No Doubt It's A Climate-Change Drought, Scientists Say

It was raining in San Francisco on the damp December morning that three scientists gathered at the offices of Climate Nexus to hold a press conference about the drought. It had been raining regularly for more than a week, in fact, and Stanford University had just recorded its rainiest day ever on campus.
These three drought experts had gathered to swim upstream against all that rain and evaporate any false optimism it might be washing into California.
"I’m happy to be here and see some rain here in San Francisco," said Jonathan Overpeck, who had flown in from Tucson, where he is a founding codirector of the Institute of the Environment at the University of Arizona. "Of course everyone knows California’s drought has been for three years, rain so far has been helpful, there’s a snowpack in the Sierra Nevada’s that is about 50 percent of normal thanks to recent precipitation, but that hasn’t stopped the drought. The drought is still going to be the story at the end of the year, I think.
"To frame the drought we should be mentioning that much of the southwest and west has been in drought now for nearly 15 years, since 1999. This is important for California because a big part of that story is the Colorado River. The major storage, the biggest reservoirs in the United States are only half full now, and precipitation this winter has been lower than average there."
The scientists had gathered in part because a recent study from NOAA has been interpreted to suggest the drought derives from the natural variability of the climate. But these three scientists say that interpretation derives from NOAA's focus on only one aspect of the drought—mean rainfall. When you look at the drought as an extreme event, they said, and when you look at its probability of recurring, and when you look at not only rainfall but also temperature and evaporation, there's no doubt what's behind the drought.
"One of the things that is certainly making it worse is climate change," Overpeck said. "If we really want to tackle the water problem in the west we need to tackle the climate change problem."
Overpeck and Stanford Professor Noah Diffenbaugh said the NOAA study is sound but does not consider all of the factors that reveal the influence of anthropogenic climate change.
"I agree that all of these studies that in some cases appear at first glance to be conflicting are good science, but they all have their own focus. But one of the reasons we’re doing this press event today is that we’re trying to give you the big picture."
To see the big picture, one has to look beyond mean rainfall, at other places climate signals may be recorded, the scientists said. For example, high temperatures worsen droughts by causing moisture to evaporate more quickly.
"If you’re a betting person you can bet your car, you can bet your house that over time in the future it’s gonna get hotter and hotter until we curb the emissions of greenhouse gasses to the atmosphere, you can bet as much as you want on that," Overpeck said.
When higher temperatures reduce soil moisture, vegetation suffers. So vegetation is another place to look for climate signals.
California blue oaks are some of the most climate-sensitive trees on earth. Scientists from the Woods Hole Oceanographic Institute recently published results of an analysis of blue oak tree rings, said Valerie Trouet, a tree-ring scientist from the University of Arizona, that reveals the unusual nature of the current drought.
"What my colleagues did is use the record derived from the Blue Oaks to reconstruct drought and precipitation independently over the past 800-1,200 years," Trouet said. "What they found is that when you look at drought as Jonathan described earlier, the current drought that California’s in now is the most severe drought that we’ve witnessed over the last 1,200 years. There has been no drought that’s been as severe as this in the record."
That tree-ring study also reveals how long severe droughts usually last in California. Almost half lasted four years or longer, and some lasted nine years, Overpeck said, compared to three years for the current drought. But droughts in the Colorado River Basin can last much longer—as long as 50 years.
That makes it difficult to anticipate that relief will fall from the sky.
"That’s important to not just the greater southwest and Arizona where I’m from, but California’s one of the largest users of Colorado River water, so we have to keep in mind, as temperatures go up, that these droughts could go on and on, and the unprecedented severity would go on."