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Research Article

Anthropogenic warming has increased drought risk in California

Noah S. Diffenbaugh, Daniel L. Swain, and Danielle Touma
  1. aDepartment of Environmental Earth System Science and
  2. bWoods Institute for the Environment, Stanford University, Stanford, CA 94305

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PNAS March 31, 2015 112 (13) 3931-3936; first published March 2, 2015; https://doi.org/10.1073/pnas.1422385112
Noah S. Diffenbaugh
aDepartment of Environmental Earth System Science and
bWoods Institute for the Environment, Stanford University, Stanford, CA 94305
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  • For correspondence: diffenbaugh@stanford.edu
Daniel L. Swain
aDepartment of Environmental Earth System Science and
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Danielle Touma
aDepartment of Environmental Earth System Science and
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  1. Edited by Jane Lubchenco, Oregon State University, Corvallis, OR, and approved January 30, 2015 (received for review November 22, 2014)

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Significance

California ranks first in the United States in population, economic activity, and agricultural value. The state is currently experiencing a record-setting drought, which has led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Our analyses show that California has historically been more likely to experience drought if precipitation deficits co-occur with warm conditions and that such confluences have increased in recent decades, leading to increases in the fraction of low-precipitation years that yield drought. In addition, we find that human emissions have increased the probability that low-precipitation years are also warm, suggesting that anthropogenic warming is increasing the probability of the co-occurring warm–dry conditions that have created the current California drought.

Abstract

California is currently in the midst of a record-setting drought. The drought began in 2012 and now includes the lowest calendar-year and 12-mo precipitation, the highest annual temperature, and the most extreme drought indicators on record. The extremely warm and dry conditions have led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Analyzing historical climate observations from California, we find that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm. We find that although there has not been a substantial change in the probability of either negative or moderately negative precipitation anomalies in recent decades, the occurrence of drought years has been greater in the past two decades than in the preceding century. In addition, the probability that precipitation deficits co-occur with warm conditions and the probability that precipitation deficits produce drought have both increased. Climate model experiments with and without anthropogenic forcings reveal that human activities have increased the probability that dry precipitation years are also warm. Further, a large ensemble of climate model realizations reveals that additional global warming over the next few decades is very likely to create ∼100% probability that any annual-scale dry period is also extremely warm. We therefore conclude that anthropogenic warming is increasing the probability of co-occurring warm–dry conditions like those that have created the acute human and ecosystem impacts associated with the “exceptional” 2012–2014 drought in California.

  • drought
  • climate extremes
  • climate change detection
  • event attribution
  • CMIP5

Footnotes

  • ↵1To whom correspondence should be addressed. Email: diffenbaugh{at}stanford.edu.
  • Author contributions: N.S.D., D.L.S., and D.T. designed research, performed research, contributed new reagents/analytic tools, analyzed data, and wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • See Commentary on page 3858.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1422385112/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Global warming increases California drought risk
Noah S. Diffenbaugh, Daniel L. Swain, Danielle Touma
Proceedings of the National Academy of Sciences Mar 2015, 112 (13) 3931-3936; DOI: 10.1073/pnas.1422385112

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Global warming increases California drought risk
Noah S. Diffenbaugh, Daniel L. Swain, Danielle Touma
Proceedings of the National Academy of Sciences Mar 2015, 112 (13) 3931-3936; DOI: 10.1073/pnas.1422385112
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  • Climate change and California drought
    - Mar 23, 2015
Proceedings of the National Academy of Sciences: 112 (13)
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