Paper Reviewed
Hatchett, B.J., Boyle, D., Putnam, A.E. and Bassett, S.D. 2015. Placing the 2012-2015 California-Nevada drought into a paleoclimatic context: Insights from Walker Lake, California-Nevada, USA. Geophysical Research Letters 42: 8632-8640.

In this timely study, as they describe it, Hatchett et al. (2015) strove to determine whether the hydro-climatic conditions that occurred during the 2012-2015 (hereafter current) California-Nevada drought were "within the range of natural variability documented by paleo-proxy indicators," which they hoped could lead to the "disentanglement of the relative roles of natural versus anthropogenic forcing factors as causative agents." So what did they do? And what did they learn?

The four U.S. scientists say they "employed a coupled water balance and lake evaporation model to simulate the magnitude of climate changes required to generate the prolonged medieval western U.S. droughts identified in the paleoclimate record," while they also "examined the response times of the watershed-lake system to persistent changes in climate." Last of all, they examined "the current drought and compared results to the medieval drought simulations."

This work revealed that the Stine (1990, 1994) droughts "provide evidence that extended drought episodes with precipitation anomalies on the order of those being experienced today are within the range of natural climate variability." And they add that this record "is sufficient to support this conclusion from [both] precipitation and duration perspectives."

Hence, it would appear there has been nothing unusual, unnatural or unprecedented about either the length or the severity of the California-Nevada Drought of 2012-2015.

References
Stine, S. 1990. Late Holocene fluctuations of Mono Lake, eastern California. Palaeogeography, Palaeoclimatology, Palaeoecology 78: 333-381.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during mediaeval time. Nature 369: 546-549.