Geoscience Reference
In-Depth Information
According to present assessment, every year up to 17 million ha of tropical
forests are removed, with
6 million ha falling on the basin of Brazil Amazon. By
the end of 1988, 21 million have been deforested, and 10 years later this area
reached 27.5 million ha, which exceeds the territory of Great Britain.
The WTF destruction is important from the viewpoint of the impact on the
global carbon cycle, since there is a danger of transformation of the WTF zone from
the sink to the source of carbon for the atmosphere. Not less substantial are the
ecological aspects of WTF elimination, in view of ecological uniqueness of the
tropical forests of Central and South America. According to available estimates,
deforestation only in the basin of Brazil Amazon (over the area 360 million ha) will
result in the annual sink of carbon up to 0.56 billion t, and on global scales the level
of this source can reach 4 billion t C per year. Bearing in mind that in 1988, as a
result of forest
*
fires, the tropical forests were burned over an area of about 9 million
ha, then due to only this source the atmosphere could get 1
2 billion t C.
Over the virgin tropical forests, about 75 % of the incoming solar radiation are
spent on evapotranspiration. Therefore, removal of WTF will result in radical
changes of energy exchange and global atmospheric circulation. Changes of local
climate should be even more substantial, especially from the viewpoint of precip-
itation, which can be reduced by 65 %. Of key importance is the fact that the
threshold level of WTF elimination that determines the loss by the ecosystem of its
self-support ability, remains unclear. For instance, if it is 20 %, then this threshold
has been exceeded already.
Bengtsson (1999) drew attention to the fact that since non-linear processes
exhibit the prevailing impact on climate system
-
is variability, it is impossible to
establish any simple connection between external forcings (e.g., the growth of the
GHGs content or variability of extra-atmospheric insolation) and response of the
climate system to such forcings. With the unpredictability of some factors of cli-
mate taken into account, the dif
'
culty of distinguishing between anthropogenic and
natural variability of climate becomes apparent and even increases due to the fact
that both internally and externally forced modes of climate variability are deter-
mined by the same mechanisms and feedbacks.
Although a considerable progress has been recently achieved in numerical
modeling of the climate system, it refers mainly to the atmosphere, which is testi
ed
by correspondence of the results of numerical modeling of atmospheric circulation
to observational data. Results of the
numerical experiments indicate
that the 3D atmospheric circulation in the tropics is mainly determined by the
impact of boundary conditions, whereas in high latitudes the impact of atmospheric
dynamics prevails. The simulation of water cycle in the atmospheric turned out to
be rather realistic.
A considerable progress in modeling of the interactive atmosphere-ocean system
has made it possible to successfully predict the seasonal and inter-annual variability
and, in particular, El Ni
“
ensemble
”
of events. An adequate consideration of the processes on
land surface has ensured a substantial increase of reliability of hydrological fore-
casts (including the river run-off).
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