The first panel of Figure 3.1 shows the major components
of the carbon cycle, estimates of the current storage in the active compartments,
and estimates of the gross fluxes between compartments. The second panel shows
best estimates of the additional flux (release to the atmosphere - positive;
uptake - negative) associated with the human perturbation of the carbon
cycle during the 1980s. Note that the gross amounts of carbon annually exchanged
between the ocean and atmosphere, and between the land and atmosphere, represent
a sizeable fraction of the atmospheric CO2 content - and are
many times larger than the total anthropogenic CO2 input. In consequence,
an imbalance in these exchanges could easily lead to an anomaly of comparable
magnitude to the direct anthropogenic perturbation. This implies that it is
important to consider how these fluxes may be changing in response to human
To understand how the changing global environment may alter the carbon cycle, it is necessary to further analyse the fluxes and examine the physicochemical and biological processes that determine them. The remaining two panels of Figure 3.1 indicate the main constituent fluxes in the terrestrial and marine systems, with current estimates of their magnitude. The following sections explain the controls on these fluxes, with special reference to processes by which anthropogenic changes may influence the overall carbon balance of the land and oceans on time-scales from years to centuries.
Box 3.1: Measuring terrestrial carbon stocks and fluxes.
Estimating the carbon stocks in terrestrial ecosystems and accounting
for changes in these stocks requires adequate information on land cover,
carbon density in vegetation and soils, and the fate of carbon (burning,
removals, decomposition). Accounting for changes in all carbon stocks
in all areas would yield the net carbon exchange between terrestrial ecosystems
and the atmosphere (NBP).
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