unit11 desertification by 4o2Oob8

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DESERTIFICATION


E d i t e d by J o e N a u m a n n
    X. Desertification
       A. Deserts: areas of low rainfall…..
    Arid:         less than 10 cm (4 in)
    Semi-arid:    less than 25 cm (10 inches)




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    X. Desertification
        A. Deserts
        B. Drylands: What life cares about.
    Effective Moisture: the ratio of precipitation
    (PPT) to potential evapo-transpiration (PET).
    Hyperarid: PPT/PET <0.05
    Arid:      PPT/PET 0.05 to 0.20
    Semi-arid: PPT/PET 0.20 to 0.5
    Dry sub-humid:        0.5 to 0.65
    Humid:           PPT/PET >1
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    X. Desertification
       A. Deserts
       B. Drylands
       C. Distribution
              1. Sunbelt
              2. Rainshadow
              3. Continental interior
              4. Coastal
       Adjacent to these real deserts are the
       sensitive drylands.
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    X. Desertification
       A. Deserts
       B. Drylands
       C. Distribution
       D. Desertification:the degradation of land
       in arid, semi-arid, and dry sub-humid
       areas.
       Can be caused by human activity or
       climate change.


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    X. Desertification
       D. Desertification:the degradation of land in
       arid, semi-arid, and dry sub-humid areas.
         1. Causes of Desertification: Complex
       interplay of physical and ecological
       processes, and developmental and socio-
       economic forces. For example:
               a. brief period of excess rainfall
                  “the rain follows the plow”
               b. over irrigation (salanization)
               c. ground-water depletion
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    X. Desertification
       D. Desertification: the degradation of land
       in arid, semi-arid, and dry sub-humid areas.
         1. Causes of Desertification: Complex
       interplay of physical and ecological
       processes, and developmental and socio-
       economic forces. For example:
         2. Drought: rainfall significantly below
       normal for an extended period.
              several years
              more than 25% less than normal
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    Dryland regions of the world (yellow)
    These are sensitive to desertification
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    D. Desertification:
     4. The Sahel as an example

    climate
    climate change
    agricultural practices
    political process
    population


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     D. Desertification:
       3. The Sahel as an example
            a. Sub-saharan rainfall index shows
     drought from 1970 to 2000.
            b. Impact on human activity in the
     Sahel.




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     D. Desertification:
       3. The Sahel as an example
            a. Sub-saharan rainfall index shows
     drought from 1970 to 2000.
            b. Impact on human activity in the
     Sahel.
            c. Possible explanations for the Sahel




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Remember what drives the monsoon circulation.

• Strong seasonal pressure gradient between land
    and sea (hot land mass)
• Warm oceans to supply moisture.
• Sufficient recycling of rainfall by plants
    (evapotranspiration) to allow moisture to
    reach deep into the interior.



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  D. Desertification:
    c. Possible explanations for the Sahel
• Natural climate variability:
  Monsoon activity controled by sea surface
  temperatures, related to ENSO variability.
• Inappropriate technology/misguided foreign
  aid.
• Aerosols produced by industrialization in
  Europe cool Sahel, reducing strength of the
  African Monsoon.
    d. Positive feedbacks
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     D. Desertification:
      c. Possible explanations for the Sahel
      d. Positive feedbacks


     Fuel wood gathering and stock overgrazing
     reduces vegetation cover. Two impacts:
     Albedo increases: cooling the region in summer
     and weakening monsoon circulation.
     Less vegetation = less evapotranspiration. So less
     water vapor in the atmosphere to fuel the monsoon
     and deliver precipitation over the Sahel region.

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     D. Desertification:
      4. The Sahel as an example
      5. Desertification in the US




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 Precursers
    10-fold increase in population between
    1860 and 1920.
    Deep plowing and monoculture destroyed
    soil structure and increased sensitvity to
    erosion.
 Additional factors:
    Great Depression: no $ for prairie farmers
    Out migration “Okies”
    True drought.

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    D. Desertification:
5. Desertification in the US: The Dust Bowl of the
    1930s.
    Precursers
    Causes
    Consequences: Changed farming practices
      Plowing techniques
            Shallow, along contours
            Established windbreaks
            Crop rotation
            Irrigation
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    D. Desertification:
 5. Desertification in the US: The Dust Bowl of
    the 1930s.
    Precursers
    Causes
    Consequences: Changed farming practices
    Better? Yes, but….



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26   Phoenix, Arizona in the early 1970s
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