Source: http://arstechnica.com/science/2015/03/c...

Since 2012, California has been in the midst of a record-setting drought, with extremely warm and dry conditions characterizing the last three years in that state. A new paper published in the Proceedings of the National Academy of Sciences concludes that warming caused by humans is responsible for the conditions that have led to this California drought.

This analysis found that the statewide warming in California occurs in climate models that include both natural and human factors, but not in simulations that only include natural factors. It's a difference with a very high (0.001) level of statistical significance.This study, published by scientists affiliated with the Department of Environmental Earth System Science and the Woods Institute for Environment at Stanford University, used historical statewide data for observed temperature, precipitation, and drought in California. 

In their discussion and conclusions, the investigators state that their results strongly suggest that anthropogenic (human-caused) warming has increased the probability of co-occurring temperature and precipitation conditions that have historically led to California’s droughts. They also state that continued global warming is likely to lead to situations where every future dry period, whether it’s seasonal, annual, or multiannual, will be accompanied by historically warm conditions.

This projected increase in dry/warm conditions may have serious impacts on human and natural systems. The authors posit that high temperatures during spring and autumn could deplete stores of snowpack and increase wildfire risk, severely affecting the state’s ecology. They also hypothesize that increased dry periods will increase the risk of water shortages for humans and ecological systems and even raise the threat of species extinction due to drought.

This is one of a number of studies to have looked at California’s drought, and they’re coming to similar conclusions: historically it’s not unusual for the state to have either warm or dry winters, but the recent drought is notable for having both at once.