FIGURE 1.3 The global carbon cycle and changes in the sizes of CO2 reservoirs over the last two decades. All values are billions of metric tons of carbon. Arrows show annual fluxes. Values for the 1990s are in blue; those for 2000-2008 are in red. SOURCE: Le Quéré (2009), International Geosphere-Biosphere Programme/Global Carbon Project. Data from Le Quéré et al. (2009).

FIGURE 1.3 The global carbon cycle and changes in the sizes of CO2 reservoirs over the last two decades. All values are billions of metric tons of carbon. Arrows show annual fluxes. Values for the 1990s are in blue; those for 2000-2008 are in red. SOURCE: Le Quéré (2009), International Geosphere-Biosphere Programme/Global Carbon Project. Data from Le Quéré et al. (2009).

individual molecule in the atmosphere, but by the small imbalance that the pulse creates between the uptake and removal rates. For example, CO2 molecules spend thousands of years in the oceans once they have been transported into the abyss. Consequently, dissolved CO2 in the deep oceans reflects the atmospheric abundance before the industrial revolution, rather than the increased abundance caused by fossil-fuel burning in the last 200 years. The upshot is that a fraction of the fossil-fuel CO2 emitted is taken up rapidly by the upper ocean and biosphere, but the remainder of the perturbation acts like a very long-lived gas, requiring thousands of years to decay away (Denman et al., 2007).

Methane is short-lived in the atmosphere relative to CO2. A molecule emitted into the atmosphere is oxidized to CO2 in an average of about 8 years, but chemical feedbacks extend this time scale to 12 years (Prather, 1994). This means that the current abundance of methane is derived from the last several decades of emissions. Nitrous oxide has an average residence time of 114 years in the atmosphere before it is photochemically decomposed in the stratosphere. The average atmospheric lifetimes of the other gases considered in this report range from 45 to 1,700 years for CFCs, 1 to 270 years for HFCs, 3,200 years for SF6, and tens of thousands of years for PFCs (Forster et al., 2007).



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