Tree rings in the American Southwest provide evidence of megadroughts that hit the region hundreds of years ago. According to a new study from Cornell University, these droughts have been linked to the demise of civilizations, and changing climate conditions virtually assure that another one is on its way. If precipitation decreases, stays them same, or even increases slightly in the coming years, there’s a 99 percent chance of a megadrought hitting the region, the study says.