// ARS TECHNICA — MODA & SOCIETÀ
El Niño is now stronger than at any point in the last 1,000 years, study finds
“One more warning sign here of what we’re facing in a warmer world.”
Every few years, El Niño or its cool counterpart, La Niña, shifts rainfall across the tropics, dries out Australia, floods parts of Peru, and rearranges the weather on most of the planet. The two are the warm and cold swings of a single system, the El Niño–Southern Oscillation, or ENSO, and that system is the largest source of year-to-year climate variation on Earth. Its swings sit atop the steadily rising temperatures driven by global warming.
But we didn’t know whether global warming was making it stronger. It’s possible that the added energy in the atmosphere and oceans was causing its swings to be more dramatic.
“We’ve been seeing some very strong El Niño events in the late 20th and early 21st century, and what we didn’t know was how unusual those are,” said Julie Cole, an environmental scientist at the University of Michigan. Because models disagree with one another and the instrumental data are too short to separate a trend, Cole’s team reconstructed a thousand-year-long ocean surface temperature record by analyzing fossilized Galápagos corals.
It turns out that El Niño in the eastern Pacific has never been as violent as it is now. “The events don’t look anything like what we see in the pre-industrial era,” Cole said. “That was exciting. And a little surprising, actually.”
The Galápagos sit in the bullseye of the eastern Pacific ENSO pattern, where the biggest events have their strongest impact. It is also one of the few options to look for fossilized corals in the region. “The Galápagos are located at 90 degrees west, and you have to go all the way to 160 degrees west to find another island in that near-equatorial zone,” Cole said.
Corals there grow one to two centimeters a year, laying down a calcium carbonate skeleton with a chemistry that depends on the water it formed in. In these skeletons, Cole and her colleagues looked at the strontium-to-calcium ratio and at different isotopes of oxygen because both track temperature. “As temperatures get warmer, we get less strontium in the coral,” Cole said. Sampling the extracted cores of fossilized corals millimeter by millimeter yields more than a dozen measurements per year—a monthly temperature log for the life of the coral colony.
Living colonies cover recent decades and can be checked against instrument data. The deeper history comes from dead coral heads, which can be dated using uranium-thorium methods to within a few years, even as far back as 500 years. “We were also able to find corals that are dead and washed up on the beach,” Cole said. “They may be 100 years old, 200 years old, or 900 years old, or even older.”
The result of sampling corals was a patchy but long temperature log—28 time-separated series from five islands, covering scattered years back to around 1100 CE. Set against that baseline, the last four decades look rather strange.
Temperature variability since 1984 runs 36.5 percent higher than it was during the preindustrial stretch from 1000 to 1850 CE and is still 16.2 percent higher than during the period from 1851 to 1982. All three intervals are statistically distinct, and the trend only goes up. Those figures, Cole explained, mean that events that once ran a degree or two above normal in the Galápagos now run three or four.