It is both. A warming climate, which we have now, can change areas from moderate to desert. The Sahara used to be fertile but was changed to desert when the last ice age ended.
However, mankind can also cause immediate desertifcation by over-farming and over-logging. This has happened all too often. Sometimes it happens in a single generation and sometimes it takes longer. Also, if you believe that humans are causing global warming, which I do, then creation of deserts by climatic change can also be tied to humans.