The Sahara Desert region of Africa has long been a region of unreliable rainfall and the areas prone to desertification have increased and decreased over the last 6000 years.
The recent expansion of the Sahara Desert has impacted the diffusion of agriculture in the Sahel region in a number of ways.
Firstly, the lack of reliable rainfall (that is, high levels of rainfall variability) coupled with marginal soil and natural vegetation conditions means it has become harder to grow a crop which has the ability to be harvested and resown. In fact cultivating the soil will lead to the erosion of the soil, as there is little vegetation to bind the soil. It will also deplete the soil of any vital minerals it may contain to allow plant growth.
As Europeans colonized the region, boundaries and borders were set that paid little attention to the interests of locals. Herders for example were now not allowed to take their herds where once they had. The impact of this was overgrazing and resultant erosion from overstocking of the available land.
A feature of this part of Africa has also been many conflicts. Refugees have fled the rural landscapes to urban areas, which has led to agriculture's diffusion being halted. The migration of people from rural to urban areas has also put the existing natural vegetation under pressure as much has been taken for firewood or for building shelters. This has a negative impact on the soil through erosion and supplying leaf litter to the soil.
Overpopulation has led to more marginal areas being used to grow an ever increasing yield. This often fails and continues the cycle of poverty. Commercial levels of crop production have failed to keep up with population growth let alone for export purposes.