THE CARBON CYCLE
First I want to review some relevant information regarding the global carbon cycle and the processes that affect atmospheric concentrations of carbon dioxide. There are vast reservoirs of carbon in the system (see
Figure 3.1) that can exchange fairly rapidly with the atmosphere, which contains about 750 gigatons (1 gigaton = 109 tons) of carbon (GtC). The terrestrial biosphere and soils contain about 2,000 GtC; the mixed layer of the ocean contains about 1,000 GtC; and the deep oceans, 38,000 GtC.
These numbers—averages for the 1980s estimated by the Intergovernmental Panel on Climate Change (IPCC, 1995)—are reasonably well determined, especially in the context of this subject. (There are also larger reservoirs in which exchanges occur on geological time scales, but I do not discuss those.)
Exchange fluxes of carbon among these systems must also be considered. The human contribution to emissions to the atmosphere from combustion of fossil fuels—about 5.5 GtC per year in the 1980s and about 6.3 GtC per year in the 1990s—is reasonably well known to within about ±10%. However, estimates of net emissions from tropical deforestation, shown here as 1.6 GtC per year, are far less reliable. Again, the values I am citing are from IPCC (1995), in the case for the decade of the 1980s. Figures for land use change as a whole, especially considering reforestation in the middle latitudes, are less certain. Nonetheless, the consequences of tropical deforestation are important.
In addition, there are vast natural cycles involving two-way exchanges of CO2 into and out of the terrestrial biosphere through respiration, photosynthesis, and decay. Carbon dioxide is also exchanged into and out of the ocean through the mixed layer, by thermodynamic processes of gaseous invasion and
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FIGURE 3.1 The carbon cycle. SOURCE: IPPC (1995).