A storm is gathering. Most likely a very big one. El Niño is a natural climate phenomenon occurring every two to seven years, towards the end of the year, but it comes in various different strengths, depending on the prevailing conditions.
El Niños are considered strong if the surface temperature of the water off the west coast of South America is more than 1.5 degrees Celsius above average. In July this year, the US National Oceanic and Atmospheric Administration (NOAA) was already observing temperature increases of greater than three degrees Celsius in the eastern and central Pacific.
The German Meteorological Service believes this "could in all likelihood develop into one of the strongest El Niño events since records began."
But a recent article by the Max Planck Institute for Meteorology states that whether or not strong El Niño events will intensify as a result of climate change "remains the subject of ongoing research."
The data gap: What corals can tell us about El Niño
The problem is that the data on ocean surface temperatures is only truly reliable from the 1980s onward, with the start of satellite observation. And this data doesn't tell us how El Niño behaved before industrialization and the burning of fossil fuels.
US researchers say corals can help to close this knowledge gap. Julia Cole, a climate scientist and paleoclimatologist at the University of Michigan, and her team recently published a study on this subject in the journal Science.
"We've known for a long time that the last 40 or 50 years has really strong El Niño events," says Cole. "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 analysis points firmly in one direction: Unusual. Not normal.
Corals: A repository of 1,000 years of climate history
Cole and her team examined fossilized and living corals around the Galapagos Islands off Ecuador in the eastern Pacific. Corals grow between one and two centimeters per year, and in doing so form a structure similar to growth rings on trees. "We made use of the fact that corals record ocean temperatures as they grow, in the chemical composition of their skeleton," Cole explains.
Corals are living creatures that construct external skeletons through calcification, adding a band of new growth each year with calcium carbonate precipitated from the surrounding water. In this way, over decades and centuries, corals have formed entire reef systems. Other chemical elements are also incorporated into the skeleton during the calcification process — strontium, for example. Sometimes more is deposited, sometimes less, depending on the temperature of the water.
"In colder temperatures, it [the skeleton] absorbs more strontium, in warmer temperatures less," says Cole. The ratio of strontium to calcium in the skeleton provides information about changes in temperature.
Specific isotopes of oxygen in coral skeletons provide clues, as well. "The rare oxygen-18 and the common oxygen-16 are also taken up in a ratio that reflects temperature," says Cole. In colder conditions, more oxygen-18 is absorbed than in warmer temperatures.
In this way, researchers were able to reconstruct the changes in ocean temperatures over the past 1,000 years, according to the age of the corals. Their conclusion was that El Niño events have become markedly stronger compared to the pre-industrial period.
Corals give indications, not conclusive answers
Cole's study has confirmed a long-held suspicion. "These developments correspond to the predictions of the best climate models. They predict a quantifiably higher likelihood of extreme eastern Pacific El Niño events in the 2030s," says Jens Zinke, professor of paleobiology at the University of Leicester in England.
However, other scientists are more circumspect in assessing the study's significance. "Coral data does achieve a high temporal resolution — monthly to seasonal — but its accuracy is dependent on local conditions, meaning that direct comparisons with modern measurement data are fraught with uncertainty," says Andreas Fink, professor of meteorology at the Institute for Meteorology and Climate Research in Karlsruhe. According to Fink, the significance of the study is undermined by its regional limitation to the eastern Pacific.
Cole is also aware of her study's limitations. "Our data does fill a gap, but I want to be clear that our record isn't continuous," she says. "We don't have every single year over the last millennium. We have a little bit under half of the years covered."
Floods, droughts, record heat — the consequences of stronger El Niños
Although the epicenter of the El Niño phenomenon is the Pacific, it changes the weather across the globe. In countries like Ecuador and Peru it can cause heavy rains and flooding, while Australia and Indonesia may experience a lack of rain, leading to drought and wildfires. El Niño also causes global temperatures to rise for a while.
If the predictions prove right and this El Nino does in fact become a Super El Niño, the coming year, 2027, will probably be another of record heatwaves, according to Thomas Felis. Felis leads the research group on coral paleoclimatology at the MARUM Center for Marine Environmental Sciences at the University of Bremen, and has conducted a similar study to Cole's.
Heat-stressed oceans: El Niño is a threat for corals
A year of record heat "will very probably lead to another global coral bleaching," says Felis. This has happened before, in the years 2023–24 and 2015–16, which were also El Niño years.
Corals get their characteristic colors from certain algae with which they live in symbiosis. Through photosynthesis, the algae provide the energy the corals need to live. If the temperature of the ocean is too high, the algae start to produce toxins, and the corals end the partnership. The coral remains, in its skeleton.
It can survive like that for a few weeks, and if the temperature drops again, the algae return. "But if this state, with abnormally high summer temperatures, goes on for too long, the coral becomes so sick that it eventually dies," Felis explains.
Worldwide coral death would mean not just the end of a unique ecosystem, but the destruction of an archive that preserves the history of the oceans.
