When Daniel Griffin first heard media reports earlier in the fall that California's drought was the worst in the past few centuries, he didn't quite buy it.

Griffin, an assistant professor in the Department of Geography, Environment and Society at the University of Minnesota, had a decade of experience studying paleoclimate and environmental science using tree-ring data. From his research on blue oak trees, he knew that periods without a lot of precipitation weren't that uncommon in the state's history.

"I was kind of skeptical that this year would be different," he said.

Still, Griffin was curious to see if the blue oaks he was studying could provide clues about the state's three-year drought. The trees are particularly sensitive to changes in water availability, and their rings clearly showed changes in moisture levels over the trees' lifetime.

"When it's very dry, they grow very slow; when it's a wet year, they grow like gangbusters," he said.

The trees are long-lived. They can be up to 500 years old, and deadwood like stumps can stay in the ground for around 700 years, providing an extended record of water availability over centuries. They are also native to Southern and Central California, where the drought has been the most severe.

Griffin, along with Kevin Anchukaitis, an assistant scientist at Woods Hole Oceanographic Institution, began taking pencil-thin tree-ring samples from trees to see how they were responding to the drought compared to previous years.

The researchers compared their tree-ring data with National Oceanic and Atmospheric Administration precipitation measurements from between 1920 and 2014, and used that information to reconstruct precipitation history before rain gauge measurements began.

To compare the present drought to the magnitude of past events, the researchers used the Palmer Drought Severity Index, a proxy for soil moisture that includes temperature effects, from the North American Drought Atlas, which was developed by the Lamont-Doherty Earth Observatory at Columbia University and extends back 1,200 years.

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.

A record that may not last for long

"Low precipitation is compounded by record high temperatures to create extreme drought," said Griffin.

Griffin and Anchukaitis published their findings in Geophysical Research Letters.

"This study really established how exceptional and severe this last drought was even in the last thousand years," said Benjamin Cook, a climate scientist at NASA's Goddard Institute for Space Studies who studies the drought on the West Coast. He was not involved in the study.

According to Cook, the modest declines in precipitation levels have not been enough to explain why the drought in California has been so bad. The study teased out how precipitation and temperature interacted to make dry conditions worse.

"The conclusion that I think is the most compelling is that warming from anthropogenic greenhouse gas emissions really made the drought more severe than it would have been," Cook said. "California is on track for the warmest year on their record."

The findings also raise concerns about what California could experience in the future, as well as how the state should plan to conserve its limited groundwater resources, said Park Williams, a professor at the Lamont-Doherty Earth Observatory.

"Regardless of how much of this year's heat was man-made or natural in origin, 2014 serves as an important reminder that heat can seriously exacerbate drought events," wrote Williams in an email. "If temperatures continue rising, we should expect record-breaking drought years to become increasingly common."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500