Why are savannahs replaced by deserts?

Deserts annually absorb more and more territories from the natural zones of savannahs. This happens for several reasons. One of the reasons is human activity, who takes a lot of water from natural reservoirs for his needs. Therefore, the vegetation in the savannah dries up. The second reason is the huge number of cattle that graze in the savannas and destroy perennial plants along with the roots, which is why nature does not have time to restore the vegetation cover. Global warming is also an important factor.