It can be one of two things.
First, it can be due to global warming.
Second, it can be due to the rotation of the sun. When the sun rotates, it can change it latitude causing it to hit at different points onto different areas of the world.
Some of the physiographic features that cause extreme aridity may be found together within a <span>single desert, but t</span>he deserts found in the western united states are primarily the result of being located within the rain shadow of one or more mountain ranges.
