El Niño events are becoming stronger as the world warms

By Andrew Freedman, CNN

(CNN) — The climate pattern that drives El Niño is becoming more erratic as the planet warms, and El Niño itself is becoming more intense, according to a comprehensive new study published Thursday.

The research sheds new light on the developing Super El Niño gathering strength in the tropical Pacific, which is forecast to become the strongest such event since at least 1950 — and potentially long before that.

The study, published in the journal Science, finds that El Niño events during the past 40 years have become 36% stronger than those before the industrial revolution, when people began to generate large volumes of climate pollution. They were also about 16% stronger than the El Niño events that occurred during the 20th century.

The research relies on records of past temperature variability gleaned from living and ancient fossil corals in the Galápagos Islands. These islands are located in an all-important eastern section of the tropical Pacific that is key to El Niño’s formation and intensification.

Corals are the time machines of the sea, said study lead author Julia Cole of the University of Michigan.

“The corals preserve a history of the conditions under which they grew in the chemistry of their skeleton, and we’re able to extract that history by measuring the chemistry,” she said.

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Researchers found that the El Niño events of the past few decades are a marked departure from those of the previous 1,000 years prior to the industrial revolution.

“Something is going on now that is highly unusual and very likely due to human-caused climate change,” said study co-author Jonathan Overpeck, the dean of the School for Environment and Sustainability at the University of Michigan and a coauthor of the new study.

The new research utilizes a combination of coral records extending back 1,000 years plus results from a suite of 12 state-of-the-art climate models to rule out natural climate variability as the cause for El Niño’s recent changes. Overpeck and other study authors said the El Niño events beginning with the 1982-1983 Super El Niño depart dramatically in their intensity from the events that showed up in the millennium-long coral record.

“We are looking at a system that has been perturbed,” Cole said. “We can see that it’s not behaving the way it used to.”

The study stops just shy of formally attributing the shifts in El Niño specifically to human-caused climate change. Further research is needed to pinpoint that causation, study coauthors said.

However, the results leave human-caused global warming as the most likely culprit for strengthening the El Niño events during the past few decades. The study adds to other research that also points to climate change-induced shifts in El Niño’s characteristics.

El Niño events are characterized by hotter than average sea surface temperatures in the equatorial tropical Pacific Ocean, along with corresponding shifts in the atmospheric circulation throughout that region. These events can increase global average surface temperatures and alter the weather globally, including triggering extreme weather events worldwide.

Corals reveal El Niño has changed

The data shows that the swings from El Niño to La Niña (its cooler-water sibling) and back again are increasing in amplitude, and that this is because the cycle is increasingly skewed toward more intense El Niño phases.

The study’s generation and use of a millennium-long coral climate record is extremely valuable to climate scientists, said Kim Cobb, a coral researcher at Brown University, who was not involved in the new study.

“This is a timely and important addition relevant to the question of climate change impacts on El Niño strength,” she said. “While we wait to see just how large the 2026 El Niño will become, this study provides fresh evidence that man-made global warming may be responsible for a portion of its chart-topping size.”

Cobb referred to the study’s reconstruction of El Niño activity as “close to the gold standard.” Specifically, Cole and her colleagues were able to learn about what ocean temperatures were like at the study locations during the past millennium by analyzing the ratio of certain isotopes present in the coral reef core samples.

“The right choice of archive, using corals, in the right place, where El Niño packs its largest punch,” Cobb said.

The study comes as the rapid warming of our current El Niño is outpacing all but the most intense historical events, and it is already altering weather conditions and contributing to weather extremes around the world.

Vanishing climate evidence

The warmer water from El Niño, coupled with long-term warming, is likely to harm the very corals that provided the new insights, Cole said. Corals thrive within a narrow temperature range, and when the ocean water becomes too hot, they can expel the organisms that give them their vibrant colors — turning the coral skeletons a ghostly white in what is known as a coral bleaching event. If the ocean water remains above a certain temperature band for a prolonged period, coral death can ensue.

Worldwide coral bleaching events have coincided with major El Niño events, including the most recent Super El Niño from 2015 to 2016, and scientists fear another one is coming this year.

This means that scientists like Cole and her colleagues are involved in a high-stakes race to obtain as many coral samples as possible before there are no more healthy corals to take samples from, much as glaciologists are working to secure ice cores from fast-receding mountain glaciers before they disappear.

Cobb noted that the study has important implications for future El Niño events, since the results imply that stronger ones are becoming the norm.

“As we pile up the evidence for a fossil fueled-intensification of El Niño, we contemplate a future of worsening El Niño-related weather and climate extremes on top of a baseline of worsening weather and climate extremes,” Cobb said. “This would have massive implications for communities and ecosystems around the world.”

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