"Their analysis showed that a number of other droughts in California's history had less precipitation than the one the state is currently experiencing. However, the most recent drought stood out because of how exceptionally hot it was compared to other droughts over the past 1,200 years. Even when they accounted for errors associated with combining the different data sets, they saw that "what's really different is the record high temperatures," Griffin said. "That kind of knocked my socks off, I wasn't expecting that result," he said. The "hot drought" was worse because the heat drew more moisture from the soil into the atmosphere, according to Griffin. For every 1 degree Celsius increase in air temperature, the atmosphere's capacity to retain moisture increases by 7 percent, as defined by the Clausius-Clapeyron equation." - Todd Hoff

So worst means a combination of dryness + high temperature. That's what makes now unique. - Todd Hoff