Deserts cyclically expand and contract, reflecting global environmental
changes. Many civilizations on the planet are thought to
have met their demise because of desertification of the lands they
inhabited, and their inability to move with the shifting climate zones.
Desertification is defined as the degradation of formerly productive
land, and it is a complex process involving many causes, including
climate change and misuse of the land. Climates may change, and
land use on desert fringes may make fragile ecosystems more susceptible
to becoming desert. Among civilizations thought to have
been lost to the sands of encroaching deserts are several Indian
cultures of the American southwest such as the Anasazi, and many
peoples of the Sahel, where up to 250,000 people are thought to
have perished in droughts in the late 1960s. Expanding deserts are
associated with shifts in other global climate belts, and these shifts
too are thought to have contributed to the downfall of several societies,
including the Mycenaean civilization of Greece and Crete, the
Mill Creek Indians of North America, and the Viking colony in
Greenland. Many deserts are presently expanding into previously
productive lands creating enormous drought and famine conditions.
For instance, Ethiopia, Sudan, and other countries in the horn
of Africa have suffered immensely in the past few decades with the
expansion of the Sahara desert into their farmlands.
Desertification is the invasion of a desert into nondesert
areas, and it is an increasing problem in the southwestern United
States, in part due to human activities. Most notably, water is being
moved in huge quantities into California, and people are moving
into desert areas in vast numbers, all seeking water from limited
groundwater supplies. This decreases water supply, vegetation,
and land productivity, with the result being that about 10 percent of
the lands in this country have been converted to desert in the last
100 years, while nearly 40 percent are well on the way. Desertification
is also a major global problem, costing hundreds of billions of
dollars per year. China estimates that the Gobi Desert alone is
expanding at a rate of 950 square miles per year (2,460 km2/yr), an
alarming increase since the 1950s when the desert was expanding
at less than 400 square miles per year (1,035 km2/yr). The expansion
of the Gobi is estimated to cost $6.7 billion a year in China and
affects the livelihood of more than 400 million people through
decreased crop yields and forced migrations of people from formerly
productive areas to cities.
Desertification is beginning to drastically alter the distribution
of agriculture and wealth on the globe. If deserts continue to
expand, within a couple of hundred years the wheat belt of the central
United States could be displaced to Canada, the sub-Saharan
Sahel might become part of the Sahara, and the Gobi Desert may
expand out of the Alashan Plateau and Mongolian Plateau.
Desertification is a multistage process, beginning with
drought, crop and vegetation loss, and then establishment of a
desert landscape. Drought alone does not cause desertification,
but misuse of the land during drought greatly increases the
chances of a stressed ecosystem reverting to desert. Desertification
is associated with a number of other symptoms, including
destruction of native and planted vegetation, accelerated and high
rates of soil erosion, reduction of surface and groundwater
resources, increased saltiness of remaining water supplies, and
famine. Desertification can be accelerated by human-induced
water use, population growth, and settlement in areas that do not
have the water resources to sustain the exogenous population.
Eastern California, Nevada, Arizona, and New Mexico all are experiencing
problems related to rapidly increasing populations settling
into regions with scarce water supplies, leading to desertification
of fragile ecosystems.
Drought often presages the expansion of desert environments,
and regions like Africa’s sub-Saharan Sahel have experienced
periods of drought and desert expansion and contraction,
several times in the past few tens of thousands of years. At present
much of the Sahara is expanding southward, and peoples of the
Sahel have suffered immensely.
Droughts may begin imperceptibly, with seasonal rains often
not appearing on schedule. Farmers and herdsmen may be waiting
for the rains to water their freshly planted fields and to water their
flocks, but the rains do not appear. Local water sources such as
streams, rivers, and springs may begin to dry up until soon only
deep wells are able to extract water out of groundwater aquifers.
This is typically not enough to sustain crops and livestock, so they
begin to be slaughtered or die of starvation and dehydration. Crops
do not grow, and natural vegetation begins to dry up and die.
Brushfires often come next, wiping away the dry brush. Soon people
start to become weak, and they cannot manage to walk out of
the affected areas, so they stay and the weak, elderly, and young of
the population may die off. Famine and disease may follow, killing
even more people.
One recent example of this is highlighted by the Sudan, where
years of drought have exacerbated political and religious unrest,
and opposing parties raid Red Cross relief supplies, sabotage the
other side’s attempts at establishing aid and agriculture, and the
people suffer. The Sudan is in the sub-Saharan Sahel, which is a
large region between about 14° and 18° N latitude, characterized by
scrubby grasslands, getting on average 14–23 inches (36–58 cm) of
rain per year. In the late 1960s this amount of rainfall had fallen to
about half of its historical average, and the 25 million people of the
Sahel began suffering. One of the unpleasant aspects of human
nature is that slow-moving, long-lasting disasters like drought tend
to bring out the worst in many people. War and corruption often
strike drought-plagued regions once relief and foreign aid begins to
bring outside food sources into regions. This food may not be
enough to feed the whole population, so factions break off and try to
take care of their own people. By 1975 about 200,000 people had
died, millions of herd animals were dead, and crops and the very
structure of society in many Sahel countries were ruined. Children
were born brain-damaged because of malnutrition and dehydration,
and corruption had set in. Since then the region has been plagued
with continued sporadic drought, but the infrastructure of the region
has not returned and the people continue to suffer.
severe droughts in recent times. The Middle East and parts of
the desert southwest of the United Sates are overpopulated
and the environment is stressed. If major droughts occur in
these regions, major famines could result and the land may be
permanently desertified.
Location and Formation of Deserts
More than 35 percent of the land area on the planet is arid or
semiarid, and these deserts form an interesting pattern on the
globe that reveals clues about how they form. There are six
main categories of desert, based on their geographic location
with respect to continental margins, oceans, and mountains.
Trade Wind or Hadley Cell Deserts
Many of the world’s largest and most famous deserts are
located in two belts between 15° and 30°N and S latitude.
Included in this group of deserts are the Sahara, the world’s
largest desert, and the Libyan Desert of North Africa. Other
members of this group include the Syrian Desert, Rub a’Khali
(Empty Quarter), and Great Sand Desert of Arabia; the
Dasht-i-Kavir, Lut, and Sind of southwest Asia; the Thar
Desert of Pakistan; and the Mojave and Sonoran Deserts of
the United States. In the Southern Hemisphere, deserts that
fall into this group include the Kalahari Desert of Africa and
the Great Sandy Desert of Australia, and this effect contributes
to the formation of the Atacama Desert (South
America), one of the world’s driest places.
The location of these deserts is controlled by a largescale
atmospheric circulation pattern driven by energy from
the Sun. The Sun heats equatorial regions more than high-latitude
areas, which causes large-scale atmospheric upwelling
near the equator. As this air rises, it becomes less dense and
can hold less moisture, which helps form large thunderstorms
in equatorial areas. This drier air then moves away from the
equator at high altitudes, cooling and drying more as it
moves, until it eventually forms two circum-global downwelling
belts between 15 and 30°N and S latitude. This cold
downwelling air is dry and has the ability to hold much more
water than it has brought with it on its circuit from the equator.
These belts of circulating air are known as Hadley Cells
and are responsible for the formation of many of the world’s
largest, driest deserts. As this air completes its circuit back to
the equator, it forms dry winds that heat up as they move
toward the equator. The dry winds dissipate existing cloud
cover and allow more sunlight to reach the surface, which
then warms even more.
Deserts formed by global circulation patterns are particularly
sensitive to changes in global climate, and seemingly
small changes in the global circulation may lead to catastrophic
expansion or contraction of some of the world’s
largest deserts. For instance, the sub-Saharan Sahel has experienced
several episodes of expansion and contraction of the
Sahara, displacing or killing millions of people in this vicious
cycle. When deserts expand, croplands are dried up, and livestock
and people can not find enough water to survive.
Desert expansion is the underlying cause of some of the
world’s most severe famines.
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