Atmospheric Sciences Research Spotlight

California's 2012–2014 Drought Unusual for Last Millennium

Soil moisture estimates, inferred from thousands of tree rings spanning the past 12 centuries, highlight the severity of the recent record-breaking drought.


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.

© 2015. The authors. CC BY-NC 3.0
  • Travis Speed

    “The researchers believe that record high temperatures explain the recent drought’s severity, even though precipitation was not unprecedentedly low.” The inference of this article is that, while this isn’t the worst drought in our history, we do need to take precautions and attempt to mitigate the effects of the drought, as it is nothing to be taken lightly. Climate has been proven to have been changing more severely than ever before with C02 levels in the atmosphere becoming scarily high, and this drought should definitely be considered a potential result of global warming. We as humans rely on water for a large majority of what we need to survive, in terms of hydration, sanitation, and energy. It is important to take this drought seriously and to constantly be taking the steps necessary to get California back to a safe water level that is appropriate for our survival. #UCIGCB2015

  • We have watched sever drought and excessive floods, and the devastation they cause, in many countries around the world but this time it is the eighth biggest economy in the world. The drought itself is not so unusual but the extra heat from climate change is really doing the damage. With California producing such a high proportion of the US’s food it will be interesting to see how it all pans out.

  • Jack Wolf

    It seems that drought years in CA since 2000 have outnumbered non-drought years. It’s the new normal. And, they had fair warning from the scientists. Now the hammer falls.

  • Wanda Allen

    This is happening because of geoengineering programs that are ongoing and causing devistation around the world. Known and stated side effects, drought in some areas,floods in others, food disruption and ozone depletion.

    • Jack Wolf

      No, it’s not due to some grand conspiracy, but due to fossil fuel driven climate change.