Corals reveal El Niño events are intensifying beyond natural cycles

A storm is building in the Pacific, and scientists studying ancient corals believe it will be unlike anything in recorded history. In July, ocean temperatures off South America's west coast had already climbed more than three degrees Celsius above normal—well beyond the 1.5-degree threshold that defines a strong El Niño. Germany's meteorological service warned the event could rank among the most intense since measurements began. But the real question isn't what's happening now. It's whether what's happening now is normal at all.

El Niño arrives every two to seven years, a natural oscillation in ocean and atmosphere that reshapes weather across the globe. Heavy rains flood Ecuador and Peru. Australia and Indonesia dry out, their forests catching fire. Global temperatures spike. Scientists have long suspected that recent decades—packed with unusually powerful El Niños—represent something different from the natural rhythm of the past. The problem was proving it. Reliable ocean temperature data only stretches back to the 1980s, when satellites began continuous observation. Everything before that was darkness.

Corals, it turns out, are time machines. Julia Cole, a paleoclimatologist at the University of Michigan, and her team published research in Science showing how. Corals grow roughly one to two centimeters each year, laying down annual bands much like trees. As they build their skeletons from calcium carbonate, they incorporate chemical traces of the water around them. Strontium and calcium exist in ratios that shift with temperature—more strontium in cold water, less in warm. Oxygen isotopes tell the same story: the rare oxygen-18 accumulates more heavily when temperatures drop. By reading these chemical signatures in corals collected near the Galápagos Islands, researchers reconstructed ocean temperatures across the past thousand years.

The conclusion was stark. El Niño events have grown markedly stronger since industrialization began. "We've known for a long time that the last 40 or 50 years has really strong El Niño events," Cole said. "It's been hard to say whether the recent strong El Niño that we've had is part of a natural cycle, or if it's something a little bit unusual related to being in a warmer than normal world." The coral data pointed firmly toward unusual. The findings align with predictions from climate models, which forecast a quantifiably higher likelihood of extreme eastern Pacific El Niños in the 2030s.

Yet the study has limits that matter. Cole herself acknowledged that her coral record covers only about half the years in the past millennium—gaps exist. Andreas Fink, a meteorologist in Karlsruhe, noted that coral data achieves fine temporal resolution but depends heavily on local conditions, making direct comparison with modern measurements uncertain. The research also focuses narrowly on the eastern Pacific, leaving questions about whether the pattern holds globally. Other scientists remain cautious about drawing sweeping conclusions from a single region's ancient corals.

What makes the research urgent is not its certainty but its implications if it proves right. If 2027 brings the predicted super El Niño, Thomas Felis of the University of Bremen expects record heatwaves and, almost certainly, another global coral bleaching event. Corals depend on symbiotic algae that give them color and energy through photosynthesis. When ocean temperatures soar, the algae produce toxins and the partnership breaks. The coral expels them, turning white. It can survive weeks in this state if temperatures fall again. But prolonged heat stress kills the coral entirely. The bleaching events of 2015–16 and 2023–24, both El Niño years, offered a preview. Widespread coral death would erase not just a unique ecosystem but destroy the very archive that revealed how El Niño has changed—the historical record locked inside the skeleton would be gone.