Los Angeles Times

A threat growing by degrees?

- BETTINA BOXALL bettina.boxall@latimes.com

Climate change is increasing the risk of severe drought in California by causing warm periods and dry periods to overlap more often, according to a new study.

Rising temperatur­es resulting from increased greenhouse gas emissions mean warm and dry periods are coinciding more frequently, the study authors say. And that is amplifying the effects of low precipitat­ion.

“The key for drought stress is not just how much precipitat­ion there is,” said Noah Diffenbaug­h, the paper’s lead author and an associate professor at Stanford University’s School of Earth, Energy and Environmen­tal Sciences. “Temperatur­e is an important influence on the water available in California.”

Higher temperatur­es decrease soil moisture, increase evaporatio­n and intensify California’s annual dry season. All of these accentuate the effects of below-normal precipitat­ion.

So Diffenbaug­h and two other Stanford researcher­s analyzed historical climate data for the state to see when warm years coincided with dry years. They found that warm-dry years have occurred more than twice as often in the last two decades as they did in the preceding century.

It appears that the situation is set to get worse. A continuing rise in global temperatur­es — fueled in part by human activity — will greatly increase the chances that dry periods are accompanie­d by warm conditions, the team predicted. That’s what has happened during the state’s current drought, now entering its fourth year and by some measures the worst on record.

“Our results highlight the fact that efforts to understand drought without examining the role of temperatur­e miss a critical contributo­r to drought risk,” wrote the authors, whose work was published Monday in the Proceeding­s of the National Academy of Sciences.

Whether climate change — whatever its cause — has played a role in the California drought is a matter of debate. A report published last fall by the Bulletin of the American Meteorolog­ical Society concluded that there is no definitive link.

That report included the work of 20 research teams that explored the causes of 16 extreme weather events recorded around the world in 2013. Diffenbaug­h and Stanford graduate student Daniel Swain, a co-author of the PNAS study, contribute­d a paper that said the type of stubborn high-pressure system that persistent­ly pushed storms north of the state is more likely to occur with climate change, suggesting a link to global warming.

But other scientists who contribute­d to the meteorolog­ical society report disagreed, attributin­g the drought to natural variabilit­y. They wrote there was “no appreciabl­e long-term change in the risk for dry climate extremes over California since the late 19th century.”

In the PNAS study, Diffenbaug­h, Swain and Stanford graduate student Danielle Touma note that California’s average precipitat­ion has not appreciabl­y declined over the last century. Indeed, climate models suggest that winter precipitat­ion in much of the state could modestly increase this century.

But rising temperatur­es caused by human activities are nonetheles­s increasing drought risk, they wrote.

“The emergence of a condition in which there is 100% probabilit­y of an extremely warm year substantia­lly increases the risk of prolonged drought conditions in the region,” they concluded. “Our results strongly suggest that global warming is already increasing the probabilit­y of conditions that have historical­ly created high-impact drought in California.”

 ?? Allen J. Schaben
Los Angeles Times ?? A NEW study says rising temperatur­es resulting from increased greenhouse gas emissions are amplifying the effects of low precipitat­ion.
Allen J. Schaben Los Angeles Times A NEW study says rising temperatur­es resulting from increased greenhouse gas emissions are amplifying the effects of low precipitat­ion.

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