What Global Warming means is that climate zones shift, and that includes all climate zones. At the same time the northern boundary of the desert bands moves north, the southern boundary also moves north. In effect, the tropics expand. So the Sahel is where you´re expected to see it, and if they´re seeing it now in the Sahel as the article seems to say, then that is just as much in line with Global Warming expectations as the desertification of Spain.