Figure 1.2 shows the main components of the natural carbon cycle with the various storages represented in terms of Pg (billions of tonnes) of carbon (C) and fluxes (Pg/year) estimated for the 1980s. The thick black lines represent the most important fluxes, while the thinner lines denote smaller yet significant transfers of carbon. These natural fluxes into and out of the atmosphere are approximately balanced over the course of a year. Note that the CO2 emissions from fossil fuel burning and cement production are estimated to have increased from 5.4 Pg C/year in the 1980s to 6.3 Pg C/year in the 1990s. As we will see, this human perturbation to the global carbon cycle is still relatively small compared to the huge natural fluxes, but its cumulative effect on concentrations of CO2 in the atmosphere, and therefore on global climate, is substantial.
In considering the global balance, or imbalance, of carbon we should first examine the natural sources of CO2.
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