Desertification - OceanWorld

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The 5-D’s and the historical geography of
          land degradation in arid, semi-arid and dry sub-humid areas
          resulting from various factors including climatic variations and
          human activities (UNEP)
• Drylands cover about 40% of Earth’s land

• Home to more than 38% of the total global
  population of 6.5 billion (about 2.4 billion)

• Land degradation is present on 10 to 20% of the
  global drylands

• Land degradation indirectly affects about 250
  million people in the developing world
         Sahelian Desertification

• Drought in the Sahel (1970-1997?)
• Consequences (environmental, social)
• 1977: United Nations Conference on
  Desertification (UNCOD) held in Nairobi,
• 1993: Convention to combat desertification-
• 2006: International Year of the Desert and
From Nicholson, 2005
            Sahelian Desertification


• the physical geography of arid lands
   – transitional region between savanna and desert
   – variability in precipitation
   – natural vegetation
   – the Sahel and ancient stabilized sand seas (ergs)
   – the nature of desert winds – Harmattan
      • drought and dust
The Sahel and southern Sahara—limits of active and relict vegetation-stabilized dunes. (A) Landsat
image of Sahara–Sahel transition in southern Mauritania. Note the change in dune trend and
increasing vegetation cover to the south. (B) Aster image of relict dunes and river systems in Mali.
(C) Vegetation-stabilized linear dunes west of Lake Chad.

From Lancaster, 2007
From Lancaster, 2007
From Middleton and Goudie, 2001
            Sahelian Desertification

•   long term climatic change since post-glacial times
•   short term drought and desiccation cycles
•   greenhouse warming?
•   atmospheric circulation changes
     – multi-decadal variations in SST
     – reduction in the strength of the African Monsoon
     – shifting of the ITCZ
     – land-atmosphere feedbacks
From Wells and Haragan, 1983
From Rognon, 1991
From Tchakerian, 1999
Mitchell, T. (June 2005) Sahel Rainfall Index.
Mitchell, T. (June 2005) Sahel Rainfall Index.
 Complex feedbacks. The recent Sahel drought was likely initiated
       by a change in worldwide ocean temperatures, which
 reduced the strength of the African monsoon, and was exacerbated
     by land-atmosphere feedbacks through natural vegetation
       and land cover change. Land use changes by humans
may have also played an important role. SST, sea surface temperature;
               ITCZ, inter-tropical convergence zone.
       Sahelian Desertification

•   grazing
•   cultivation
•   firewood
•   salinization and waterlogging
•   migration
•   Sahelian population is doubling
    every 20 years
From Thomas and Middleton, 1994
From Thomas and Middleton, 1994
From Thomas and Middleton, 1994
    A Recent Greening of the Sahel?
• Olsson et al., 2005 and Nicholson, 2005
• The wettest years were 1998, 1999, 2003, 2005 and
  2007(?) – 5 of the last 10 years show above average
• Increasing rainfall
• Improved land management - very limited increases in
  agricultural production (only Mali and Burkina Faso)
• Increased rural to urban migration and the growth of
  urban centers
• political unrest and armed conflicts
   – Darfur
   – Chad
   Fig. 6. Population growth rates for six Sahelian countries (Data for Chad and Ethiopia are missing),
   expressed as (% year) of (a) national growth rate (left column), (b) urban growth rate from official source
   (FAO 2003), (central column) and (c) growth rates of city populations from different sources with
   minimum and maximum values of data indicated by the error bars (right column). Different sources have
   been used and after consistency check the following data were selected: for Burkina Faso (1985 and 1996), (1998), (2002), for Mali (1976 and 1987), (1998 and 2003), for Mauretania (1977, 1988 and 2000), (2003), for Niger: www.citypopulation.
   de (1977, 1988 and 2000) (2003), for Sudan
   (1983 and 2003), for Senegal (1976 and 2001).

Olsson et al., 2005
From Goudie, 2002
From Thomas and Middleton, 1994
• Evidence of human occupation in the Sahel
  dates from about 600,000 BP. Since that time,
  selective hunting and gathering, bush fires,
  agriculture, herding, charcoal production, the
  destructive exploitation of forest products, and
  other activities have contributed greatly to the
  modification of the Sahelian ecosystems. No
  areas, however remote from human settlement,
  have been left undisturbed (National Research
  Council, 1983)

From Thomas and Middleton, 1994
Fig. 1. The focus on global drylands is
shifting from an emphasis on negative
images of desertification (upper: drought-
stricken cattle on an eroded grassland in
central Australia. Photo: M. Stafford Smith)
to a more forward-looking perspective
concerning human livelihoods, based on
interactions between and among human
activities and natural-world processes
(lower: farmer spraying organic pesticide
on domesticated quinoa in southern
Bolivia. Photo: J. Reynolds). Either way,
great challenges to the future security of
some 250 million people remain.

Reynolds et al., 2007
   Drylands Development Paradigm - DDP
• P1: H-E systems are coupled, dynamic and
  coadapting, with no single target equilibrium
• P2: A limited suite of “slow” biophysical and
  socioeconomic variables are critical
  determinants of H-E system dynamics
• P3: Thresholds in key “slow” variables define
  different states of H-E systems
• P4: Coupled H-E systems are hierarchical,
  nested, and networked across multiple scales
• P5: The maintenance of a body of up-to-date
  LEK or hybrid EK is key to functional
  coadaptation of H-E systems

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