ES 1111

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ES 1111

• Lecture 4 - Desertification

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Desertification

• Desertification is the expansion of desert-like
  conditions into areas where they did not
  previously exist
• Most regions undergoing desertification lie on
  the fringes of existing deserts
• Not a smooth process can be halted in wet years
• Each year, 27,000 square miles turns to desert
• Total area threatened is ¼ the land of Earth

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Areas Prone to Desertification

• Figure showing the extent of the land surface
  vulnerable to desertification. Many of the
  regions border existing deserts

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Onset of Desertification

• Semi-arid locations have sparse vegetation
• Desertification is accomplished primarily through
  the loss of the stabilizing natural vegetation
• If vegetation gets reduced,
• More soil is exposed to erosion by wind and water
• Runoff increases because the ground becomes hard
  and soil pore spaces are sealed off by compacted
  clay, resulting in decreased soil water storage
  and further decrease of vegetation
• Albedo increases
• Less moisture results in less latent heat flux,
  which would yield higher temperatures
• Increased albedo would result in a cooling trend

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Transition to a Desert (text)

• Models indicate that the cooling trend from a
  higher albedo offsets the warming trend due to a
  lower latent heat flux
• Cooling at the surface would induce large scale
  sinking air from above (subsidence)
• Large-scale subsidence would inhibit cloud
  formation and precipitation
• Aridity would increase as a result

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Initiation of Desertification

• Removal of vegetation was the primary cause that
  started the process
• Removal of vegetation can happen by
• Climatic changes (drought)
• Recall that semi-arid regions in the subtropics
  get their precipitation from the meandering ITCZ
• Droughts occur in years where the ITCZ fails to
  appear at that location and a lack of rainfall
  occurs
• Over-grazing by livestock (numbers increase with
  population)
• Planting of unsuitable vegetation by man (removes
  native plants)
• Firewood gathering (primary source of fuel in
  poor areas)
• Overirrigation of crops (raises water table,
  salts left behind in evaporation, salts are toxic
  to crops)

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Example 1 The Sahel

• South of the Sahara Desert lies grasslands
• Nomadic herdsmen with increasing number of goats
  removed the grasses at an increasing rate
• With the loss of the grasses, weathering
  increased and soil moisture decreased, killing
  more grasses
• Grasslands turned to sandy desert (200 miles
  south of previous position of Sahara desert)
• Led to widespread famine in the 1960s through
  1980s 250,000 deaths (millions of livestock)
• Overgrazing combined with drought combined to
  induce this incident of desertification

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Example 2 The Dust Bowl

• Great Plains of the United States suffered a
  drought in the 1930s
• Originally, drought-resistant grasslands were in
  this region
• With the arrival of settlers, crops such as
  cotton and wheat were planted (drought-prone
  crops)
• Loss of rainfall led to loss of crops, which left
  the top soil prone to erosion
• Severe dust storms removed top soil rapidly
• The planting of crops better suited for the
  geography, along with improved measures to
  preserve topsoil, have resulted in no repeat
  performance of the Dust Bowl era

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Preventing Desertification

• Climate shifts can initiate desertification, and
  there isnt anything we can do to steer the
  ITCZ over drought areas
• We can prevent desertification that is in part
  due to man by
• NOT overgrazing grasslands
• NOT overirrigating crops
• NOT planting unsuitable vegetation
• NOT chopping down trees for firewood
• Preventing wind erosion of topsoil (snow fences,
  trees)