South Pole, where measurements have been made since 1957. Most of the remaining emissions of C02 that have been removed from the atmosphere have been taken up by land ecosystems, with a small contribution from the oceans (Tans and White, 1998).

Figure 14.13 shows C02 concentrations measured in ice cores at the Byrd Station in Antartica from 5000 years before the present (bp) to 40,000 years bp (Anklin et al., 1991). The use of ice core data for elucidating atmospheric composition is discussed by Delmas (1992) and in more detail in Section E.l. As seen in Fig. 14.13, atmospheric C02 concentrations about 5000 years ago were only ~280 ppm. (Note that interpretation of such ice core data must be carried out with care since there is evidence that in some cases, C02 can be produced in the ice from decomposition of carbonate; e.g., see Smith et al., 1997.)

In short, it appears that C02 concentrations prior to the industrial age were even smaller than those measured starting in the late 1950s.

Year

FIGURE 14.12 Measured C02 concentrations at Mauna Loa, Hawaii, from 1958 to 1994. The line represents the atmospheric C02 expected if 55.9% of the cumulative emissions of C02 from fossil fuel combustion and cement production remained in the atmosphere (adapted from Keeling et al., 1995).

Year

FIGURE 14.12 Measured C02 concentrations at Mauna Loa, Hawaii, from 1958 to 1994. The line represents the atmospheric C02 expected if 55.9% of the cumulative emissions of C02 from fossil fuel combustion and cement production remained in the atmosphere (adapted from Keeling et al., 1995).

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