A desert often refers to a certain area of land that is barren due to little precipitation. These conditions create a harsh environment for flora and fauna. A lot of the surface of the ground lacks vegetation and therefore exposed to the process of denudation. It is important to note that at one third of the earth in arid or semi-arid in nature. Deserts are sometimes classified according to the amount of precipitation they receive in addition to the temperature that is constant in that area. Another factor of classification is the causation of desertification based on geographical location. Therefore, there are hot and cold deserts. An example of a hot desert is the Atacama and a cold one, the Arctic. An example of a desert that experiences less than 9.8 inches of precipitation per year is Phoenix in Arizona partly because of its aridity (lack of vegetation cover). AlaskaÆs Brooks Range also receives the same amount of precipitation and is classified as cold desert. Other regions of the world that have cold deserts include the Himalayas as well as other arid areas located in high altitude areas. Polar deserts such as the Arctic and Antarctic are considered so because of their lack of vegetative cover and inability to support human populations .
These climatic variations can be accounted for through changes in solar radiation that come about because of slow shifts in the earthÆs orbit. The summer monsoon rains are strengthened at the beginning of the Holocene period. These rains increased the vegetative cover and brought about two significant changes: an increase in the ability to recycle water back to the atmosphere through a process known as evapotranspiration. Secondly, there was a reduction in the surface reflectance . These effects contributed to the additional summer rains fueled by the monsoon winds. Most of Africa maintained a green vegetation especially in the Sahara until the middle Holocene that led to the sudden rise of the desert current Sahara desert. The process of aridization further led to desertification as there were instances of decreased vegetative cover and moisture into the atmosphere. This further precipitated the arid conditions .

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The Sahara, one of the hot deserts in the world and the third largest in size, covers large parts of Egypt, Mali, Libya, Chad, Niger, Morocco, Sudan, Western Sahara and Mauritania. It is about 9 million square kilometers in size which represents about 31% of Africa. This area receives less than 250mm of rain. It is considered to be hyper arid with small hints of vegetation. In the northern and southern regions of the desert, there are sparse populations of desert shrub and trees where moisture collects. The desert has numerous sub-divisions due to its monumental size and these include the Tanezrouft and the Libyan Desert. These areas are vulnerable to aridity because they do not receive rainfall for many years and the only source of rainfall in this region is the Intertropical Convergence Zone. This a belt of continuous low-pressure systems specifically at the equator that contribute to the sparse and erratic rainy season in the Sahel. Rainy seasons are often accompanied by various impediments such as the physical terrain and atmospheric conditions of the desert that minimize instances of precipitation. Therefore, this region has high sunshine duration and unreliable yet low rainfall periods. The temperatures are also extremely high in the day and very cold in the night (contributing to its high diurnal temperature variation). Additionally, the area also experiences very low rates of relative humidity and significant rates of evaporation ever recorded in the world

With gradual and concrete changes in climatic conditions in the world, the factors for desertification have shifted from geographical factors to human-based processes. The immediate loss of vegetative cover can be attributed to a number of factors such overpopulation, overgrazing and exploitation of soil covers due to increased and continuous tillage of the land. Other factors include deforestation for human settlement. Instances of soil erosion are decreased significantly when there is increased vegetative cover and a decline in human activity. Surfaces that are left bare are vulnerable to winds that blow away the soil leaving infertile layers which then become unproductive. The encroachment of deserts as a result of the processes is rapidly increasing. There have been controversial suggestions that controlled human and animal activity can reverse desertification. This is because as the process of desertification continues, the landscape undergoes major changes and only slopped terrain is spared. However, a well panned out strategy in reversing the effects would be to increase agricultural activities that would create vegetative cover that subsequently attracts rain. With the increased industrial and chemical uses in the world, the effects of climate change are being felt immensely. This is evident in the change of composition of hot and cold deserts. For instance, the acreage of the Sahara is increasing due to unmitigated human activities while in the Artic, industrial processes are leading to the melting of glaciers and in return making these areas much warmer than they were before. Therefore, rather than geographical processes and factors that shaped deserts occurring, they have been replaced with human centered activities. Maybe in the centuries to come, there will be the emergence of more deserts than they currently are.

    References
  • Hurrell, James W., Yochanan Kushnir and Geir and Visbeck, Martin Ottersen. An Overview of the North Atlantic Oscillation. American Geophysical Union, 2003.
  • Laity, Julie J. Deserts and Desert Environments: Volume 3 of Environmental Systems and Global Change Series. John Wiley & Sons, 2009.
  • United Nations Environmental Programme. Glacial Periods and the Evolution of Modern-day Deserts. UNEP, Jan. 2006, www.unep.org. Accessed 16 November 2016.
  • Waghorn, Terry. Holistic Land Management: Key to Global Stability. Forbes, Dec. 2012, http://www.forbes.com/sites/terrywaghorn/2012/12/20/holistic-land-management-key-to-global-stability/#1f303e966b8c. Accessed 16 November 2016.
  • Wickens, Gerald E. Ecophysiology of Economic Plants in Arid and Semi-Arid Lands. Springer, 1998.