Trends in the Carbon Cycle

Throughout this chapter many of the arguments are based on an assumption of steady state. Before the agricultural and industrial revolutions, the carbon cycle presumably was in a quasi-balanced state. Natural variations still occur in this unperturbed environment the Little Ice Age, 300-400 years ago, may have influenced the carbon cycle. The production rate of varies on time scales of decades and centuries (Stuiver and Quay, 1980,1981), implying that the pre-industrial radiocarbon distribution may not have been in steady state. 

Measurements of CO2 concentrations in air bubbles trapped in glacial ice (Berner et ah, 1980 Delmas et ah, 1980 Jouzel et ah, 1993 Raynaud et ah, 1993) show that atmospheric Pqo was about 200 ppmv toward the end of the last glaciation 20000 years ago (Fig. 11-19). 

The projection of future emissions of fossil fuel CO2 is subject to a number of uncertainties. The growth rate has already shown great variations and the reserves of fossil fuels are not 

The atmospheric CO2 content increased by about 1 ppmv per year during the period 1959-1978 (Bacastow and Keeling, 1981) with the South Pole Pco increase lagging somewhat behind the Mauna Loa (19.5°N,155.6 W) data. This difference is consistent with our knowledge of interhemispheric mixing times and the fact that most fossil fuel emissions occur in the northern hemisphere (see also Conway et al, 1994a). 

Fossil fuel emissions alter the isotopic composition of atmospheric carbon, since they contain no C and are depleted in C. Releasing radiocarbon-free CO2 to the atmosphere dilutes the atmospheric C content, 3delding lower C/C ratios ( the Suess effect ). From 1850 to 1954 the C/C ratio in the atmosphere decreased by 2.0 to 2.5% (Fig. 11-23) (Suess, 1965 Stuiver and Quay, 1981). Then, this downward trend in C was disrupted by a series of atmospheric nuclear tests. Many large fission explosions set off by the United States with high emission of neutrons took place in 1958 in the atmosphere and the Soviet Union held extensive tests during 

20000 years ago. In Fig. 11-15, an estimated range of atmospheric CO2 during the past 40 000 years is shown. The steady CO2 level maintained during the past 10 000 years supports the steady-state assumption often invoked in modeling. 

The terrestrial biota seems unable to take up much of the excess CO2. In fact, a careful assessment of the impact of deforestation and land-use changes indicates that the terrestrial biota has been a considerable source of CO2 during the past century (Bolin, 1977 Woodwell et al., 1983). A complex effort to deduce mankind s impact on terrestrial biota using a book-keeping model based on historical records on land use in all parts of the world (Moore et al., 1981 Houghton et al., 1983 Woodwell et al., 1983) gives the curves in Fig. 11-22. Woodwell et al. 

---