Environmental Engineering Reference
In-Depth Information
Some of the buried carbon from biomass is returned to the surface as carbon dioxide by
natural pathways of volcanism, weathering, and oxidation. However, when left to its own
devices, most of the carbon remains buried with two important consequences:
1.
Carbon dioxide is taken out of the atmosphere.
2.
Oxygen is released into the atmosphere.
In other words, carbon has been taken out of the atmospheric system for millions of years
with the production of oxygen in the process. The combined action of decreasing carbon diox-
ide and increasing oxygen created the atmosphere that the planet had in the preindustrial
period.
The short-term carbon cycle shares some characteristics with the long-term cycle but
excludes deep burial. It involves photosynthesis, respiration, accumulation in soils,
decomposition/fermentation, and interchange of carbon dioxide between the atmosphere and
the oceans. The short-term carbon cycle is represented by Equation 2.2 and the following
chemical reactions:
2n CH O
→+
n CO
n CH
[2.3]
2
2
4
CH
+↔ +
2O
CO
2H O
[2.4]
4
2
2
2
CO (atmospheric)
↔
CO (dissolved)
[2.5]
2
2
CO (dissolved)
+
H O
↔
H CO
[2.6]
2
2
2
3
Once more, photosynthesis transforms atmospheric carbon dioxide into biomass
(Equation 2.2) by primary producers. Some biomass serves as food for consumers and the
remaining accumulates as living biomass that eventually dies and gets buried into the soil and
aquatic sediments. From here, buried biomass is decomposed into carbon dioxide and meth-
ane (CH
4
) by anaerobic fermentation (Equation 2.3), and into carbon dioxide via aerobic reac-
tions (Equation 2.2, reverse direction). Methane by oxidation is transformed into carbon
dioxide (Equation 2.4), which is then returned to the atmosphere.
Consumers use the biomass produced by primary producers that oxidizes through the
respiration process with elimination of carbon dioxide in the process (Equation 2.2, reverse
direction). After consumers expire, they are decomposed according to aerobic and anaerobic
pathways explained before and the carbon is returned to the atmosphere.
In oceans, carbon dioxide from the atmosphere gets dissolved (Equation 2.5) and then
transformed into carbonic acid according to Equation 2.6. Some carbonic acid reacts with
calcium, magnesium, and other metal ions and transforms into insoluble carbonates (CO
3
=
),
which precipitate into the sediments and eventually enter the long-term carbon cycle
(Fig. 2.2).
The modern carbon cycle
The modern long-term and short-term carbon cycles have the same basic components as the
preindustrial one with the addition of two elements: emissions of carbon dioxide from fossil
fuel burning and large-scale deforestation, which alters the long- and short-term carbon cycles,
respectively.
It is estimated that burning fossil fuels increases the release of carbon into the atmosphere
by 100-fold in reference to what would be released by normal oxidation and volcanism
(Intergovernmental Panel on Climate Change [IPCC], 2001), thus significantly disrupting the
