Ii the American Southwest heading for a megadrought? A projection based on climate models suggests this could be the outcome as global warming alters the Pacific weather patterns that normally bring rain to the region. Now tree rings are revealing just how dry things could get.

Climate models and satellite observations both indicate that the total amount of water in the atmosphere will increase at a rate of 7% per kelvin of surface warming. However, the climate models predict that global precipitation will increase at a much slower rate of 1 to 3% per kelvin. A recent analysis of satellite observations does not support this prediction of a muted response of precipitation to global warming. Rather, the observations suggest that precipitation and total atmospheric water have increased at about the same rate over the past two decades.