"Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand polewards1. Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P−E, the net flux of water at the land surface, leading to, for example, a decline in Colorado River flow8. This would cause widespread and important social and ecological consequences."
Climate Change Making Southwest U.S. Steadily Drier
Source: Nature Climate, 12/26/2012