Paper Reviewed
Mao, Y., Nijssen, B. and Lettenmaier, D.P. 2015. Is climate change implicated in the 2013-2014 California drought? A hydrologic perspective. Geophysical Research Letters 42: 2805-2813.

Writing as background for their work, Mao et al. (2015) state that "California has experienced severe drought in 2012-2014 (which appears to be continuing into 2015), with especially low winter precipitation and mountain snowpack in winter 2013-2014," but they say that "the extent to which climate change is implicated in the drought, if at all, is not clear." And, therefore, they constructed "a historical record of California snowpack, runoff, and other hydrological variables of almost 100 years in length and used the reconstructed records to analyze climate trends in the Sierra Nevada and their impact on extreme drought events in the historic record."

This work revealed, as they continue, that "water year 2014 was among the five driest of our 95-year historic record, but it was not the most severe hydrologic drought, from either a single-year or multi-year perspective." Instead, the three researchers report that "water year 1977 was the most severe by most measures" and that "it occurred immediately after water year 1976, which was among the five most severe years of record by most measures." And, therefore, while the recent drought was -- and still is -- severe, the three researchers demonstrate that it was "not without precedent" and that "similar droughts have occurred in the last century, before the recent period of pronounced warming," which clearly suggests that the recent period of pronounced warming (which is itself debatable) need not have been responsible for the recent severe drought.