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二
):Desert Formation
TPO-2-2
：
Desert
Formation
The
deserts, which already occupy approximately a
fourth of the Earth's land
surface,
have
in
recent
decades
been
increasing
at
an
alarming
pace.
The
expansion of desert-like
conditions into areas where they did not
previously exist
is called
desertification. It has been estimated that an
additional one-fourth of the
Earth's
land surface is threatened by this process.
Desertification
is
accomplished
primarily
through
the
loss
of
stabilizing
natural
vegetation
and
the
subsequent
accelerated
erosion
of
the
soil
by
wind
and
water. In some cases the loose soil is blown
completely away, leaving a stony
surface. In other cases, the finer
particles may be removed, while the sand-sized
particles are accumulated to form
mobile hills or ridges of sand.
Even in the areas that
retain a soil cover, the reduction of vegetation
typically
results in the loss of the
soil's ability to absorb substantial quantities of
water. The
impact of raindrops on the
loose soil tends to transfer fine clay particles
into the
tiniest
soil
spaces,
sealing
them
and
producing
a
surface
that
allows
very
little
water
penetration.
Water
absorption
is
greatly
reduced;
consequently
runoff
is
increased,
resulting
in
accelerated
erosion
rates.
The
gradual
drying
of
the
soil
caused
by
its
diminished
ability
to
absorb
water
results
in
the
further
loss
of
vegetation, so that a
cycle of progressive surface deterioration is
established.
In
some regions, the increase in desert areas is
occurring largely as the result
of a
trend toward drier climatic conditions. Continued
gradual global warming has
produced an
increase in aridity for some areas over the past
few thousand years.
The
process
may
be
accelerated
in
subsequent
decades
if
global
warming
resulting from air pollution seriously
increases.
There
is
little
doubt,
however,
that
desertification
in
most
areas
results
primarily from human
activities rather than natural processes. The
semiarid lands
bordering the deserts
exist in a delicate ecological balance and are
limited in their
potential to adjust to
increased environmental pressures. Expanding
populations
are subjecting the land to
increasing pressures to provide them with food and
fuel.
In wet periods, the land may be
able to respond to these stresses. During the dry
periods
that
are
common
phenomena
along
the
desert
margins,
though,
the
pressure
on
the
land
is
often
far
in
excess
of
its
diminished
capacity,
and
desertification results.
Four
specific
activities
have
been
identified
as
major
contributors
to
the
desertification
processes:
overcultivation,
overgrazing,
firewood
gathering,
and
overirrigation.
The
cultivation
of
crops
has
expanded
into
progressively
drier
regions as population densities have
grown. These regions are especially likely to
have
periods
of
severe
dryness,
so
that
crop
failures
are
common.
Since
the
raising
of
most
crops
necessitates
the
prior
removal
of
the
natural
vegetation,
crop failures
leave extensive tracts of land devoid of a plant
cover and susceptible
to wind and water
erosion.
The
raising of livestock is a major economic activity
in semiarid lands, where
grasses are
generally the dominant type of natural vegetation.
The consequences
of an excessive number
of livestock grazing in an area are the
reduction of the
vegetation
cover
and
the
trampling
and
pulverization
of
the
soil.
This
is
usually
followed by the drying of the soil and
accelerated erosion.
Firewood
is
the
chief
fuel
used
for
cooking
and
heating
in
many
countries.
The
increased
pressures
of
expanding
populations
have
led
to
the
removal
of
woody
plants
so
that
many
cities
and
towns
are
surrounded
by
large
areas
completely lacking in trees and shrubs.
The increasing use of dried animal waste
as a substitute fuel has also hurt the
soil because this valuable soil conditioner
and source of plant nutrients is no
longer being returned to the land.
The
final
major
human
cause
of
desertification
is
soil
salinization
resulting
from overirrigation. Excess water from
irrigation sinks down into the water table. If
no
drainage
system
exists,
the
water
table
rises,
bringing
dissolved
salts
to
the
surface.
The
water
evaporates
and
the
salts
are
left
behind,
creating
a
white
crustal layer that prevents air and
water from reaching the underlying soil.
The
extreme
seriousness
of
desertification
results
from
the
vast
areas
of
land and the tremendous numbers of
people affected, as well as from the great
difficulty
of
reversing
or
even
slowing
the
process.
Once
the
soil
has
been
removed
by
erosion,
only
the
passage
of
centuries
or
millennia
will
enable
new
soil
to
form.
In
areas
where
considerable
soil
still
remains,
though,
a
rigorously
enforced program
of land protection and cover-crop planting may
make it possible
to reverse the present
deterioration of the surface.
译文：
TPO-2-2
：沙漠的形成
沙漠已经占据了地球陆地面积约四
分之一，
而且最近几十年正以惊人的速度
扩张。
沙漠化是指类似沙漠的环境漫延到原本并非沙漠的区域。
据估计，
地球表
面另外四分之一的地方正面临沙漠化威胁。
沙漠化主要通过以下过程实现：<
/p>
首先自然植被不断减少，
随后风力和雨水加
速了土壤的腐蚀。有的时候松散的土壤全部被风刮走，留下石质化的表层
;
其它
情况下细小的沙粒可能会被吹走，
而正常沙粒
大小的砂子不断堆积，
从而形成移
动的沙丘或者沙脊。