Daniel Griffin, an assistant professor in the Department of Geography, Environment and Society at the University of Minnesota, and Kevin Anchukaitis, an assistant scientist at Woods Hole Oceanographic Institution, asked the question, “How unusual is the ongoing California drought?" Watching the severity of the California drought intensify since last autumn, they wondered how it would eventually compare to other extreme droughts throughout the state's history.
To answer those questions, Griffin and Anchukaitis collected new tree-ring samples from blue oak trees in southern and central California. “California’s old blue oaks are as close to nature’s rain gauges as we get,” says Griffin. “They thrive in some of the driest environments where trees can grow in California.” These trees are particularly sensitive to moisture changes and their tree rings display moisture fluctuations vividly.
As soon as the National Oceanic and Atmospheric Administration (NOAA) released climate data for the summer of 2014, the two scientists sprang into action. Using their blue oak data, they reconstructed rainfall back to the 13th century. ... Griffin and Anchukaitis found that while the current period of low precipitation is not unusual in California’s history, these rainfall deficits combined with sustained record high temperatures created the current multiyear severe water shortages. "While it is precipitation that sets the rhythm of California drought, temperature weighs in on the pitch," says Anchukaitis.
“We were genuinely surprised at the result,” says Griffin, a NOAA Climate & Global Change Fellow and former WHOI postdoctoral scholar. “This is California--drought happens. Time and again, the most common result in tree-ring studies is that drought episodes in the past were more extreme than those of more recent eras. This time, however, the result was different.” While there is good evidence of past sustained, multi-decadal droughts or so-called “megadroughts”' in California, the authors say those past episodes were probably punctuated by occasional wet years, even if the cumulative effect over decades was one of overall drying. The current short-term drought appears to be worse than any previous span of consecutive years of drought without reprieve....