carbon cycle (Fig 5.3.2; Keeling et al. 1996). During burning, a fixed amount of oxygen is consumed in relation to carbon. If the recent consumption of fossil fuels is known, it is possible to calculate the corresponding requirements for oxygen and the C02 produced and the concentration to be expected if the C02 formed remains in the atmosphere. Observations in the atmosphere show a significantly lower C02 concentration than expected from the use of fossil fuels. This difference is caused by photosynthesis of land plants in which oxygen proportional to C02 consumption is released into the air. At the surface of oceans more C02 is consumed than 02, as the solubility of C02 in water at 20 °C (0.878 ml C021"1 H20) is much greater than that of 02 (0.0311 1 02 l"1 H20; Sestak et al. 1971). Calculated from these turnover rates ca. 50% of C02 from fossil fuels remains in the atmosphere, 30% is taken up by the oceans, and 20% is assimilated by plants (Keeling et al. 1996).
It is to be expected that the increased concentrations of C02, methane, N20 and other anthro pogenic trace gases will affect climate because of their specific energy absorption characteristics (see Fig. 2.1.2). This is confirmed by geological events, in which a sudden release of marine methane hydrate caused a 5-7 K warming of the lower atmosphere (Norris and Röhl 1999). At the transition from the Palaeocene to the Eocene (55 million years ago), 1200-2000 Gt CH4 was released at the continental shelf in the north Atlantic. This corresponds to the release of C02 expected from fossil fuels. In this process stable methane hydrates, formed under cooler conditions and buried under sediments, were released as gases as the water warmed, such as might occur in an interglacial period. At the foot of the continental shelf sediments slipped and the methane gas was released in a chain reaction (Kennet et al. 2000). Methane concentration in the atmosphere was high for 120,000 years until the gas was re-assimilated and the atmosphere cooled again. There are indications that the evolutionary separation of man from other apes occurred during this time of enormous warming at the transition from the Palaeocene to the Eo-
Fig. 5.3.3. Influence of trace gases and aerosols on the solar radiation balance of the earth. (Schimel et al. 1997)
Tropospheric ozone i
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