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Drought: California's dry spell is the worst of the last 1,200 years, study finds

Blue oak trees up to 500 years old thrive in California's drought-scorched foothills. Their rings provide a unique record of drought and wetness for the Golden State
Courtesy of Daniel Griffin
Blue oak trees up to 500 years old thrive in California's drought-scorched foothills. Their rings provide a unique record of drought and wetness for the Golden State

California's current drought is the worst of the last 1,200 years, according to a new analysis from the University of Minnesota and the Woods Hole Oceanographic Institution.

Not only is the weather severely dry, but the extreme heat of the last several years has baked much of the moisture out of the soil, creating intense drought conditions, the report finds.

As of Thursday, the dry spell, which dates back to 2012, has left nearly 80% of the state in a condition known as "extreme drought," according to the U.S. Drought Monitor.

Fifty five percent of the state is considered to be in "exceptional drought," which is the most severe classification there is.

The new study was published by the American Geophysical Union.

It found that in particular, the water year for 2014, which ended in September, was most likely the driest period examined, said researcher Kevin Anchukaitis. 

That includes 1580, 1782 and 1829 which were all famously dry years.

"2014 appears to be from our best estimate, slightly drier than these," he explained.

The study looked at precipitation levels and accumulated soil moisture using data from the National Oceanic and Atmospheric Administration and a computer climate model called the North American Drought Atlas.

Using the model, Anchukaitis and his research partner Daniel Griffin were able to create a picture of California's past weather dating back to the year 800 A.D.

In addition, researchers took tree ring samples from centuries-old blue oak trees scattered throughout California's Central Valley.

(A cross section of an oak tree. Patterns of narrow and wide rings correspond to years of drought and wetness. Image used with permission from Daniel Griffin.)

Using blue oaks, the team was able to find information about dry and wet years dating back as far as 1293.

Thicker rings correspond to wetter years when trees were able to grow faster, Kevin Anchukaitis explained.

Luckily, researchers don't have to cut down these ancient plants to study them.

Anchukaitis says he and his team simply inserted a small hollow drill in the trunk and pull out a core containing a small sliver of the tree.

"It's like for you and me getting a paper cut."

(Pencil-like tree-ring cores are collected non-destructively using an increment borer. Image by Daniel Griffin.)

However, the information from these trees only tells part of the story of California's droughts, noted USC Earth Sciences researcher Sarah Feakins, who was not involved with this study.

According to Feakins, blue oak trees grow only in select parts of the state and they are rare in Southern California, leaving some gaps in the tree ring data.

Still, she says the report is a "timely analysis" that brings clarity to the unique factors influencing the current dry conditions.

One thing the researchers noticed was that rainfall during the current drought isn't abnormally low for a dry period.

Rather, it seems to be the region's intense heat that has made the drought so unusual, Anchukaitis said.

As the temperature rises, more soil and plant moisture is lost to evapotranspiration. This worsens the effects of the drought.

He noted that man-made climate change is expected to send temperatures across the state even higher in the future, which could mean more so called "hot droughts" are on the way.

"This is going to be an increasing stress on water resources in California," he said.

This story has been updated.