How unusual is the 2012-2014 California drought?

Daniel Griffin1,2, Kevin J Anchukaitis2
Corresponding author: Daniel Griffin, Department of Geography, Environment and Society, University of Minnesota, griffin9@umn.edu.
1. Department of Geography, Environment and Society, University of Minnesota, MN,
USA.
2. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

Discussion and Conclusions

Evaluated using an integrated soil moisture metric like PDSI, 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. In contrast, the precipitation deficits of 2014 and the three-year period are not unique in the paleoclimate record (Figure 4). A simple modeling exercise (Figure S6), calculating the average division 4–7 PDSI with observed [Vose et al., 2014] vs. climatological mean temperatures, suggests that temperature could have exacerbated the 2014 drought by approximately 36%. Based on these complementary lines of evidence, we infer that the severity of the 2014 drought is a result of both anoma- lously low – yet not unprecedented – water year precipitation (Figure S4) and record high temperatures (Figure S3). The 2014 JJA PDSI value is ∼3.5 standard deviations below the long-term (800-2014) mean (Figure 1b) and the cumulative 2012-2014 drought is the worst unbroken drought interval of the last millennium (Figure 3a, Figure 4). Precipitation for 2012-2014 was indeed low, but is less than 1.5 standard deviations below the reconstructed long-term normalized regional mean and not unprecedented over the last centuries, neither on the annual nor 3-year time scale. These observations from the paleoclimate record suggest that high temperatures have combined with the low but not yet exceptional precipitation deficits to create the worst short-term drought of the last millennium for the state of California.

A return to normal rainfall levels may partially relieve some of the immediate water resources pressures imposed by the current drought, perhaps through an El Ninõ event or a series of landfalling atmospheric rivers [Dettinger, 2013]. However, the blue oak precipitation reconstruction reveals that the climate system is capable of natural precipitation deficits of even greater duration and severity than has so far been witnessed during the comparatively brief 2012-2014 drought episode. Attribution of anthropogenic influence on California rainfall and Pacific storm tracks during the current drought is thus far equivocal [Funk et al., 2014; Swain et al., 2014; Wang and Schubert, 2014] and future rainfall patterns in California remain unclear, due both to climate model differences and internal climate system variability [Pierce et al., 2012; Kirtman et al., 2013; Deser et al., 2014]. However, projections for a continued trend toward higher mean and extreme temperatures are robust and will play an increasingly important role in 21st century hydroclimate [Seager and Hoerling, 2014]. Future ‘hot’ droughts [Overpeck, 2013; Pederson et al., 2014], driven by increasing temperatures due to anthropogenic emissions of greenhouse gases and enhanced evaporative demand, are assured and will be a substantial influence on future water resources supply and management in the western United States [Cook et al., 2014b]. Additional uncertainties remain, including the nature of land-surface feedbacks between soil moisture and temperatures [Yin et al., 2014], the sensitivity of drought metrics such as PDSI to data and methodological choices [Trenberth et al., 2014], and the multivariate influence of temperature and precipitation on paleoclimate proxy records. Further insight into California’s drought history will come from developing longer tree-ring chronologies by updating existing series to the present, expanding the use of subfossil wood to extend further into the past, and exploring the dendrochronological potential of underutilized species.

In California, droughts have and will continue to occur, as evidenced by the paleoclimate and instrumental record and as projected by Earth System Models of the anthropogenic future. While the natural environments of central and southern California have been shaped by hydroclimatic extremes, the state’s built environment reflects a complex – and at times maladaptive [Christian-Smith et al., 2014] – solution to the spatial and temporal asymmetry between surface water supplies and socioeconomic water resources demand. Future severe droughts are expected to be in part driven by anthropogenic influences and temperatures outside the range of the last millennium. However, under- standing past drought and precipitation variability through paleoclimate data may prove useful for adapting to climate change despite continuing uncertainties regarding regional precipitation in a warmer future.