Blue oak trees up to 500 years old thrive in California's drought-scorched foothills. Their rings provide a unique record of drought and wetness for the Golden State. Credit: Daniel Griffin

California’s ongoing drought, which began in 2012, is severely straining the state’s water resources, agriculture, and economy and is disrupting ecosystems. Although the drought is the worst on record since instruments began recording aridity levels about 120 years ago, scientists wonder how unusual it is over longer periods of time.

To learn more, Griffin and Anchukaitis analyzed two paleoclimatic records from tree rings to compare this drought with others from the last 1200 years. California’s long-lived trees provide a record of hydroclimate history through their wide (wet) and narrow (dry) rings.

The first record estimates the Palmer drought severity index (PDSI), a metric of soil moisture based on precipitation and temperature, using tree ring data from a large number and wide variety of sites in the region. The second record, which extends back over 700 years, estimates simple precipitation using tree rings from blue oaks (Quercus douglasii) at four sites in central and Southern California. Collectively, these two records provide long-term context for the 2012–2014 drought in terms of both soil moisture and precipitation.

Using PDSI data dating back to 800 A.D., the authors found that dry-soil periods of 3 or more years occurred in California relatively frequently. However, soil moisture deficits accumulated from 2012 to 2014 were more severe than any other short-term period in the record.

However, the blue oak tree rings from 2012 to 2014 suggest that although precipitation has been low, it was lower in numerous earlier years. The researchers believe that record high temperatures explain the recent drought’s severity, even though precipitation was not unprecedentedly low. (Geophysical Research Letters, doi:10.1002/2014GL062433, 2014)

—Puneet Kollipara, Freelance Writer

Citation: Kollipara, P. (2015), California’s 2012–2014 drought unusual for last millennium, Eos, 96, doi:10.1029/2015EO027299. Published on 3 April 2015.

Text © 2015. The authors. CC BY-NC 3.0
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