A 1000-year archive preserved in fossil corals from the Galápagos offers some of the strongest evidence yet that global warming is already making El Niño stronger. Reporting today online in Science, researchers found that temperature swings recorded in Galápagos corals—a place where the El Niño signal is strong—have intensified by 36% over the past 4 decades, far beyond anything seen in previous centuries. Along with other coral results elsewhere in the Pacific, the findings suggest “there’s something about this important source of climate extremes that has already changed in recent decades,” says Kim Cobb, a climate scientist at Brown University who was not part of the study.
The study doesn’t prove climate change is the culprit. But because the intensification coincides with rapid postindustrial warming and exceeds El Niño’s natural variability, the authors argue that human-driven warming is the leading explanation. “That’s the thing that has changed over that time,” says Julia Cole, a paleoclimatologist at the University of Michigan who led the study. “So if I get pushback on that, I would like to know what the alternative is.”
The stakes are enormous, as Cole herself witnessed when she first visited the Galápagos in 1989 and saw the devastation that spiking ocean temperatures during a 1982–83 El Niño inflicted on the islands’ coral reefs. Beyond ecological damage, the extreme weather during large El Niño events can cause trillions of dollars of economic losses. As forecasters warn that this year’s El Niño could end up the strongest in nearly a century, understanding whether climate change itself is strengthening the events has taken on new urgency.
“This system is the largest source of climate extremes on our planet,” Cobb says. “And if it is in itself becoming more extreme, that has some very serious implications for society, and it’s information that we should be acting on.”
El Niño events develop every 2 to 7 years, when weaker trade winds allow warmer waters from the western Pacific to flow east and pile up near the Galápagos. Through “teleconnections” in the atmosphere, the events have far-flung effects, for example causing flooding in South America and devastating droughts in some parts of Africa.
Diagnosing how the events might be changing has proved difficult. El Niño depends on subtle interactions among winds, ocean currents, and sea-surface temperatures that climate models struggle to reproduce. In many simulations, the eastern tropical Pacific starts out too cold. Those kinds of errors can ripple through the feedbacks that generate El Niño, causing models to disagree on how the phenomenon will respond to greenhouse warming. “Those can create uncertainty,” says Agus Santoso, a climate scientist with the World Climate Research Programme.
Historical records haven’t delivered a clear verdict either. El Niño records span little more than a century, and satellite observations of the Pacific, critical to watching the events unfold, extend back only to the 1980s. But in ancient corals scientists can trace the phenomenon over centuries. “Corals are one of the best games in town for El Niño,” says Judson Partin, a geophysicist at the University of Texas at Austin who has been teasing El Niño signals from corals in Vanuatu, an archipelago in the southwestern Pacific.
Corals build their skeletons from calcium carbonate precipitated from the surrounding water, adding a band of new growth each year. In the Galápagos, Cole and her colleagues drilled cores from corals—both on the beach and underwater. Back in the lab, the researchers measured trace amounts of strontium trapped in each carbonate layer. During El Niño events, when the Galápagos waters warm, the carbonate captures fewer of the large strontium atoms from the water.
The researchers also measured the carbonate’s ratio of heavy oxygen, O18, to ordinary O16. Warmer water favors the incorporation of lighter oxygen, lowering the ratio. The heavy rains that fall on the Galápagos during El Niños lower it further because rain is depleted in O18. Together, the signals provided a record of El Niño–driven temperature swings stretching back centuries. The team found those swings grew significantly stronger after global warming took off—far stronger than natural variability could explain.
The new findings echo an analysis of fossil corals from Kirimati in the central Pacific that Cobb and her colleagues reported in 2019, which found a 25% increase in El Niño variability. But the signal is even stronger in the Galápagos, where the warming effects from El Niño are the most extreme.
The study is unlikely to definitively settle the debate. But Santoso says newer climate models also support a link between warming and El Niño. The latest models, which will inform the next report from the Intergovernmental Panel on Climate Change (IPCC), tend to show that El Niño events will become stronger and more frequent as Earth warms, he says. “The fact that three separate analyses—models, paleorecords, and modern observations—are providing the same results is encouraging,” Santoso says.
The last IPCC report, in 2023, refrained from making statements on how global warming might impact El Niño. But Cobb, who helped draft the previous report, says the next report, expected in 2029, is likely to deliver a clearer message. “We are edging close to saying climate change has already had a considerable impact on the strength of El Niño.”
