III. The Carbon
Cycle
The biogeochemical cycle—or simply, the carbon cycle is the cycle by which
carbon is transferred between the four main reservoirs that hold carbon
compounds within them. The
four carbon-reservoirs are 1) the biosphere (plant and animals, large
to microbic or planktonic), 2) the geosphere (chalk, limestone and marble),
3) the hydrosphere (the lakes and oceans, and finally 4), the atmosphere. The term “global carbon budget” refers to the
balance of exchanges (incomes versus losses) of carbon (in all its various
organic compounds, C, CaC03, C02, etc) between any one of these reservoirs
to another. Careful scientific
examination of any one carbon-reservoir can reveal whether that reservoir
is or has become a source or sink for carbon dioxide (C02). A reservoir which has become a source is giving off C02, while
a sink is a reservoir which is taking in and storing C02.
Carbon exists in the Earth’s atmosphere mainly in the form of
C02, a gas (about 0.04 percent, but rising). The other two atmospheric gases that contain carbon are methane
(CH4) and chlorofluorocarbons (Freon for refrigeration and ac-units). (the latter, CFC’s are artificial, while C02 and CH4 are naturally
occurring. These are greenhouse
gases which trap the sun’s infrared spectrum of light energy in the
troposphere (the lowest part of the atmosphere). Carbon in its C02 form can be transferred into the two distinct
divisions of the biosphere’s carbon reservoir, land and oceanic. On land, plants sink C02 (forests and all plants, but forests
proving more permanent sinks for C02) through photosynthesis, where
C02 and water are converted first to sugars, and then (in trees) to
cellulose (wood). The oceans, on the other hand, are massive carbon
sinks for C02, where C02 is readily dissolved in cold or cooler waters. But then the C02 which has been dissolved readily
into the oceans is then, slowly but massively absorbed first by the
tiny one-celled plants called phytoplankton, which converts C02 and
water into sugars, just like their plant counterparts on land. Then each night zooplankton, some single-celled,
some multi-celled sea animals, some resembling tiny shrimp, rise to
near the oceans surface and feed on the plentiful phytoplankton, converting
the carbon, now in the form of sugars, into the calcium carbonate (CaC03)
which makes up their tiny shells or skeletons. When the zooplankton die, they sink to the ocean’s floor, which
slowly builds up to form chalk, which under time and pressure turns
to limestone and then marble. Thus,
this process sinks tons of carbon dioxide, now in the form of calcium
carbonate (chalk, limestone and marble) into a more permanent form,
locked safely out of Earth’s atmosphere. As we have read previously in this article,
multiple giga-tons of atmospheric C02 have been and are continually
being locked away as CaC03 on the ocean floors of the Earth. It is by way of these “living oceans” that C02
is “scrubbed” from Earth’s atmosphere. On each nuclear submarine in the U.S. Navy are two “C02 scrubbers”. They are designed to “scrub”, chemically remove,
all the C02 that could possibly build up via the crew’s respiration
(and yes, smoking) with plenty of margin for safety (unexpected fires,
a real danger on any submarine). But
should all the ship’s C02 fire extinguishers be discharged and emptied
all at the same time into the sub’s atmosphere, C02 levels would rise
beyond the sub’s C02 scrubber’s capability of handling. They would be helpless to stop the rise of C02 levels, and would only be capable of lowering the C02 levels
over a period of time. The sub
would have to surface and emergency ventilate. But with Earth, there is no way of “surfacing and emergency ventilating.” We have to ride it out. If
the best scientists in their respective fields are recording a steady
parts per million rise in C02 levels in Earth’s atmosphere it means
the Earth’s bio-oceanic “C02 Scrubber” system has reached it’s saturation
point, its limit for scrubbing excessive amounts of C02 being introduced
into the atmosphere. The atmospheric
reservoir has now become a positive sink for C02 instead of a source
which remains in balance.
Carbon (in all its forms) can be released back into the atmosphere
in these 7 ways:
1) Respiration via plants and animals (yes, land plants breathe
out C02 and breathe in Oxygen at night.): They both break down glucose
into C02 and water. But plants,
especially trees, end up storing far more C02 than they give off, so
they continue to be effective carbon dioxide sinks.
2) Decay of animal and plant matter via breakdown through
fungi and bacteria: This process
releases C02. If air is present
aerobic decay takes place, releasing C02. If air is not present, methane is produced (CH4),
called anaerobic decay. Swamps,
compost piles, peat bogs in warm weather, can produce either kind of
decay. Sanitary land-fills produce
methane through anaerobic decay.
3) Combustion of organic material: This releases C02 and water vapor. Some of those materials are wood, coal, petroleum and natural
gas.
4) Limestone decay: Decaying limestone (chalk, limestone, marble)
via erosion by water (especially acid rain, etc) releases C02 and carbonic
acid.
5) Production of cement: Cement production heats limestone, which releases
a substantial amount of C02.
6) Warming ocean surfaces: Warming ocean surfaces cause C02 to be released
back into the atmosphere. (Let
a glass of Ginger Ale stand on the counter in the sunlight and watch
it bubble. Those bubbles are the C02 molecules dissolved
in the water of the Ginger Ale
out-gasing back into the atmosphere.
7) Volcanic eruptions: Volcanic eruptions release a lot of C02, water
vapor and sulfur dioxide (S02) into the atmosphere.
As we have seen, carbon
dioxide, C02, leaves the biosphere, on land in the form of peat and
in forests (cellulose) and in the geosphere on ocean bottoms when animal
shells and skeletons composed of CaC03 become chalk, then limestone,
and then marble via the process of sedimentation. Models of the carbon cycle have been incorporated into global
climate models. This allows that the interactive responses of
atmosphere, oceans, biosphere and geosphere to rising C02 levels to
be plotted. As accuracy of measurement
techniques steadily improve, so will the modeling. These successively more accurate models are
all showing a positive feedback between global temperatures and C02
levels.
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