And still, people will insist it is all natural.
Here is an abstract from their paper:
A comparison of historical climate data from California with different climate models has shown the influence of human-driven climate change. Noah Diffenbaugh, Daniel Swain, and Danielle Touma of Stanford University examined the state's records of temperature, rainfall, and drought conditions and evaluated the severity of different periods of warming and drought using the Palmer Hydrological Drought Index and the Palmer Drought Severity Index. They then compared those periods with a range of global climate models. The models that included only naturally occurring variations did not accurately depict California's warm periods, but the models that included anthropogenic warming did. The researchers believe that anthropogenic warming has increased the simultaneous occurrence of periods of both warming and decreased rainfall, which has resulted in an increase in the frequency and severity of droughts across the state.