MANY elements cycle from the atmosphere to living things, the
oceans or the soil and then back to the atmosphere, but the cycling
of carbon atoms is particularly important. One reason is that
carbon dioxide (CO2) plays a key role in trapping heat
in the atmosphere—one of the basic mechanisms behind the
greenhouse effect, which raises temperatures near the earth’s
surface.
Another factor that makes the cycling of carbon important is
that carbon plays a central role in
combustion—burning—and in the last 200 years we have
dramatically changed the carbon cycle through burning fossil fuels,
which has released large volumes of CO2 into the
atmosphere. Also, substantial areas of forest have been cut down,
removing a pathway for CO2 absorption (see diagram
below). Consequently, recent times have seen the amount of
CO2 in the air increase and the amount of oxygen
decrease.
Because there is so much oxygen in the air, the oxygen drop is
hardly noticeable, but as there is very little CO2
(0.038% of the atmosphere) the extra CO2 from burning
and deforestation has caused a dramatic rise as shown in the graph
at right. This has contributed to an enhanced greenhouse effect in
recent decades. Given the potentially serious consequences for the
earth’s climate of this enhanced greenhouse effect, great
importance is now placed on ways of reducing CO2
emissions and on reducing the CO2 already in the
air.
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The global carbon cycle where black numbers indicate the annual
flows of carbon in pre-industrial times and red numbers indicate
recent human-caused annual flows. The flows and stores are in
billions of tonnes or Gigatonnes of carbon GtC. Source: IPCC AR4
2007
Atmospheric CO2 concentrations over the last 20,000
years from various sources including ice cores. The grey bars show
the reconstructed ranges of natural variability for the past
650,000 years. Note the dramatic increase in recent times.
Modified from IPCC AR4 2007.
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