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It's true that building a large city in an arid place is not the smartest thing in the world but the fact is that an expected effect of global warming is expansion of the sub-tropics as the sinking region of Hadley Cells moves northward. This is expected to make areas such as the southwest and Texas even drier than they currently are exacerbating the problem.
http://en.wikipedia.org/wiki/Hadley_cells#Hadley_cell_expansion