A megadrought is the name for a decades-long dry spell. Researchers have known since 1998 that megadroughts can occur in the region. Under normal conditions, dry periods of several decades or more come once or twice in a millennium, but the changing climate has made the danger much more imminent. Now, a new study has determined there is a 50 percent chance that a drought as long as 35 years will occur in the next century. And the chances of another decade-long drought are as high as 80 percent.