It is believed
that most of the carbon now on Earth was originally released from the
interior of the Earth as CO2, a gas which now makes up about
0.03 to 0.04 percent by volume of air, and is responsible for maintaining
the Earth as a greenhouse with temperature conditions suitable for life.
CO2 is the most available form of carbon for living organisms.
Molecules containing carbon may keep the carbon fixed over millions of
years or may cycle the carbon through quickly. The atmospheric cycling
and effects of CO2 on climate are discussed in the Atmospheric
exists in the biosphere as the central element of life, in the lithosphere
as coal (carbon) or limestone (Calcium Carbonate, CaCO3 ),
in the atmosphere as CO2, in the hydrosphere as dissolved CO2
, as well as in other complex forms. The versatility of carbon compounds
and the vital role of carbon as the basis of life is described in Biological/Ecological
Systems. The atmosphere contains about 750 billion tons of carbon
in the form of CO2. Photosynthesis by plants removes about
120 billion tons of carbon from the air per year, but plant decomposition
returns about the same amount. Living plants and animals contain 560 billion
tons of carbon (mostly forest trees). Plant remains and organic matter
buried in the soil contain about 1400 billion tons. About 11,000 billion
tons are trapped in compounds which are complexes of methane (CH4)
and water, found on ocean floor. The oceans contain another 38,000 billion
tons of carbon, most of it in the form of dissolved CO2.
onset of the Industrial Revolution about 200 years ago, we began burning
massive amounts of fossil fuels and releasing large amounts of the earthbound
carbon into the atmosphere, primarily as CO2. The burning of
fossil fuels adds about 22 billion tons of CO2 per
year (?), containing about 6 billion tons of carbon. Deforestation
adds a further 1.6 to 2.7 billion tons, by not removing this amount. The
rapid growth of synthetic organic chemicals contributes to the amount
of CO2 released.
reservoirs for carbon are sedimentary rocks, fossilized organic carbon
including the fossil fuels, the oceans, and the biosphere. Carbon goes
primarily through three cycles with different time constraints:
- A long-term
cycle involving sediments and the depths of the lithosphere.
- A cycle
between the atmosphere and the land.
- A cycle
between the atmosphere and the oceans.
two cycles are faster and subject to human intervention.
Cycle One: Long-term Cycle
between atmosphere, oceans, and sediments involve a slow dissolution of
atmospheric carbon and carbon from rocks via weathering into the oceans.
In turn, the oceans deposit sediments, and then some of the sediments
are thrown back into the atmosphere through volcanic action.
occurs over hundreds of millions of years. A larger portion of sediments
is calcium carbonate (CaCO3) because the ocean contains large
amounts of calcium.
Cycle Two: Air and Land Cycle
cycle between the atmosphere and biosphere occurs over different time
scales ranging from days to decades. Carbon dioxide is the basic "food"
of the biosphere and thus the biosphere is the agent for this cycling.
Photosynthesis (synthesizing starches and sugars using light) is a main
mechanism for cycling carbon by the biosphere. The chemical reaction of
photosynthesis may be represented as:
represents a unit of organic matter; six of the CH2O unit would
be C6H1206 which makes the simple sugar
(glucose or fructose) and 11 of these units make C11H22O11,
a more complex sugar, sucrose, formed by the combination of one
glucose and one fructose. Thousands of glucose molecules combine to form
a molecule of starch, or of cellulose. (Molecule
photosynthesis takes the atmospheric carbon in CO2 and "fixes"
it into the biosphere. The subsequent cycling of the carbon in the biomass
C2: Carbon Cycle Two.
750 Gt-C in the atmosphere cycling at the rate of 80 Gt C/yr means that
the lifetime of the carbon in the atmosphere reservoir is about 9 years.
the organic matter is oxidized through respiration, the reverse of photosynthesis
releases CO2 into the atmosphere. Respiration and photosynthesis occur
at nearly equal rates over one year. Buried biomass--eventually becoming
fossils, including coal--have historically had an effect of keeping the
carbon in the land. The accelerated burning of fossil fuels is, however,
releasing these large stores into the atmosphere as combustion products.
of biomass-based fuels such as methanol and ethanol has been suggested
an alternate to fossil fuel combustion. Biomass fuels have no net release
of carbon dioxide. The effects of fossil fuel burning is discussed in
the Atmospheric System.
Cycle Three: Air and Sea Cycle
oceans contain much more carbon than the atmosphere. Carbonates washed
down from the rocks, over thousands of years, dissolved CO2,
and carbon in the oceanic biomass constitutes this reservoir. The carbon
from the top layers of the ocean cycles faster whereas the carbon in deep
waters may take thousands of years.
summary of the three cycles is shown in Figure C3.
C3: Three Carbon Cycles combined.