The part of West Africa called the Sahel, which I believe includes the range of these elephants, has gotten steadily drier over our lifetimes. We have before and after pictures taken by satellites that show the Sahara Desert expanding southward. You may also remember the famine in this region during the 1970s and 80s.
Some of this desertification has been caused by mismanaged agriculture, like the “Dust Bowl” we had on the Great Plains in the 1930s. And the last tree in northern Niger was killed when a Libyan drove a truck into it. But I believe the most important factor is the general warming trend that the earth has undergone since the end of the ice age. If man-made global warming was the culprit, we’d probably see the opposite effect, with some parts of the Sahara getting greener.