Scientists strengthen link between climate change and drought

In early August, California Gov. Jerry Brown held a news conference at Cowboy Camp, a spot of BLM land popular with equestrians near Clear Lake, northwest of Sacramento. For more than a week, amid warm, dry conditions, the Rocky Fire had been aggressively racing through the nearby hills, jumping a highway, forcing thousands to evacuate, and scorching dozens of homes. Brown was there to console families, thank firefighters, and also to deliver a blunt message about climate change.

"Unlike the East, where climate change seems to be adding more storms, here in California and the Southwest it's more dryness. We've got more dryness, less moisture and more devastating fires," he said, according to the L.A. Times. He went on, the paper reported, to issue a challenge to Republican presidential hopefuls, climate leaders none of them: "California is burning," he said. "What the hell are you going to do about it?"

This is the point in the story where reporters deliver an obligatory caveat: It is difficult to tie any single fire to climate change. Still, going forward, the warming of the earth will make such events more likely in California and much of the arid West. While that's true, it's also true that scientists are getting better at teasing out the extent to which climate change is or isn't influencing extreme events, from heat waves to floods to blizzards.

Researchers have also begun to define the role climate change has played in the current California drought. In public discussions, climate change and drought are often linked, even if research hasn't yet demonstrated that one is causing the other in the present, says John Abatzoglou, an associate professor who studies weather and climate at the University of Idaho, and co-author of a new study on the subject, published online today in the journal Geophysical Research Letters. "So we set out to put some numbers on how much more severe this drought in California has been as a result of the warming that we have seen."

The new study concludes that the lack of rain in California the past few years has been primarily a function of natural variability, not climate change. At the same time, other research has shown that climate change has increased the probability of rare ridges of extreme high-pressure in the atmosphere over the Northeast Pacific, the so-called "Ridiculously Resilient Ridge" that has blocked storms from reaching California and is the underlying cause of the drought.

That's not the same as saying that climate change had a role in causing the current ridge – research hasn't demonstrated that – but it is one indication of how climate change is increasing drought risk. Most models agree that climate change is not likely to significantly affect mean precipitation in the future, says Noah Diffenbaugh, a climatologist at Stanford University (and, full disclosure, the editor of the journal the new study is published in). It is, however, likely to increase the frequency of extremely low precipitation years, he adds.

But the amount of moisture that falls from the sky is not the only thing that determines a drought's severity. Temperature, wind speed, solar radiation and humidity also matter because they influence how rapidly moisture in plants and soils evaporates into the atmosphere. 

The new study focuses on these other factors, and researchers found that unusually hot temperatures attributable to anthropogenic climate change intensified the current drought. Think of drought like a debt of moisture instead of money, explains lead author Park Williams, a bioclimatologist at Columbia University's Lamont Doherty Earth Observatory. The goal of this study was to figure out how much various factors contributed to California's debt between 2012 and 2014. Anthropogenic warming, Williams and his co-authors concluded, was responsible for between 8 and 27 percent of it. 

"Global warming is not causing the drought to occur," says Williams. "Instead, it's amplifying a drought that would already be in place. Global warming is important and it's going to become increasingly important."

Some of the most striking findings in Williams' study come in a supplemental analysis that looks at the difference between the actual drought conditions, and what they likely would have been without the influence of climate change. Williams calculated how much climate change had shifted the baseline conditions that droughts are measured against on a tool called the Palmer Drought Severity Index. On the index, a value of zero indicates normal conditions, while a value of -1 indicates mild drought; the further you get below -1, the more severe the drought. So far, warming has shifted the baseline on the index from zero to about -0.5, meaning it now takes a slightly less dramatic dip in precipitation to cause a drought. 

But the math gets a lot more troubling in a few decades. Climate models that simulated warming in California from 1895 to 2014 did a good job of predicting actual conditions, says Williams, indicating that they can also reliably project the trend into the future. If warming continues at its current pace, Williams found, by about the 2050s it will have shifted the index's baseline value to right around -1. In other words, within just a few decades, even when California receives what we today consider normal amounts of rain and snow, it will be teetering on the edge of drought simply because it's hotter. In the absence of global warming, Williams explains, values below -1 are likely to be reached in California during about 33 percent of years. With global warming, in the 2050s values below -1 become likely to occur around 54 percent of the time. Very severe droughts, represented by values of -3 or lower, go from occurring in about 5 percent of years to 15 percent. 

The study's conclusions are consistent with other recent research, including a study Diffenbaugh published in PNAS earlier this year that found that the probability of drought in California has doubled compared to the previous century. That's happened due to higher temperatures, and without any increase in the probability of low precipitation years. "With a lot of extreme events, it doesn’t take a lot of change to have a big impact," Diffenbaugh says. "It only takes a degree of warming or so and we’re already seeing a doubling of the drought risk."  

"The warming signal has definitely emerged from the noise, and we can start making very reliable estimations of where we'll be in 10, 20 or 30 years," Williams adds. "It will be a pretty different world than it is even right now."