Californians are living through the most severe three-year drought in the last 1,200 years, according to a new study published this week in an American Geophysical Union journal.
Although other three-year periods have been drier, 2012-14 stands as perhaps the worst drought in a millennium due in part to "anomalously low" precipitation and "record-high" temperatures, the study said.
And viewed through the lens of history and the changing climate, this current severe event won't be the last, the study's co-author said.
"One thing is clear," said Daniel Griffin, an assistant professor in the University of Minnesota's department of geography. "Drought is going to continue to happen. This is the kind of thing we get to see in the future."
Griffin has been doing research on tree rings, moisture and drought in California for a decade. He teamed with co-author Kevin J. Anchukaitis, a paleoclimatologist at the Woods Hole Oceanographic Institution, to see how the state's current precipitation levels and drought severity compared with centuries past.
To measure precipitation levels, the researchers studied the rings of blue oak trees in Central and Southern California, which have a strong growth relationship with rain. Then they compared their tree-ring data with National Oceanic and Atmospheric Administration rain records and built a model reconstructing precipitation levels hundreds of years into the past.
To measure the severity of the current drought, researchers then turned to the North American Drought Atlas, which provides historical drought severity data based on tree rings that go back at least 1,200 years. The Atlas generates Palmer Drought Severity Index data, which measure drought severity by considering soil moisture conditions, which are in turn affected by precipitation, temperature and evaporation.
Their study concluded that, in terms of "cumulative severity," drought during the three-year period from 2012-14 "stands out in the context of the last millennium" and was more extreme than droughts that stretched even longer – from four to nine years. Over the last 1,200 years, researchers estimated that three-year droughts occurred about 37 times.
But the lack of precipitation between 2012 and 2014 was not "unprecedented," the study said. There were times near the turn of the 20th century and the early 16th century when there was less rain in a three-year drought period. Higher temperatures, though, may have exacerbated the current drought by as much as 36%, according to the authors. Combined with low rainfall, those conditions made this drought California's worst in more than a millennium.
"It was a surprise," Anchukaitis said of the severity finding. "I don't think we expected to see that at all."
Griffin acknowledged that there was a "lingering question" about whether one of the decade-long droughts in the medieval era may have been more severe, saying there was not "a perfect way" to measure."
The bad, if obvious, news for water-starved California?
Researchers found about 44% of three-year droughts go on to last four years or longer.