Why does climate change generate desertification?
Most arid and semi-arid regions are normally far inland. Often those also have terrain features that don't allow for the moist air to reach, such as mountain ranges, etc.

So now if global temperatures increase, the evaporation in semi-arid regions increase as well, reducing the amount of water available for plants. At the same time moisture coming from the ocean still precipitates as rain before reaching those regions.