If the history books that I've read are correct, then during the time of Alexander the Great, the northern coast of Africa (outside of the areas with higher human population density) was a functioning dry-forest ecosystem. During the time of the Roman Empire, people cut down large areas of forest in order to plant grass – wheat and barley for human consumption, and pasture grass for cows and goats. Without sufficient forest cover to pull in moisture from the Mediterranean, the rains stopped coming, the remaining vegetation died off, and the Sahara Desert advanced to the coast. This was a case of human-caused climate change.

Forests precede civilisations, and deserts follow them.

Something to keep in mind the next time that you hear/read that non-forest ecosystems are "natural" and should be preserved as they are.