If less rain means that the relative deserts of California and Arizona can't be irrigated/subsidized as heavily, the resulting price increases will shift agricultural production back to more natural locations in the Midwest. There was once a dairy industry here, for one thing. We couldn't compete with the desert alfalfa of the Salt River Valley, but then again maybe their advantage wasn't an environmentally sustainable one. It requires a certain perspective to bemoan the fact that deserts aren't great places for agriculture.
This is not a possible shift to an environment with a recent (human-timescale) analogue. If it comes to pass, we will not be able to farm or live in the same places or at the same volumes as we are currently capable of.
