President Obama and Governor Brown believe the science is settled and carbon emissions lead to droughts. Before we test the veracity of their beliefs, consider that many of the warmest places on Earth, such as rainforests, are both warm and wet. Further, some of the driest places on Earth, such as Antarctica and Siberia, are also the coldest. The coldest city in the world, Oymyakon, Russia, has a mean annual temperature of -15.5 °C and gets only 8.3 inches of precipitation per year. Antarctica's McMurdo Dry Valleys, obviously cold, are the world's driest locations. So we can't just assume that warmer equals drier.
Droughts are defined by reduced precipitation and increased evaporation, and California's 2011-2015 drought had both. What happened over the last three winters was a major reduction in precipitation (a reduction of 0.9 mm per day) with only a minor increase in evaporation (less than 0.1 mm per day). In short, the reduction in precipitation was an order of magnitude more than the increase in evaporation. "California lost essentially one full year of precipitation," according to Richard Seager, a climate model specialist at Columbia University's Lamont-Doherty Earth Observatory. "The [reduced] precipitation was the essence of this drought," added Marty Hoerling, a meteorologist at the NOAA Earth System Research Lab: "Farmers were praying for rain, not cooler temperatures."