DesertificationDesertification

The 5-D’s and the historical geography of desertification

DRYLANDS

DROUGHT

DESICCATION

DEGRADATION

DESERTIFICATIONland degradation in arid, semi-arid and dry sub-humid areas resulting from various factors including climatic variations

and human activities (UNEP)

IntroductionIntroduction

• Drylands cover about 40% of Earth’s land surface

• 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

http://earthtrends.wri.org/maps_spatial/maps_detail_static.php?map_select=459&theme=9

Sahelian DesertificationSahelian Desertification

• Drought in the Sahel (1970-1997?)• Consequences (environmental, social)• 1977: United Nations Conference on

Desertification (UNCOD) held in Nairobi, Kenya

• 1993: Convention to combat desertification-CCD

• 2006: International Year of the Desert and Desertification

http://earthobservatory.nasa.gov/Study/Desertification/desertification.html

http://earthobservatory.nasa.gov/Study/Desertification/desertification.html

http://earthobservatory.nasa.gov/Study/Desertification/desertification.html

From Nicholson, 2005

Sahelian DesertificationSahelian Desertification

A: BIOGEOMORPHIC

• 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

http://earthobservatory.nasa.gov/Study/Desertification/desertification.html

http://www.miamisci.org/ecolinks/mapbiosphere.html

The Sahel and southern Sahara—limits of active and relict vegetation-stabilized dunes. (A) Landsat 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 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. 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.(C) Vegetation-stabilized linear dunes west of Lake Chad.

From Lancaster, 2007

From Lancaster, 2007

From Middleton and Goudie, 2001

http://www.gsfc.nasa.gov/topstory/20010824redtides.html

Sahelian DesertificationSahelian Desertification

B: CLIMATIC

• 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, 1983From Wells and Haragan, 1983

From Rognon, 1991

From Tchakerian, 1999

http://jisao.washington.edu/data_sets/sahel/Mitchell, T. (June 2005) Sahel Rainfall Index.

http://jisao.washington.edu/data_sets/sahel/Mitchell, T. (June 2005) Sahel Rainfall Index.

Complex feedbacks. Complex feedbacks. The recent Sahel drought was likely initiatedThe recent Sahel drought was likely initiatedby a change in worldwide ocean temperatures, whichby a change in worldwide ocean temperatures, which

reduced the strength of the African monsoon, and was exacerbatedreduced the strength of the African monsoon, and was exacerbatedby land-atmosphere feedbacks through natural vegetationby land-atmosphere feedbacks through natural vegetation

and land cover change. Land use changes by humansand land cover change. Land use changes by humansmay have also played an important role. SST, sea surface temperature;may have also played an important role. SST, sea surface temperature;

ITCZ, inter-tropical convergence zone.ITCZ, inter-tropical convergence zone.

Sahelian DesertificationSahelian Desertification

C: ANTHROPOGENIC

• grazing

• cultivation

• firewood

• salinization and waterlogging

• migration

• Sahelian population is doubling every 20 years

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From Thomas and Middleton, 1994

From Thomas and Middleton, 1994

http://www.unesco.org/bpi/pdf/memobpi40_desertification_en.pdf

A Recent Greening of the Sahel?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 precipitation

• 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

AGRHYMET MONTHLY BULLETIN REPORTS

AGRHYMET MONTHLY BULLETIN REPORTS

http://earthobservatory.nasa.gov/Study/Desertification/desertification.html

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 withminimum and maximum values of data indicated by the error bars (right column). Different sources havebeen used and after consistency check the following data were selected: for Burkina Fasowww.citypopulation.de (1985 and 1996), populations.com (1998), www.library.uu.nl (2002), for Maliwww.citypopulation.de (1976 and 1987), www.world-gazetter.com (1998 and 2003), for Mauretaniawww.citypopulation.de (1977, 1988 and 2000), www.world-gazetter.com (2003), for Niger: www.citypopulation.de (1977, 1988 and 2000) www.world-gazetteer.com (2003), for Sudan www.world-gazetter.com(1983 and 2003), for Senegal www.citypopulation.de (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 - DDPDrylands Development Paradigm - DDP• P1: H-E systems are coupled, dynamic and

coadapting, with no single target equilibrium point

• 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