Cayan et al., 2010. “Future dryness in the southwest US and the hydrology of the early 21stcentury drought, PNAS, Vol. 107, December 14, 2010, pp 21271-21276:
Although the recent drought may have significant contributions from natural variability, it is notable that hydrological changes in the region over the last 50 years cannot be fully explained by natural variability, and instead show the signature of anthropogenic climate change.
Dai, A. 2013. “Increasing drought under global warming in observations and models,” Nature Climate Change, Vol 3., pp. 52-58. DOI:10.1038/nclimate1633
Trenberth, K. E., A. Dai, G. van der Schrier, P. D. Jones, J. Barichivich, K. R. Briffa, and J. Sheffield, 2014: Global warming and changes in drought. Nature Climate Change, 4, 17-22, doi:10.1038/NCLIMATE2067.
Wehner et al., 2011. “Projections of future drought in the continental United States and Mexico,” Journal of Hydrometeorology, Vol. 12, December 2011, pp 1359-1377.
All models, regardless of their ability to simulate the base-period drought statistics, project significant future increases in drought frequency, severity, and extent over the course of the 21st century under the SRES A1B emissions scenario.
Pederson et al., 2011. “The unusual nature of recent snowpack declines in the NorthAmerican Cordillera,” Science, Vol. 333, 15 July 2011, pp 332-335.
Over the past millennium, late 20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains and in their north-south synchrony across the cordillera. Both the snowpack declines and their synchrony result from unparalleled springtime warming that is due to positive reinforcement of the anthropogenic warming by decadal variability. The increasing role of warming on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western United States.
See also the conclusions from:
The 2013 report of the IPCC’s Working Group I, The Science Basis (pdf)
The Technical Summary from IPCC WGI 2013 report (pdf)
(1) Trenberth, K. E., A. Dai, G. van der Schrier, P. D. Jones, J. Barichivich, K. R. Briffa, and J. Sheffield, 2014: Global warming and changes in drought. Nature Climate Change, 4, 17-22, doi:10.1038/NCLIMATE2067.