Geoscience Reference
In-Depth Information
in the river channels, in reservoirs or even on farmland if
floods have occurred. Previously navigable rivers have
become silted up in Madagascar, where deforestation and
soil erosion are major problems. During the wet season
the coastal waters change colour as vast quantities of
sediment are transported to the Indian Ocean.
Climate
. Deforestation can affect climate on the local,
the regional and perhaps even the global scales. The local
effect is obvious, as surface characteristics, such as the
albedo, are being changed. The dark green of the rain
forest is replaced by lighter greens of growing crops or
light browns of bare soil. Solar radiation is able to reach
the surface without much interruption and moisture is
more easily evaporated from the exposed surface. As less
moisture is stored in the sponge-like forest floor, there
is a loss of water via surface run-off rather than as
evaporation back to the atmosphere. It is believed that this
reduction of moisture can reduce the rainfall regime,
leading to drier conditions. Soil temperatures which were
previously relatively stable will fluctuate more widely.
Heating of the forest soils can speed up the processes of
hardpan formation and nutrient leaching which quickly
render them useless for agriculture.
On a regional scale, extensive deforestation can change
surface temperatures and even regional air circulation
through the albedo change and alter the hydrological
properties. Increased run-off, increased evaporation,
decreased soil water storage and increased temperatures
can lead to a cycle of drying. Cloud cover may be reduced
as less moisture is returned to the atmosphere, giving
a positive feedback effect to higher surface tempera-
tures. Modelling studies of a complete deforestation of
Amazonia indicate irreversible climate change with
implications in other parts of the world. More realistic
assumptions of partial clearance give less consistent
predictions though most agree on a reduction of
precipitation.
Less obviously, tropical forests may have an impact on
the gaseous composition of the atmosphere, which in
turn can affect surface climate. As plants photosynthesize,
they use carbon dioxide from the atmosphere and
therefore contribute to the balance of this gas. If the
forests are cleared, the subsequent vegetation growth is
likely to have lower biomass and so extract less carbon
dioxide. Burning of the forest will directly add carbon
dioxide too. It has been estimated that if all the world's
rain forests were burned between 1986 and 2000 the
carbon dioxide concentration in the atmosphere would
have risen by up to 20 per cent.
Methane is another greenhouse gas which would
increase through forest clearance. The main sources of
methane are rice growing, biomass burning and cattle
ruminating (see
Chapter 9).
All these activities may
increase as forest is removed, and methane is a more
efficient greenhouse gas than carbon dioxide.
These effects lead us to the largest scale of climate
impact, as the addition of greenhouse gases would add
to global warming. Enhanced rates of clearance would
exacerbate the problem. General circulation models
(GCMs) have been used to predict what the impact of
total forest clearance would be at both the regional and
the global scale. The consensus view is that the direct
effects of deforestation on regional climate may be large
but the impact on global climate would be relatively small,
perhaps warming Earth by about 0·3
C.
Forest management
We have seen that tropical rain forests are rich ecosystems
which are being threatened by extensive clearance. At a
local scale, the change in land use may benefit a small
number of individuals, but at a world scale we are facing
a major crisis of the wholesale extinction of species and
habitats, which in turn may affect global climate. Can or
should anything be done about it?
Much deforestation takes place far away from the
centres of national government. In order to address the
problem we have got to examine why clearance is taking
place and what the alternatives may be. Passing legislation
which cannot be enforced is meaningless. We have also
got to be economically realistic. Brazil has one of the
largest areas of this natural resource. For Brazil to cease
development of that resource for the global good is not
feasible. It is often pointed out that many developed
countries cleared their own forests centuries ago. Why
should developing countries not benefit from the use of
available forest resources? Most of them are heavily
overburdened by debt to the developed countries or the
World Bank, so much of the revenue earned from forest
exploitation goes into interest payments.
One suggestion to conserve forests has been the
designation of national parks or nature reserves. By 1990
there were about 550 tropical forest parks which account
for about 4 per cent of all tropical forests. There has also
been a scheme to offset debts to developed countries in
exchange for retaining forest lands. In 1991 Mexico agreed
a debt-for-nature swap with Conservation International,
which agreed to purchase and write off US$4 M worth
of Mexican debt from foreign creditors. In return the
Mexican government agreed to invest US$2·6 M in rain
forest conservation. These schemes have declined in
popularity recently as they need strong government
