How much worse is our carbon spew making California’s record-breaking drought?

About 15 to 20 percent worse, according to a new study that is the first to put a number on climate change’s impact on the state’s dry spell.

“We weren’t looking at the length of the drought but the severity of it,” said A. Park Williams, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study, which was published Thursday in the journal Geophysical Research Letters. “In the absence of global warming, there would still be a drought in California; it would be just be less severe.”

The findings are part of a growing body of evidence that people are playing a major role in the state’s four-year-long drought. Noah Diffenbaugh, professor of environmental earth system science at Stanford University, said the latest study complements his ongoing research on a persistent high-pressure system called the “ridiculously resilient ridge.” The Triple-R, as it’s been dubbed, has been lingering over the Pacific Ocean and steering storms away from California’s coast. Research shows that the phenomenon is more likely to occur as temperatures increase.

“The historical warming trend we’re seeing is really doubling the probability of severe drought conditions across the state, and this new paper is a confirmation,” Diffenbaugh said.

To get a figure—instead of just an “influence”—on how much climate change is shaping the state’s drought, Williams and his colleagues broke California into a grid of 24,000 regions, or “buckets.” They then looked at a century’s worth of historical precipitation levels—the main influencer of drought—for each bucket, along with other drought influencers such as temperature, wind speed, humidity, and net radiation.

“Through our models, we can go back and manipulate the climate scenarios, and by just teasing out the temperature increases that we’ve already seen, we were able to show that this drought is substantially worse due to global warming,” Williams said.

Diffenbaugh and other Stanford scientists are now investigating just how dry the state will become if the current rate of carbon emissions continues.

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“Look at the last 120 years of data, and you’ll see that California used to have about half of its years above average temperatures and half below average, and half wet years and half dry—but over the past 20 years, that’s not the case,” Diffenbaugh said. “California’s warmed so much that more than 80 percent of the years now are warmer than average.”

Even if temperatures are kept from rising more than 2 degrees Celsius—the threshold for avoiding catastrophic climate change—California will likely experience longer, more severe, and more frequent droughts.

“Precipitation will vary up and down in California, but the real risk is that the temperature effect becomes so strong that it melts snowpack quicker, draws water out of soil and plants, and puts much of the state in perpetual drought conditions,” Williams said.

In other words, drought could become the new norm for the Golden (Brown) State—something Diffenbaugh says California is not prepared for. “Water storage and water rights in the state are based on the climate of 100 years ago, not for the conditions we’re going to be facing,” he said.