Climate change could bring twice as many big El Niños

Scientists have long wondered whether rising global temperatures will lead to an increase in intense El Niño events, like the one that could bring major storms to Southern California this winter. According to a groundbreaking study published Monday, the answer is a resounding, “Yes.”

If human beings don’t slow their emission of planet-warming greenhouse gases, extreme El Niños could nearly double in frequency — from once every 28 years to once every 16 years on average, the new study found. That could spell trouble for California, which experienced mudslides, flooding and $500 million in damage during an intense El Niño in the late 1990s.

The idea that global warming will lead to stronger El Niños makes perfect sense, said Bill Patzert, a NASA climatologist who was not involved in the study. That’s because El Niños are driven largely by warming water in the tropical Pacific Ocean, which will get especially hot as the world temperatures rise.

“Already, 90 percent of all the heat is taken up by the oceans. More heat will accumulate in the tropics,” Patzert said. “Intuitively, this makes an awful lot of sense.”

If a strong El Niño materializes this winter, as scientists have predicted, it could make a dent in California’s historic four-year drought. But in the long term, experts cautioned, an increase in extreme El Niños won’t offer much relief from the longer, more frequent and severe droughts that climate change is expected to bring.

An increase in extreme El Niño events will probably be accompanied by an increase in extreme La Niña events, which lead to dry winters in California. Additionally, El Niño events contribute a small fraction of California’s overall water supply, and that won’t change much even if strong El Niños happen more often.

“The background climate state has a trend in California — and most of the Western U.S. — toward increased drought,” said Kim Cobb, a climate scientist at the Georgia Institute of Technology and co-author of the new study. “That’s just how climate change marches forth, decade by decade, in California. That’s going to the major force.”

El Niño events also put critical infrastructure, including major highways, at risk. State officials list about 450 bridges as potentially unstable during intense floods, including five in the Coachella Valley — two of which are traversed by 80,000 drivers per day.

During the last two major El Niños, Patzert noted, there were no droughts, so there was nothing to be hopeful about.

“This event, if it delivers, people are thinking in terms of drought relief,” Patzert said. “But that’s not how it played on the marquee in the past couple of big events. It played as mudslides, flooding, general havoc.”

The new study, “ENSO and greenhouse warming,” was led by Australian researchers and published in the peer-reviewed journal Nature Climate Change. More than a dozen scientists surveyed existing research on El Niño, finding widespread agreement among the latest models that climate change will lead to an increase in extreme El Niño events.

The lead researcher on the study was Wenju Cai, a climate scientist at Australia’s Commonwealth Scientific and Industrial Research Organisation. Cai called the relationship between climate change, El Niño and La Niña a prime example of what scientists mean when they say climate change is causing more extreme weather events of all stripes.

“Each kind of extreme event will occur more often,” Cai said in a Skype interview Monday. “A huge La Niña would tend to cause big droughts in the southwest U.S., but El Niño will give you floods.”

Researchers noted it’s difficult to say with certainty how complex global weather patterns like El Niño and La Niña will impact specific regions. Cai’s group defined “extreme” El Niño events as those involving average winter rainfall of at least 5 millimeters per day in the eastern equatorial Pacific, but there’s no guarantee those conditions would lead to intense storms in California.

Another key point: A doubling of extreme El Niño events isn’t set in stone.

Researchers based their projections on a “business as usual” climate change scenario, in which human beings continue spewing carbon into the atmosphere unabated. If world governments take strong action on climate change — and global emissions stop rising around 2060 — the increase in extreme El Niños could be limited to about 30 percent, Cai said.

Sammy Roth writes about energy and water for The Desert Sun. He can be reached at sammy.roth@desertsun.com, (760) 778-4622 and @Sammy_Roth.