Final answer:
The desertification of the Sahel is largely due to global warming and subsistence farming, which lead to climate change, overcultivation, deforestation, and overgrazing.
Step-by-step explanation:
The desertification of the Sahel has been influenced by several factors. Among these, global warming (a) and subsistence farming (c) have played significant roles. Global warming leads to changing climate conditions and can exacerbate droughts, thereby increasing the desertification process. Subsistence farming, especially when compounded by increasing populations, can lead to overcultivation of land, deforestation for firewood, and overgrazing, all of which significantly contribute to land degradation and desertification. Heavy rains (b) are not typically associated with desertification, and a decreasing population (d) would likely reduce pressure on the land, making it an unlikely contributor to desertification.