Study: California drought a symptom of Earth’s fever

Tree rings show the current combination of dryness and heat makes this the worst drought in 1,200 years

Researchers expect drought to become frequent and last longer. MAP COURTESY IPCC.
Researchers expect drought to become frequent and last longer. MAP COURTESY IPCC.

Staff Report

FRISCO — California’s current drought is already going down as one of the worst in recorded era, and a new tree-ring study by scientists shows it may be the driest period for the region in 1,200 years.

Researchers with the University of Minnesota and Woods Hole Oceanographic Institution 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,” said University of Minnesota professor Daniel 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,” Griffin said.

Using recent National Oceanic and Atmospheric Administration climate data for the summer of 2014, the two scientists reconstructed rainfall back to the 13th century and scaled it using modern drought measurement tools like the Palmer Drought Severity Index. Their model also included a spatial tree-ring based reconstruction of drought developed by scientists at Columbia University’s Lamont-Doherty Earth Observatory.

Griffin and his colleagues said the current period of low precipitation is not unusual in California’s history. But combining the rainfall deficits with months of record-high temperatures is making this drought epic and is probably a symptom of human-caused global warming, with hot droughts becoming more common in the future.

“We were genuinely surprised at the result,” said 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.

The scientists said there is room to wonder how California will bounce back.

“There is no doubt that we are entering a new era where human-wrought changes to the climate system will become important for determining the severity of droughts and their consequences for coupled human and natural systems,” said co-researcher Kevin Anchukaitis, an assistant scientist at Woods Hole Oceanographic Institution.

The results of their study are published this week in Geophysical Research Letters in the article, “How unusual is the 2012-2014 California Drought?



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