Sahara is a desert land form that covers one third of
African continent. Several factors such as sudden temperature changes, rainfall
and geographical features are some reasons for the formation of the desert land forms.
Based on the geographical location desert can be
divided into four types.
(i) Polar deserts:
This part of the region has inability to contain
water vapor. Because air to cold or moisture to hold the water vapor.
Examples – Arctic and Antarctic regions
(ii) Subtropical desert:
This type of deserts spread out across the two hemispheres.
During summer time desert change too hot and dry and during winter it falls
into cold and continuous dry. Therefore, rain evaporates quickly before it reaches
to the ground as a result of hot dry climate conditions.
Examples – Sahara, Thar desert
(iii) Continental or Coastal deserts:
Distance from the sea is the main factor to create
this type desert. When the air pass over the continents before it reaches to these
regions it lost moisture and become as a dry air.
Examples – Nambian desert (Africa), Attacama desert
(Chili)
(iv) Cold winter desert:
This desert known as semi-arid desert. This type of
desert contains long dry summers and cold winters.
Examples – Great Basin in America, Colorado plateau,
Red desert
Gobi desert – China
These regions receive only very lack of rainfall. Rain
shadow effect is the main reason for this little rainfall. Because when the air
rises over the mountain it prevents rain carrying clouds. Therefore leeward
side become as a dry region.
Examples – Himalaya Mountain range act as a big
barrier to stop the rainfall to reach Gobi desert.
Sahara
desert
Sahara is a world’s
largest hot desert. It covered one third of the African continent and
characterized as subtropical desert.
In ancient timed, this
desert was a vast green grassland area. Scientists provided following evidences
that several thousand years ago, Sahara desert had grassland with moisture:
Ø In
1990’s German scientists created thousand years of Earth’s climate model based
on computer simulation. According to this simulation they have mentioned,
climatic transition of the Sahara took place within 300 years.
Ø The
radar image, that had taken by Columbia shuttle in 1981 using radar reveals Western
Sahara desert sand topography has been characterized by wide river valleys that
located aside river valleys.
Additionally
this radar image reveals that beneath the desert sand topography with wide
black channels cut by meandering of an ancient river.
Ø Beneath
the Sahara desert, the radio carbon dating reveals, very porous and permeable
type of rock known as the Nubian sandstone, is about 900m thick and stores
enormous quantities of ground water around 18,000 cubic kilometers.
These analyses shows,
the existence of 35,000 years old ground water and indicating that this water
is result of precipitation fell in last ice age.
Desertification
of Sahara
(1) Changes in the
Earth’s orbit
Scientists suggest Sahara
desert dried up due to the changes of Earth’s orbit. These changes affect the
amount of electromagnetic energy which receives by Earth from sun.
In a simple world, the
shining of sunlight on a particular place depends on geographical location,
time of day, seasons, landscape and local weather.
Climatologist Gavin
Schmidt (NASA’s Goddard Institute for space studies) suggests that 8000 years
ago, Earth’s orbit slightly different from today’s position. Earth tilt changes
from 24.10 to 23.50. Additionally Schmidt explained, Earth’s
closest approach to the Sun in Northern hemisphere took place in August at
summer and today’s this movement happen in January.
Earth’s orbit tilt gets
changes by gravitational force that reveals by other bodies in solar system.
In every year earth’s
tilt get changes from 22 to 25 degree.
(2) Climatic changes
Green Sahara suddenly
became to end in 5,500 years ago. Schmidst believed that slowest changes in
Earth’s orbit disturbed that ecosystem.
A timeline of Sahara occupation:
Ø 10,500
- 9000 years ago: Sahara turned as a habitable zone by monsoon rainfall.
Ø 9000
– 7,300 years ago: Continuous rainfall, growth of vegetation, migration of
animals leads to development of human settlement.
Ø 7,300
– 5,500 years ago: Retreating of rainfall reason for human migration. End of
rainfall and desert conditions took place in 5,500 years ago.
References:
(3) National Institute
of Science Research/ Eagle.com
(5) The Encyclopedia of
Earth (2008); Weldon Owen pty Ltd; Australia
