INPUTS OF CO2 TO THE ATMOSPHERE
- Inputs of carbon dioxide to the atmosphere are
long term (geological) and short term (biological or anthropogenic) (Fig. 1).
The geological inputs can be either juvenile or recycled,
and they can arise from volcanic eruptions or from rock weathering.
Early geological inputs occurred through the degassing of the Earth
as it cooled in the first one billion years of its development (Henderson-Sellers, 1983).
- Igneous rocks are the source of juvenile inputs,
whereas sedimentary rocks produce recycled inputs.
- Volcanic eruptions, which have been occurring for the past 2.8 billion
years, contribute large quantities of inorganic materials
to the atmosphere, among them, carbon dioxide,
sulphur dioxide, and water vapor (Cattermore, 1989).
Rock weathering has been occurring since the time of their formation.
The weathering of rocks is a very slow process;
consequently, it releases
relatively small quantities of carbon dioxide to the atmosphere.
- The combustion of inorganic matter adds carbon dioxide to the atmosphere,
but the amounts are too small to be of practical significance.
On the other hand, the combustion of organic matter
constitutes the main input of carbon dioxide to the atmosphere.
Hereafter, we use the word {\em combustion} to refer only to organic combustion.
- Combustion can be either natural or artificial.
Natural combustion takes place as
respiratory combustion, i.e., the respiration of animals, or as
nonrespiratory combustion, i.e., the burning of vegetation,
such as in naturally occurring forest fires.
- Artificial combustion, which is that produced by humans in the course of
pursuing their livelihood, can be direct or indirect.
Direct artificial combustion is the actual burning of organic matter
to satisfy basic energy needs such as cooking,
lightning, heating/air conditioning, and transportation.
Direct artificial combustion is accomplished
through biomass or fossil-fuel combustion.
Biomass fuels such as firewood, charcoal, animal dung, and trash-to-energy
are recyclable, i.e., replenishable in the short term.
Fossil fuels such as oil, coal, and natural gas are nonrecyclable,
i.e., once used, they cannot be replenished.
- Indirect artificial combustion occurs when humans act to reduce
the biological or geological processes
by which carbon is removed from the atmosphere.
An example of indirect artificial combustion is
the clearing of forests or rangelands to use
the land for animal husbandry, agriculture, and other economic activities.
The clearing of forests may involve only indirect
combustion (e.g., logging), or both direct and indirect combustion
(e.g., the extraction of firewood).
- Indirect artificial combustion can be temporary or permanent.
Temporary indirect artificial combustion reduces the
effectiveness of the carbon sink for a relatively short period
(years to decades). Reducing the effectiveness of the carbon sink
decreases the net biological productivity of an ecosystem.
Net biological productivity is the difference between the sum of
gross productivities and the sum of respirations,
taking the entire food chain into account.
This definition applies to the biosphere as a whole;
transfers between neighboring ecosystems would need to be accounted
for in regional biochemical mass or energy budgets.
- An example of temporary indirect artificial combustion
is slash-and-burn agriculture,
a practice which prevails in many primitive societies around the world.
After several years of exploitation, lands subjected to slash-and-burn are abandoned,
and through secondary succession, they eventually return to the original ecosystem.
- Permanent indirect artificial combustion reduces the effectiveness
of the carbon sink for a relatively long period (centuries to milennia).
Examples of permanent indirect artificial combustion
are wet-to-dry land-use changes such as:
- converting forests to grasslands,
- converting grasslands to farmlands,
- converting farmlands to urban lands, and
- draining wetlands.
- Irrigation, a dry-to-wet land-use conversion,
increases the biological productivity of the ecosystem,
although at the cost of importing water,
an activity which may or may not entail combustion.
Anthropogenic land degradation practices such as
overcultivation, overgrazing, desertification, and salinization also lead to
permanent indirect artificial combustion.
However, the eutrophication of water bodies, while amounting to pollution,
may actually {\em increase} (although quite unintentionally)
the effectiveness of the carbon sink.
- Figure 1 shows the circulation of carbon through the ecosphere.
The juvenile inputs constitute sources of fresh inorganic carbon,
which are responsible for the inception of the carbon cycle.
Recycling through the biosphere (solid lines) is comparatively fast,
whereas recycling through the lithosphere (dashed lines) is characteristically slow.
- Two inputs are nonrecyclable: fossil-fuel combustion and
permanent indirect artificial combustion.
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