Fig. 2.5. Major Reservoirs and Exchanges in the CO2 Cycle The importance of man's contribution to the global carbon cycle cannot be underestimated. It has been estimated that the burning of fossil fuels now releases almost 2.0 x lO^^ kg of carbon dioxide per year. This is of the same order of magnitude as the carbon dioxide annually consumed in photosynthesis (11 x 1013 kg), and, more importantly, it is considerably larger than the carbon dioxide annually consumed to produce new fossil carbon (loH kg) Thus, the reservoir of fossil carbon is being depleted more than one hundred times faster than it is being renewed. Plants remove CO2 from the biosphere by photosynthesis and release it during respiration. The carbon content of short-lived biomass (1-10 years) is only 10% of that of the atmosphere, but the long-lived biomass (10-100 years) contains twice as much carbon as the atmosphere.31 In response to increasing atmospheric CO2 levels, the biosphere should grow faster and increase its storage of carbon. However, the ability of the biosphere to take up this increase in CO2 is limited. An increase of 10% in the CO2 content of
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