66
ENVIRONMENT, POLLUTION, DEVELOPMENT: THE CASE OF UZBEKISTAN
form of dissolved carbon dioxide and methane.
The atmo sphere
contains approximately 760 billion tons of carbon in the form of
such compounds as carbon (II) oxide (CO), carbon (IV) oxide (CO
2
)
and methane (CH
4
). Carbon compounds have different cycling
periods at each stage of the cycle: in the atmo sphere it is quite
short – 3 years, in the soil – 25 years, in the oceans – 350 years,
while the cycling period of carbonate rocks is more than hundreds
of millions of years.
The living, photosynthetic organisms of the biosphere that conti-
nu ously absorb CO
2
from the atmo sphere, forming organic com-
pounds, have vital functions in the carbon cycle. CO
2
goes through
a complete cycle in the atmo sphere in a relatively short period of
time – approximately four and a half years.
Not all dead organisms and plants decompose immediately. A
small fraction of them reach the deep
parts of inland waterbodies,
seas and oceans and form sediments there. The organic material that
decomposes slowly becomes part of the sedimentary rock formation
process and can return into the atmo sphere in a natural way (for
example, in the process of erosion).
Carbon dioxide from the atmo sphere can enter waterbodies and
dissolve there. In water, algae absorb carbon dioxide in much the
same way as terrestrial plants do.
In addition, some aquatic life forms
extract calcium and carbon dioxide from water to build calcium
carbonate shells. When these organisms die off, their shells are
deposited on the bottom of waterbodies, forming limestone. In this
way, part of carbon becomes involved in the sedimentary rock cycle,
which will possibly bring it as limestone up to the Earth’s surface
again in the future. After that, the erosion process and weathering
will decompose this limestone, and dissolved it will return into the
ocean, and then,
from the ocean, carbon will be released back into
the atmo sphere.
The increase in carbon dioxide and methane emissions can
become a factor that will adversely affect the environ mental
processes and carbon cycle. Estimates show that the amount of
anthropogenic emissions of CO
2
have been increasing on average for
2.5% per year within the last hundred years (Figure 4.19). Shrinking
of forest areas also affects the concentration of CO
2
in the air. The
current amount of CO
2
anthropogenic emissions is 9.0 × 0.5 GtC per
year; however, depending on the World’s population growth scenario,
it is estimated that these emissions can increase up to 35.8 GtC per
year by 2100.
Human economic activity changes
the carbon cycle and
enhances the release of the carbon compounds accumulated in
the litho sphere into the atmo sphere. Fossil fuel combustion and
deforestation make CO
2
pass from the litho sphere and biosphere
4. HUMANS AND THE ENVIRONMENT
67
into the atmo sphere much faster than it would occur in a natural
way. At the same time, the return of CO
2
from the atmo sphere in a
natural way takes place much slower than when
it is aided by human
economic activity. In effect, the amount of CO
2
in the atmo sphere
irreversibly increases.
Methane plays a substantial role in the carbon cycle. Methane
absorbs infrared radiation more effectively than CO
2
; therefore, its
increase boosts the greenhouse effect, even if methane’s concentra-
tion in the atmo sphere is lower than that of CO
2
. Since the 60s of
the last century, when the atmo spheric methane concentration
measurements began, its total concentration
has increased for an
average of 1% per year. Part of methane is generated as a result
of rice and livestock farming, especially from cattle. Historically
methane concentration changes, like those of CO
2
, have been related
to the climate changes during the ice ages and interglacial periods.
However, in recent years research shows
that geological processes
can also be a significant source of methane. For example, such
natural phenomenon as mud volcanoes is deemed to originate almost
10% of the atmo spheric methane.
2
4
6
8
10
0
1700
1750
1800
1850
1900
1950
2000
Years
CO
2
emissions (GtC/year)
The concentration of carbon dioxide in the atmo sphere has
increased from 280 parts per million in the pre-industrial period to
385 parts per million in 2008 (Figure 4.20). Probing the composition
of the air trapped in the continental glaciers has proved that the
concentration of CO
2
today is significantly higher than it had been
during the last 650 000 years (180-300 ppm). Human activity is the
main factor contributing to the increase of CO
2
concentration in the
atmo sphere and affecting the whole carbon cycle.
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