Transcript Unit XI: Desertification

X. Desertification
A. Deserts: areas of low rainfall…..
Arid:
less than 10 cm (4 in)
Semi-arid:
less than 25 cm (10 inches)
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
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.
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.
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 socioeconomic forces. For example:
a. brief period of excess rainfall
“the rain follows the plow”
b. over irrigation (salanization)
c. ground-water depletion
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 socioeconomic forces. For example:
2. Drought: rainfall significantly below
normal for an extended period.
several years
more than 25% less than normal
Dryland regions of the world (yellow)
These are sensitive to desertification
D. Desertification:
4. The Sahel as an example
climate
climate change
agricultural practices
political process
population
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.
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
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.
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
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.
D. Desertification:
4. The Sahel as an example
5. Desertification in the US
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.
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
D. Desertification:
5. Desertification in the US: The Dust Bowl of
the 1930s.
Precursers
Causes
Consequences: Changed farming practices
Better? Yes, but….
Phoenix, Arizona in the early 1970s