What is the main cause of the spread of desert conditions in the Sahel region?

Answer 1

Long term climate changes in the Sahel are related to the variability of the African Monsoon. On geologic timescales, the monsoon is influenced by orbital cycles of the earth such as the 21 thousand year precession cycle, and the glaciation cycles that come along with that. The northern African continent became began to change to desert by 5,000 years ago. All evidence suggests that changes in the monsoon over 100 to 1000 year timescales is related to orbital cyclicity, patterns of solar insolation, ocean surface temperature, and high latitude ice changes.

Other factors are changing the monsoon behavior over shorter time scales. There is debate over the effects of internal processes like overgrazing and deforestation, and external processes like climate variation. Most of the evidence that this area is being transformed by overgrazing and deforestation is based on observations made in the early 20th century. Recent UN studies have concluded similar results, partly from looking at these historical studies. A study by Prince et al in 1998 used vegetation measurements from satellites to calculate rainfall efficiency, and found that regional drought recovery was not consistent with the idea of land degradation. It is now believed by many workers that the land changes in the Sahel in recent decades are driven by natural response of the semi-arid ecosystem to climate variability.

Here is the study with many more details on anthropogenic climate change, dust feedback systems, and vegetation feedback systems:
Brooks, N. 2004. Drought in the African Sahel: long-term perspectives and future prospects. Tyndall Centre Working Paper No. 61

link to pdf:
http://www.cru.uea.ac.uk/~e118/publications/TynWP61.pdf

Dr. Nick Brooks PhD dissertation on this topic:
http://www.cru.uea.ac.uk/~e118/thesis/thesis.html

Answer 2

Sand

Answer 3

Extensive deforestation.