Chemistry Reference
In-Depth Information
FIGURE 18.3
Principal global C pools and fluxes between them.
(From
Lal, 2008
. Copyright 2008 with permission from the Royal Society.)
approximately 40% of global CO
2
emissions, so, the geological pool is being depleted at the rate of 7.0 Gt C/y due
to fossil fuel combustion. The third largest pool is in the soil (the pedologic pool), estimated at 2500 Gt to a depth
of 1 m. It consists of two distinct components, the soil organic carbon (SOC) pool (1550 Gt) and soil inorganic
carbon (SIC) pool (950 Gt). The SOC pool includes highly active humus and relatively inert charcoal, made up of
a mixture of plant and animal residues at various stages of decomposition, whereas the SIC pool includes
elemental carbon and carbonate minerals. The fourth largest (atmospheric) pool consists of 760 Gt of mostly CO
2
,
which is increasing at the rate of 3.5 Gt/y or 0.46%/y. The smallest is the biotic pool estimated at 560 Gt. The
pedologic and biotic C pools together constitute the terrestrial C pool (together about 2860). The atmospheric pool
is connected to the oceanic pool absorbing 92.3/y and releasing 90/y with a net positive balance of 2.3/y. The
oceanic pool is expected to absorb approximately 5
1
/y by 2100. The total dissolved inorganic carbon in the
oceans is approximately 59 times that in the atmosphere. On the scales of millennia, it is the oceans which
determine the atmospheric CO
2
concentration, not the opposite. The exchange between fossil fuel and the
atmospheric pool is unidirectional, corresponding to approximately 7.0/y from fossil fuel consumption to the
atmosphere. There are hopes that the rate of fossil fuel consumption may peak by about 2025. The annual rate of
photosynthesis is 120 Gt, most of which is returned to the atmosphere through plant and soil respiration.
In the atmosphere, carbon is mostly present as CO
2
, where it only represents a small percentage of the
atmosphere (0.04% on a molar basis). Most of the terrestrial carbon in the soil and above the ground is stored in the
forests, while the oceans contain the largest active pool of carbon near the Earth's surface, although the much
larger deep ocean part of this pool does not exchange rapidly with the atmosphere.
2
Most carbon released to the
2. It can, however, contribute as a result of external influences like hydrothermal vents in locations where two tectonic plates are moving
apart, or by uncontrolled deep-water oil well leaks.
