Carbon cyclex


22 févr. 2014 (il y a 3 années et 1 mois)

62 vue(s)

Carbon flows:

The Earth maintains a natural carbon balance. When concentrations of carbon dioxide (CO
) are
upset, the system gradually returns to its natural state. This natural readjustment works slowly,
compared to the rapid rate at which humans are
moving carbon into the atmosphere by burning
fossil fuels. Natural carbon removal can't keep pace, so the concentration of CO

in the
atmosphere increases.

Carbon continually exchanges within a closed system consisting of the
atmosphere, oceans, biosphere,

and landmass. There are short

and long
term cycles at work.

Term Cycles:

Carbon is exchanged rapidly between plants and animals through respiration and photosynthesis,
and through gas exchange between the oceans and the atmosphere.

Term Cycle:

Over millions of years, carbon in the air is combined with water to form weak acids that very
slowly dissolve rocks. This carbon is carried to the oceans where some forms coral reefs and
shells. These sediments may be moved deep into the Earth by drifting

continents and eventually
released into the atmosphere by volcanoes.


Dissolving of CO

gas into the oceans and inflow of carbon carried from land
by rivers.


Return of carbon in the oceans directly back to the atmosphere as CO



Slow burial of plant and animal matter on land and on the ocean floor,
which eventually becomes limestone, coal, gas, and oil.


Slow combustion of ca
rbon compounds, producing energy within organisms and
releasing CO


Conversion of CO

into energy
rich carbon compounds by plants.

Human Impact

Like all other animals, humans participate in the natural carbon cycle, but there are also im
differences. By burning coal, oil, and natural gas, humans are adding carbon dioxide (CO
) to the
atmosphere much faster than the carbon in rocks is released through natural processes. And
clearing and burning forests to create agricultural land co
nverts organic carbon to carbon dioxide
gas. The oceans and land plants are absorbing a portion, but not nearly all of the CO

added to
the atmosphere by human activities.

The red arrow, representing rapid fossil fuel burning, indicates the main way in w
hich humans
affect the natural carbon cycle. Carbon dioxide (CO
) levels are increasing because the natural
system cannot keep pace with this new emission source. The natural processes that
permanently remove this additional carbon

ocean uptake and

extremely slowly


Flows and reservoirs of carbon in the world

voirs of carbon:

Carbon is stored on our plan
et in the following major sinks

(a) as organic molecules in living and dead
organisms found in the biosphere; (b) as the gas carbon dioxide in the atmosphere; (c) as organic matter
in soils; (d) in the lithosphe
re as fossil fuels and sedimentary rock deposits such as limestone, dolomite
and chalk; and (e) in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate
shells in marine organisms.

Table 1: Estimated major stores of carbon on the Earth.

Historical data on atmosphere carbon dioxide contents:

Over the last 30 years, the amount of carbon stored in the atmosphere has increased, which
we see as a significant increase in the atmospheric concentration of
carbon dioxide

(Figure 3). The so
Mauna Loa curve

shows that between 1959 and 2004,

concentration of carbon in the atmosphere increased from about 317 parts per million (ppm)
to 377 ppm. This increase is worrisome because the amount of carbon dioxide in the
atmosphere influences the amount of

retained, which may alter global cli
mate. Notice
too that the increase in is not steady. Within each year, the concentration of carbon dioxide
rises and falls. This intrannual cycle allows us to watch the planet “breathe.”


Action points to control the carbon cycle.

Carbon is an integ
ral part of life on Earth
. With the ongoing modifications of the carbon cycle
and the climate, marine and terrestrial ecosystems will also be affected. Inevitably there will
be winners and losers as species compete to adapt. Additional research is needed t

Amount in Billions of Metric


578 (as of 1700)

766 (as of

Soil Organic Matter

1500 to 1600


38,000 to 40,000

Marine Sediments

and Sedimentary

66,000,000 to 100,000,000

Terrestrial Plants

540 to 610

Fossil Fuel Deposits


understand the ecosystem changes and the implications of these changes for our society. For
example, to understand the implications of increasing ocean acidity resulting from
atmospheric CO
uptake we need new monitoring approaches that fully characteriz
e the
biological and chemical changes, new laboratory studies to understand the physiological
effects, and new manipulative studies to directly quantify the ecosystem responses to
increased CO
Similar studies need to be further developed on land.

Agriculture and industrial development have led to inadvertent changes in the natural carbon
cycle. As a consequence, concentrations of carbon dioxide and other greenhouse gases have
increased in the atmosphere and may lead to changes in climate. The curre
nt challenge facing
society is to develop options for future management of the carbon cycle. A variety of
approaches has been suggested: direct reduction of emissions, deliberate manipulation of the
natural carbon cycle to enhance sequestration, and captur
e and isolation of carbon from fossil
fuel use.