From the New York Times:
A 60-year drought that scorched the Southwest during the 12th century may be a harbinger of things to come as greenhouse gases warm the Earth, according to research published in the Proceedings of the National Academy of Sciences.
The study's authors used tree rings to reconstruct a portrait of droughts that struck the Southwest over a 1,200-year period stretching back to 900 A.D. They believe that understanding the droughts of the past could help water managers plan for future dry periods that are expected to become more intense as climate change worsens.
Portions of the Southwest have suffered prolonged drought since 2001.