Paper Reviewed
Griffin, D. and Anchukaitis, K.J. 2014. How unusual is the 2012-2014 California drought? Geophysical Research Letters 41: 9017-9023.

After rhetorically asking "how unusual is the 2012-2014 California drought," Griffin and Anchukaitis (2014) proceed to provide the following multi-part answer. First of all, they report that "tree ring chronologies extended through the 2014 growing season reveal that precipitation during the drought has been anomalously low but not outside the range of natural variability." Second, they state that "using an integrated soil moisture metric like PDSI [Palmer Drought Severity Index], the 2012-2014 drought is the worst in our combined NOAA-NADA estimate and 2014 is the single most arid case in at least the last 1200 years." Third, but back-tracking somewhat, they write that "precipitation for 2012-2014 was indeed low," but that it was "not unprecedented over the last seven centuries." Fourth and last, they backtrack a bit more, in that they acknowledge that "the paleoclimate record also provides unequivocal evidence of previous epochs of longer and more severe megadroughts," citing the well-known studies of Woodhouse and Overpeck (1998), Cook et al. (2007), Stahle et al. (2007), Cook et al. (2010) and Woodhouse et al. (2010).

Continuing, the two researchers further acknowledge that "attribution of anthropogenic influence on California rainfall and Pacific storm tracks during the current drought is thus far equivocal," citing the work of Funk et al. (2014), Swain et al. (2014) and Wang and Schubert (2014), while adding that "future rainfall patterns in California remain unclear, due both to climate model differences and internal climate system variability," citing Pierce et al. (2012), Kirtman et al. (2013) and Deser et al. (2014). And so, to answer to the question posed by Griffin and Anchukaitis in the title of their paper, "how unusual is the 2012-2014 California drought," it would appear the answer is not so unusual, unprecedented, or unnatural, and there is no evidence to ascribe it to rising atmospheric CO2/global warming.

References
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Cook, E.R., Seager, R., Heim Jr., R.R., Vose, R.S., Herweijer, C. and Woodhouse, C. 2010. Mega-droughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context. Journal of Quaternary Science 25: 48-61.

Deser, C., Phillips, A.S., Alexander, M.A. and Smoliak, B.V. 2014. Projecting North American climate over the next 50 years: Uncertainty due to internal variability. Journal of Climate 27: 2271-2296.

Funk, C., Hoell, A. and Stone, D. 2014. Examining the contribution of the observed global warming trend to the California droughts of 2012/13 and 2013/14. Bulletin of the American Meteorological Society 95: 511-515.

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Stahle, D.W., Fye, F.K., Cook, E.R. and Griffin, R.D. 2007. Tree-ring reconstructed megadroughts over North America since A.D. 1300. Climatic Change 83: 133-149.

Swain, D.L., Tsiang, M., Haugen, M., Singh, D., Charland, A., Rajaratnam, B. and Diffenbaugh, N.S. 2014. The extraordinary California drought of 2013/2014: Character, context, and the role of climate change. Bulletin of the American Meteorological Society 95: 53-57.

Woodhouse, C.A., Meko, D.M., MacDonald, G.M., Stahle, D.W. and Cook, E.R. 2010. A 1,200-year perspective of 21st century drought in southwestern North America. Proceedings of the National Academy of Sciences USA 107: 21,283-21,288.

Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought variability in the Central United States. Bulletin of the American Meteorological Society 79: 2693-2714.