DESERTIFICATIODESERTIFICATIONN

Edited by Joe Naumann

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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 depletion6

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 feedbacks14

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

Irrigation24

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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Phoenix, Arizona in the early 1970s26

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