Geoscience Reference
In-Depth Information
linked to atoms of other elements, usually hydrogen, oxygen or
nitrogen.
4.3.1. Carbon in large natural reservoirs
The carbon cycle involves four large natural reservoirs: the
atmosphere, the biosphere (marine and terrestrial living organisms),
the hydrosphere (ocean and freshwater) and the lithosphere (crust and
sediments, sedimentary rocks and fossil organic matter, i.e. peat, coal
and hydrocarbons, including oil and natural gas) (Figure4.3,
Tables 4.3 and 4.4; Table 4.4 details the carbon values given in
Table 4.3). Various mechanisms, including the activity of living
organisms, move carbon between different inorganic and organic
compounds and between the large reservoirs, at different time scales.
The followingparagraphsbrieflydescribe the different forms of
carbon found in the atmosphere, lithosphere, hydrosphere and
biosphere.
In the atmosphere, CO 2 represents only a small proportion of gases
in this outer envelope of the Earth (<0.1%, Table 4.2), but it greatly
contributes to the greenhouse effect (i.e. retention of heat from the
Sun in the lower part of the atmosphere). The presence of different
greenhouse gases in the atmosphere, including CO 2 and water vapor,
keeps the surface temperature of the Earth higher than if they were
absent. The greenhouse effect is essential to living systems because,
without it, the mean surface temperature on Earth would be -18°C
instead of the present +15°C (in fact, at -18°C, ice would cover the
globe and the mean temperature would plummet to -50°C). The two
most abundant carbon compounds in the atmosphere are carbon
dioxide (CO 2 ) and methane (CH 4 ). Although CH 4 is almost a hundred
times less abundant than CO 2 in the atmosphere, each molecule of CH 4
has an impact 20 times greater than a molecule of CO 2 on the
greenhouse effect. However, the lifetime of CH 4 in the atmosphere is
only approximately 10 years (CH 4 is transformed into CO 2 and H 2 O
after several chemical reactions), so its effect on the climate
essentially depends on its rate of production. The natural greenhouse
effect is therefore an essential condition for the existence of life on
Earth. Our current concern about the greenhouse effect is the rapid
Search WWH ::




Custom Search