From USA Today (Doyle Rice):
The dryness in California is only part of a longer-term, 15-year drought across most of the Western USA, one that bioclimatologist Park Williams said is notable because “more area in the West has persistently been in drought during the past 15 years than in any other 15-year period since the 1150s and 1160s” — that’s more than 850 years ago.
“When considering the West as a whole, we are currently in the midst of a historically relevant megadrought,” said Williams, a professor at the Lamont-Doherty Earth Observatory of Columbia University in New York.
Megadroughts are what Cornell University scientist Toby Ault calls the “great white sharks of climate: powerful, dangerous and hard to detect before it’s too late. They have happened in the past, and they are still out there, lurking in what is possible for the future, even without climate change.” Ault goes so far as to call megadroughts “a threat to civilization.”