Contribution of anthropogenic warming to California drought during 2012–2014

Uses a suite of climate data sets and multiple representations of atmospheric moisture demand to calculate many estimates of the self-calibrated Palmer Drought Severity Index, a proxy for near-surface soil moisture, across California from 1901 to 2014 at high spatial resolution
Finds—based on the ensemble of calculations—that California drought conditions were record breaking in 2014, but probably not record breaking in 2012–2014, contrary to prior findings
Finds that regionally, the 2012–2014 drought was record breaking in the agriculturally important southern Central Valley and highly populated coastal areas
Examines contributions of individual climate variables to recent drought, including the temperature component associated with anthropogenic warming
Finds that precipitation is the primary driver of drought variability but anthropogenic warming is estimated to have accounted for 8–27% of the observed drought anomaly in 2012–2014 and 5–18% in 2014
Results indicate that although natural variability dominates, anthropogenic warming has substantially increased the overall likelihood of extreme California droughts