California’s annual average temperature in 2015 was the second hottest since record-keeping began in 1895. The year 2014 was the all time hottest on record; 2013 the ninth hottest; and 2012 the fifth hottest.
The spike in average temperatures in recent years is no coincidence. California is experiencing warmer baseline conditions, driven by climate change, that have increased the frequency and severity of arid conditions in California, and increased the chance that low rainfall years will produce drought.
The increase in annual average temperature has also affected California’s seasons. The frost-free season length, defined as the period between the last occurrence of 32°F in the spring and the first occurrence of 32°F in the fall, increased by 19 days in the Southwestern US during 1991-2012 relative to 1901-1960.
Thanks to higher temperatures driven by climate change, droughts are increasingly temperature-driven in California. This is fundamentally different from previous mega-droughts that were driven by precipitation.
By one account, human-caused warming is estimated to have caused 8 to 27 percent of the observed drought anomaly in California from 2012 to 2014 and 5 to 18 percent in 2014.