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The news is filled with stories of precipitation and water extremes, and one such event is the continuing drought in California. The state has experienced years of below average rainfall and as it headed towards its fourth year of drought a State of Emergency was declared back in January. The drought is a result of decreased rainfall combined with a reduced winter snowpack, and associated spring melt. Attribution of individual climate extremes, such as the current California drought, to climate change is difficult. The Bulletin of the American Meteorological Society recently published its latest annual supplement 'Explaining Extreme Events of 2013 from a Climate Perspective', which includes three studies on the California drought^{1, 2, 3}. These studies don't agree on the contribution of climate change to the drought. Some of the results suggest that global warming has increased the likelihood of atmospheric conditions favourable for drought, in contrast to other findings that indicate that internal variability of the climate system, such as anomalous sea surface temperatures, is responsible.

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1. Swain, D. L. et al. Bull. Am. Meteor. Soc. 95 (Explaining Extreme Events of 2013 from a Climate Perspective special supplement), S3–S7 (2014).
2. Wang, H. & Schubert, S. Bull. Am. Meteor. Soc. 95 (Explaining Extreme Events of 2013 from a Climate Perspective special supplement), S7–S11 (2014).
3. Funk, C., Hoell, A. & Stone, D. Bull. Am. Meteor. Soc. 95 (Explaining Extreme Events of 2013 from a Climate Perspective special supplement), S11–S15 (2014).
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4. http://ca.gov/drought/topstory/top-story-13.html
5. http://www.cpc.ncep.noaa.gov/products/expert_assessment/season_drought.png
6. NOAA Climate Prediction Center ENSO: Recent Evolution, Current Status and Predictions; available via http://go.nature.com/BCTp44
7. US Department of Defense 2014 Climate Change Adaptation Roadmap; available via http://go.nature.com/XvOrPH

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