California is currently in the midst of a record setting drought. An article published in Geophysical Research Letters links the magnitude of the drought to climate change. The authors of the paper state that without the current state of climate change, the drought would be as much as 27% less severe.
It is important to note that the authors of the paper are not saying that the drought would not have happened if we were not in our current state of climate change. Climate change is simply acting as a multiplier effect. The authors also state that as the effects of climate change continues to grow, future droughts will be much more severe.
The drought is caused by natural weather variations causing a lack of rainfall. Climate change has increased this effect by adding higher temperature to the mix. The higher temperatures lead to an increased amount of evaporation, creating much more drier conditions. With the current weather conditions, California needs and additional 3.5 inches of rainfall a year than it did in 1895 in order to keep up with its water demand. Basically, the temperature are increasing, but the rainfall isn't.