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Water, Carbon and Nitrogen Cycles

The Carbon Cycle.

What is the Carbon Cycle?

The carbon cycle describes the movement of carbon
through the environment. Carbon exists in the ground, in
the sea, in plants and animals, and in the air. Carbon
moves through the processes shown in the picture below. 

How does the Carbon Cycle work?

The natural processes shown in the picture above can
release carbon dioxide into the atmosphere
(shown by red arrows) by combustion, decay and respiration,
or
remove carbon dioxide from the atmosphere (shown by blue arrows)
by dissolving in the sea (see below) and by photosynthesis,
or
"lock up" carbon in the ground (shown by green arrows)
as a fossil fuel or by forming carbonate rocks.

Carbon dioxide in the atmosphere dissolves in rain water,
forming carbonic acid (H₂CO_3(aq)).
H₂O_(l)     + CO_2(g)        H⁺_(aq)    +      HCO₃^-_(aq)

This makes rain water naturally acidic (pH = 5.5).
This acidic rain water will react with carbonates in rocks.

Rain water containing dissolved carbon dioxide
finds its way into the sea (see the water cycle).
Carbon dioxide also dissolves directly into the
surface of the sea. The huge oceans are an important
gas reservoir for carbon dioxide (they store it).

Chalk and limestone release carbon dioxide into
the atmosphere when they react with acid rain.
If chalk or limestone are forced down into magma
and heated, then carbon dioxide may be
released into the atmosphere from volcanoes.

The burning of fossil fuels is increasing the amount
of carbon dioxide in the atmosphere faster than nature
can remove it. Deforestation (cutting down forests) is
making this worse, as it reduces the removal of
carbon dioxide from the atmosphere by photosynthesis.

See also global warming and greenhouse gases.

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