A recent dendrochronology study has shown that California’s drought is the worst in at least 1200 years:

Scientists studying the severe drought that has beleaguered California for the past three years estimate that this drought is the worst one in the past 1,200 years. The new estimate comes from data they compiled on tree ring growth and soil moisture. Low (but not unprecedented) precipitation and record high temperatures are the main factors driving this drought, the scientists say. An online edition of the journal Geophysical Research Letters published this work on December 3, 2014.

It should be noted that the study primarily used the Blue Oak (Quercus douglasii) which are particularly good at showing rainfall, though, 1200 years is a lot longer than the typical life span of this tree.

My guess would be that they used other (probably less accurate) trees, and perhaps some other indicators.

It is not clear from the abstract whether there was a worse drought 1200 years ago, or if it’s just the worst in the 1200 years, and perhaps more.

If it is the former, that drought corresponds to the Mayan collapse, which many (though by no means all) archaeologists attribute (at least in part) to an extended drought.  (The Mayan collapse is a matter of some dispute in academe.)

In either case, the fact that this drought is worse than 300 year long drought that destroyed Anasazi civilization between 1150 and 1450. 

What is going on here is, to use the technical term, seriously weird climate sh%$, and it requires willful ignorance not to consider anthropogenic climate change as a primary factor.