“Megadrought” threatens parts of American West

The water level in reservoirs across the American West has been declining for decades, to the point where the region faces the threat of a “megadrought.” While scientist Park Williams says this kind of drought has been seen in Earth’s history, today’s dry spell is unique in the fact that it seems to have been caused by a combination of natural climate variability and human-imposed climate change. John Blackstone explores the troubling pattern.