Lecture 6: Desertification Flashcards
Shrinking Lake Chad
Nigeria, Niger, Chad, Cameroon
Persistent drought has shrunk the lake to about a tenth of its former size
1972: Larger lake surface area is visible in this image
2001: Impact of drought displays a shrunken lake, comparatively much smaller surface area than in 1972 image
Land cover degradation around
Lake Nakuru, Kenya
1973: The area that hosts the world’s largest concentration of flamingos
2000: Excision of forest in the Eastern Mau Forest Reserve (white lines) will most likely lead to disappearance of upper catchment forest cover
Death of a sea - Aral Sea, Kazakhstan
1973: The surface of the sea once measured 66 100 km2
1987: 60% of the volume had been lost
1999-2004: The sea is now quarter of the size it was 50 years ago
Definitions
Desertification:
transformation to desert
Unfortunate choice of word : camels in Europe
Conversion from non-desert to desert, where desert is characterised by low or null biologic production (around 0.1 Kg/m2 compared with 60 Kg/m2 for tropical zones).
Definitions Desert
vegetation cover is sparse or absent so the land surface is exposed to the atmosphere and the associated physical forces.
Rainfall is usually small in amount and infrequent or irregular in occurrence
Definitions degradation
Lowering, loss in structure and function.
Definition desertization
the spread of the deserts through natural processes
Definition aridification
the general drying of the climate
Causes of aridity
High pressure- air at the equator rises
Wind direction- Winds blowing over continental interiors have a reduced opportunity to absorb moisture
Topography- on the leeward side
Cold ocean currents-
Definitions overall
all have following conditions
- An initial arid, poor productivity or degraded landscape,
- The adverse impact of (wo-) man through agriculture and economic activity,
- Climatic variability and/or climatic change and its interaction with human impact
definitions consequences
Diminution or destruction of the biological potential of the land,
Effectively irreversible changes in the nature of the landscape,
Irreversible because of the economic cost of restoration,
Harsh agricultural, health, socio-economic and aesthetic consequences.
Physical and Human Drivers for Desertification
Desertification Caused by Drought: Reaction to a rainfall low is often to grow more crops and expect to keep livestock at levels of good years. [Physical and human always interact]
Cultivation : prolonged drought ⇒ crop death ⇒ soil exposure ⇒ wind and fluvial erosion. (Dustbowl)
Grazing : prolonged drought ⇒ reduction in carrying capacity ⇒ overgrazing (Sahel case)
Also:
Frequent climatic variability and change in semi arid areas.
Response of vegetation to climate (grasses vs shrubs, r, K strategies).
Interactions with other human pressures and GEC.
Landscape mosaic. Desertification Response Unit (DRU), not uniform.
Desertification Caused by Human Activity:
In addition to increased aridity. Without change in climate (marginal environments).
[Physical and human always interact]
Deforestation for fuel/firewood. Reduces cover, exposes soil.
- For agriculture:
destruction of soil crumb structure (overgrazing, over tillage)
humus removal (in harvested crops, dung for fuel) soil erosion (assisted by tillage esp on slopes)
difficulty in re-establishment of vegetation.
over-grazing leads to lower cover, trampling and replacement with shrubs
- Over exploitation of water-resources
- Pollution etc.
Resilience science: cup and ball models
System in metastable, good state
Forced by climate/human activity into degraded stable state
Highly sensitive system – difficult to improve but easy to degrade
Each is a metastable state held stable by given pressures (climate, grazing, agriculture)
