September 2008
576
John Rutter
Causes of Desertification
Strictly speaking, desertification
should perhaps be defined as the
making of a desert, but there is
considerable disagreement amongst
scientists over what exactly the term
means – the internet has over 100
different definitions. Confusion arises
because nobody seems sure of the
exact causes of desertification, or what
the most important causes are.
Chinese scientist Zhu, for instance,
said ‘desertification is an
environmental degradation process
created as a result of the influence of
excessive human activities’, defining
the process as an extreme form of
man-made land degradation, but
completely missing the influence of
climate change.
Scientists are reluctant to put their
necks on the line and say exactly why
desertification occurs. The situation
has not been helped by media reports
over the last 30 years that have
exaggerated the growth of the Sahara
every time there is a new drought in
Africa. The expansion and contraction
of the world’s arid areas is a situation
that has been going on for millennia,
but nowadays, with the increasing
world population, more people than
ever are at risk.
The UN has issued perhaps the
simplest definition when it says:
‘Desertification is the ... destruction of
the biological potential of the land
which can ultimately lead to desertlike conditions’. This definition leaves
open the causes of desertification and
both human and natural processes can
be examined for their respective
contributions.
A global problem
According to the UN, desertification
puts at risk the health and well-being
of more than 1.2 billion people in
more than 100 countries. Other
countries face indirect consequences,
such as mass immigration from
desertified areas. Often its most severe
influence is seen in the damaging
droughts and famines of sub-Saharan
Africa but, as Figure 1 shows, no
populated region of the planet remains
unaffected. Desertification was a
major cause of mass migration within
1930s USA and is now affecting huge
areas of the former Russian republics
of central Asia. It has been implicated
in the shrinking of the Aral Sea and is
also severe in parts of Europe
including Portugal and Spain.
While evidence on the ground shows
that, in certain areas, the loss of
productive land is taking place on a
locally significant scale, there has
recently been a great deal of debate as
to whether or not the worldwide
problem has been exaggerated. Official
Chinese statistics, for instance, say the
country’s deserts are shrinking by
7,585 sq km each year ‘due to the
efforts of local governments and
Figure 1: Areas at risk from desertification
4
3
2
1
5
Key
Severe risk
Moderate risk
Slight risk
1 Ethiopia
18% at risk
2 Sudan
23% at risk
3 Chad
30% at risk
4 Niger
42% at risk
5 Somalia
26% at risk
Source: US Department of Agriculture, Natural Resources Conservation Service, Soil Survey Division, World Soil Resources, Washington, DC
Geofile for AQA © Nelson Thornes 2008
GeoFile Series 27 Issue 1
Fig 576_01 Mac/eps/illustrator 11 s/s
NELSON THORNES PUBLISHING
Artist: David Russell Illustration
September 2008 no.576 Causes of Desertification
Figure 2: Severe rill and gully erosion
ago. The influence of climatic change
over the last few thousand years is
more obvious in the deserts of Africa,
North America and central Asia, but
even here the evidence is not from the
recent past.
Photolibrary/Oxford Scientific
More obvious is that, over the last 50
years, across the world’s
desertification risk areas, there has
been massive population growth. In
some areas, the land has been rescued
from soil erosion by protecting trees
and conserving water, but, more often
than not, it seems human
mismanagement of marginal areas is
the primary cause of desertification.
Population and poverty
Source: http://humanities.cqu.edu.au/geography/GEOG11024/week_6.htm
Figure 3: Population statistics for the countries of the Sahel (UK provided for
comparison)
Country
Population
total (2005)
Birth rate
per 1000
population
(2003)
Senegal
11,658,000
Mauritania
3,069,000
Mali
13,518,000
Burkina Faso
13,228,000
Niger
13,957,000
Nigeria
131,530,000
Chad
9,749,000
Sudan
36,233,000
Eritrea
4,401,000
United Kingdom 59,668,000
Death rate
per 1000
population
(2003)
34
34
48
43
48
43
45
33
37
12
13
15
23
19
19
18
16
10
13
10
Population
change average
% per annum
2000–2005
2.4
3.0
3.0
3.0
3.6
2.5
3.0
2.2
3.7
0.3
Source: Statistics from Collins Student World Atlas 2005 and 2007
people’, compared with an annual
expansion of 10,400 sq km at the end
of the last century. The UN has also
come under fire for putting too much
emphasis on skewed information
based on a questionnaire sent to subSaharan countries in 1982, when the
continent was in the grip of a series of
exceptionally dry years and famine.
It is evident, however, that parts of
many semi-arid countries remain at
risk from the threat of desertification,
even if only on a small scale, and the
reasons can be neatly separated into
climatic factors and the problems
caused by a growing world population.
Climate change
The media is obsessed with climate
change, providing, as it does, an
Geofile for AQA © Nelson Thornes 2008
endless source of stories of doom and
gloom. It is unsurprising, therefore,
that desertification has also been
blamed on the world getting warmer.
In recent times, long periods of
drought in Africa have been followed
by series of wet years but, apart from
global warming, the cause of these
climatic fluctuations – which have
seen rainfall 48% lower than average
in the central savanna belt over the
past 30 years – could be a general shift
in global wind patterns, or the El Niño
phenomenon.
The planet has had a variable climate
for millennia. The driest deserts, such
as the Atacama and the Namib, show
few signs of climatic change, and the
typical features of desertification such
as rills and gullies (Figure 2) were
carved into the landscape centuries
The planet’s cultivatable land is, more
or less, a finite resource being put
under pressure by one that is less finite
– the growing population. In Mali, one
of the countries of the Sahel which
forms the border between the Sahara
and the savanna lands further south,
the population increased by 3% every
year from 2000 to 2005, giving a total
increase of around two million people
in a country of 13 million. These extra
people need land and food in a country
that is already having trouble feeding
itself. The story is the same across the
Sahel (Figure 3) and in many other
LEDCs where increasing populations
put pressure on the land, causing
degradation on a large scale. There are
a number of different ways in which
this pressure is felt:
• Overcultivation: peasants farming
crops are being forced to increase
the yield from their land. Fallow
periods – leaving the land bare to
regenerate and regain nutrients –
are being ignored and the soil is
losing fertility. Rising populations
are forcing farmers into cropping
more marginal areas on the fringes
of the deserts. This is fine in years
of abundant rainfall but when the
rains fail the soils quickly degrade.
• Overgrazing: as the number of
people has increased so has the
worldwide animal population.
Herds of cows, goats and sheep
concentrate in certain areas,
stripping the vegetation back and
exposing the soil to erosion. Great
pressure is put on cultivated areas
around the boreholes and wells
where the animals drink. The
trampling of the ground by
animals also leads to soil
compaction, destroying the
structure and leaving it open to
erosion.
September 2008 no.576 Causes of Desertification
• Deforestation: rising populations
need wood for building and fuel,
leading to widespread destruction
of trees. Since 1900, for instance,
90% of the forest cover has been
cleared from the Ethiopian
highlands. Removing the trees
exposes the soil to erosion from
wind and rain. The increasing
scarcity of trees also means people
use precious animal dung for fuel,
instead of putting it on the land to
help maintain fertility.
• Cash crops: during the 20th
century many countries were
encouraged to grow cash crops –
food produced for export – as a way
of gaining foreign exchange. The
concentration on a single crop
such as cotton or rice resulted in
widespread reduction of soil
fertility. Inappropriate farming
techniques such as flood irrigation
(drowning fields with water) led to
increasing salt content in the soils
and further degradation.
Figure 4: Dust storm off the coast of Morocco
The increasing population is also
forcing more and more people, such as
the previously nomadic Rendille in
Kenya, to move off the land to live in
towns and cities. In many countries
the increased urban population has to
be supplied with scarce fuelwood from
the countryside that surrounds it.
• Rain in semi-arid and arid lands is
concentrated in intense downpours
which land heavily on the soil,
dislodging particles and, if on a
hillside, forcing them further
down the slope.
• Raindrops clog up the soil, filling
holes with water and dislodged
particles, reducing infiltration.
Sheet wash results as a very thin
film of water that flows over the
surface of the soil, washing away
the most valuable and nutrientrich topsoil.
• Serious rain erosion may also
occur in channels in the landscape,
forming features known as rills
and gullies. These eroded channels
(Figure 2) can be very numerous,
destroying soil fertility and
making the land almost impossible
to farm.
Physical processes
While the influence of man is, perhaps,
the overriding factor in leaving soils
open to erosion and degradation, it is
physical processes that are responsible
for the actual erosion itself. Once the
soil has been rendered infertile and its
structure has been broken down, it is
very susceptible to the actions of both
wind (a process known as aeolian
erosion) and rain.
• Strong winds blow over flat land
stripped of vegetation, picking up
the finest soil particles,
transporting them in suspension
and forming the huge dust clouds
associated with desert landscapes.
Saharan dust has been found in the
Caribbean and southeast England
(Figure 4), and the path of a single
dust storm can be up to 4000km.
• Larger particles are blown by the
wind in a series of short hops or
bounces, dislodging other soil
particles as they land in a process
known as saltation.
• Finally, in a strong enough wind,
the biggest particles roll or slide
along the soil surface as surface
creep (Figure 5).
Geofile for AQA © Nelson Thornes 2008
Source: NASA’s Earth Observatory
http://earthobservatory.nasa.gov/NaturalHazards/natural_hazards_v2.php3?img_id=11950
Case study – The Sahel
The classic study of desertification
comes from the Sahelian countries
where the problem was first identified
and whose plight brought the
phenomenon to worldwide attention.
The causes and effects of
desertification in the Sahel are
implicated in present-day disasters
such as the civil war in the Darfur
region of Sudan.
After a period of above-average rainfall
in the middle of the last century, the
Sahelian countries have suffered low
rainfall almost every year since 1968,
with periods of severe drought in the
early 1970s and 1980s. In between, the
production of drought-resistant foods
such as sorghum and millet had been
increasing by 1% per year.
Unfortunately at the same time
population was increasing at around
2.5% per year – food production could
not keep pace. In the area bordering
the southern Sahara the carrying
capacity of the land was only 0.3
people per sq km, but the population
density was two. South of this, where
nomadic herders share the land with
settled farmers, the carrying capacity
of 15 per sq km was still substantially
lower than the actual population
density of 20. The land has become
increasingly unable to support the
number of people living on it as the
population has increased.
Meanwhile, the governments of the
Sahelian countries have also
encouraged the production of cash
crops such as cotton and peanuts, at
the expense of food crops. This,
combined with population pressure,
has forced families to expand the
cultivation of staple food crops into
marginal lands. Fallow periods in
which the land was left bare and
traditionally allowed to recover
fertility have been ignored. Trees have
been cut down to supply fuelwood and
to open up land for more crops, and
the bare ground has been left exposed
to erosion by wind and rain.
Other problems include the increase
in numbers and herds of cattle,
traditionally a sign of wealth and
September 2008 no.576 Causes of Desertification
standing in many countries of the
region. These cattle congregate on the
best pastures, or trample ‘rings of
desolation’ around the waterholes,
destroying the soil structure. The
climate of the Sahel may also be
getting drier, thereby increasing the
prospect of land degradation.
Rich countries with poor
country problems?
Although desertification is often at its
most severe in LEDCs, there is no
doubt that many MEDCs also have
serious cause for concern. It is only 70
years since large numbers of poor
farmers in the American Midwest were
forced from their land in what became
known as the ‘dustbowl’ years. In a
series of events similar to more recent
history in the Sahel, the need for wheat
following World War One combined
with a series of years of above-average
rainfall, led to the cultivation of
marginal land. When drought hit the
weakened soil in the 1930s, huge dust
storms afflicted the region and many
were forced to seek refuge in the
expanding cities of California.
Poor farming practices were the main
driving force behind desertification in
the USA and these are still being used
in countries such as China and
Vietnam today. Monoculture planting
of the same crop (e.g. coffee in
Vietnam) strips the land of nutrients,
while ploughing destroys the soil
structure and forms channels down
which rainwater is concentrated.
Nowadays, most MEDCs are aware of
the dangers of desertification, but that
does not mean the long-term need for
soil conservation is always upheld
when short-term profit can be made
from the land.
Case study – Europe’s desert
Almeria, in southern Spain, is
Europe’s only truly arid area, with an
average annual rainfall of less than
300mm. Fifty years ago there were
large areas of irrigated farmland
growing vegetables and fruit for
subsistence use, combined with
terraced dry land growing cash crops
such as olive and carob. Working the
land was hard, however, and mass
depopulation of the countryside has
taken place since the mid–1950s.
Young people were attracted to the
bright lights and easier work in the
towns and cities, and the land was
abandoned. The terraces that
Geofile for AQA © Nelson Thornes 2008
Figure 5: Processes of aeolian (wind) erosion
Suspension – smallest particles
carried by the wind as dust storms
Surface creep – the
largest particles
roll along the ground
in the strongest winds
Saltation – larger particles
transported short distances
by the wind by ‘bouncing’
or ‘hopping’
Source: http://www.seafriends.org.nz/enviro/soil/
GeoFile Series 27 Issue 1
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protected the land from soil
erosionTHORNES
swimming
pools, its golf courses,
NELSON
PUBLISHING
for centuries quickly fell into
showers
and other tourist facilities,
Artist: David Russell
Illustration
disrepair. As walls tumbled, the short,
dramatic rainstorms that characterised
the region washed away the soil from
the exposed hillsides.
further depleting the precious water
table. In some areas, the underground
aquifers are being contaminated by sea
water.
As the farmers moved out, shepherds
moved in with goats and sheep and
stripped the land of its increasingly
sparse vegetation. Over-grazing led to
removal of plants and soil compaction,
then rain splash and run-off resulted
in the loss of topsoil.
The Spanish government has taken
steps, both large and small-scale, to
combat desertification, but this
conflicts with the need for tourist cash
– Benidorm itself provides 1% of the
country’s GDP.
Meanwhile, human activity has caused
the water table to drop. Large deposits
of gypsum rock – an absorbent strata
that formed a huge underground store
of water– have been quarried for use in
the building trade. Much of this
material has been destined for the
huge tourist resorts along the Spanish
coast. Benidorm, for instance, has
grown from its fishing village origins
40 years ago to receive four million
visitors a year. The town uses huge
amounts of water for its 30,000
F o c u s
Desertification – an ongoing
problem
While many scientists now think
desertification may not be a serious
problem on a global scale, there is no
doubt that land degradation affects
many people on a local basis. Many
initiatives are now stopping the spread
of desert-like conditions but, with an
ever-increasing population and the
threat of serious climate change, it
seems likely the problem will remain
for many years to come.
Q u e s t i o n s
1 Describe the global distribution of countries at risk from
desertification with reference to slight, moderate, and severe risk.
2 What are the main causes of desertification? Discuss whether the causes
are mainly man-made or natural in origin.
3 Compare and contrast the causes of desertification in MEDCs and
LEDCs. In which type of country is the problem more severe, and why?
You should refer to specific countries in your answer.
4 What effect has the concentration on the production of cash crops in
LEDCs had on desertification?