Defining drought is a tricky business, but I think Daniel Griffin and Kevin Anchukaitis have come up with a reasonable one – three years of persistent low soil moisture. By that measure, the drought in Central and Southern California is…. Well, I’ll let them tell it (pdf):

We demonstrate that while 3-year periods of persistent below-average soil moisture are not uncommon, the current event is the most severe drought in the last 1200 years, with single year (2014) and accumulated moisture deficits worse than any previous continuous span of dry years.

That’s from “How unusual is the 2012-2014 California drought?” They used tree rings. Published today in GRL.

update: The irony of having this paper go public on a day when it’s raining like hell in California was not lost on Dan and Kevin. Dan offered this:

Its California, drought happens. Then it rains. Sometimes heavily. Proove Steinbeck wrong California, don't forget pic.twitter.com/YyO0txPTVx

— Daniel Griffin (@locallyabsent) December 3, 2014