Paleoclimatologist Julia Cole, who studies Earth’s past climate through corals, is not surprised by the currently strengthening El Niño in the Pacific Ocean. According to the scientist, this may be part of a trend observed for a century: due to human-caused climate change warming the planet, strong El Niño events are occurring more frequently.
“This does not seem like a coincidence,” she said.
This conclusion was reached by J. Cole and colleagues at the University of Michigan in Ann Arbor after reconstructing surface temperature changes in the eastern Pacific Ocean over a thousand years before the industrial revolution.
The analysis of coral skeletons from the Galápagos Islands, published in the journal “Science,” revealed that in the past century, strong El Niño events occurred more frequently than in the previous millennium, when greenhouse gases emitted by human activity had not yet begun to significantly warm the planet.
Many climate models indicate that due to climate change, strong El Niño events will continue to become more frequent this century. They temporarily raise the average global temperature, alter weather conditions in various regions, and can cause significant damage to ecosystems and economies.
“We need to think about how to better prepare so that the impacts of these events are not so catastrophic,” said scientist J. Cole.
Unfortunately, it is not that simple – El Niño is caused by a complex interaction between the ocean and atmosphere, so it is still unclear how Pacific Ocean temperature cycles will change this century.
“Corals are like a time machine that allows us to look into the past, but they do not show the future,” the scientist said.
Ocean fluctuations
El Niño is the warm phase of the natural tropical Pacific Ocean temperature cycle called El Niño and the Southern Oscillation (ENSO). The El Niño that formed in the Pacific Ocean this June is forecasted to significantly surpass its previous records. If the forecasts are confirmed, it will be the third strong El Niño in just 11 years – such a sequence has not been recorded before.
This has reignited long-standing debates about whether human-caused climate change is making El Niño events stronger and more frequent.
Answering this question is difficult for several reasons. Computer models do not always accurately reproduce the complex interaction of winds and ocean that determine ENSO. Moreover, before the 20th century, reliable temperature measurements were almost nonexistent in remote tropical Pacific Ocean areas. Therefore, scientists find it difficult to determine how ENSO behaved before human-caused climate warming began to have a significant impact.
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To reconstruct past ENSO fluctuations, paleoclimatologists rely on indirect Pacific Ocean temperature data, such as tree rings and fossilized remains of marine organisms. Corals are especially valuable because their growth layers retain information about monthly temperature changes. This allows identification of short-term phenomena like El Niño.
Data from corals collected in the central Pacific Ocean show that strong El Niño events have become more frequent in recent decades. However, until now, data from the eastern ocean, where the impact of this phenomenon is strongest, were lacking, so the overall picture of changes remained unclear. Paleoclimatologist Julien Emile-Geay from the University of Southern California in Los Angeles compared such research to trying to measure a human heartbeat by placing a stethoscope on the ankle.
According to J. Cole, the Galápagos Islands in the eastern Pacific Ocean are an ideal place to measure the “heartbeat of El Niño.” However, due to extreme conditions, few corals grow in this region.
In 2006, the scientist learned about corals found in a remote part of the archipelago. Since then, she and colleagues have collected thousands of samples, allowing reconstruction of a thousand-year period before the industrial revolution. “This is truly a huge achievement. Scientists have long tried to obtain data from this area,” says J. Emile-Geay.
New data revealed that ENSO fluctuations in the eastern Pacific Ocean have intensified compared to the previous millennium. This change essentially coincided with the rise in average global temperature over the past century.
According to researchers’ calculations, between 1990 and 2010, the average amplitude of temperature fluctuations between cold and warm phases was 36.5% higher than during the thousand years before the sixth decade of the 19th century. Compared to the period 1851–1982, it increased by 16%. The analysis shows that ENSO fluctuations were most intensified by the increasing frequency of powerful El Niño events.
“I think this is another strong piece of evidence that climate change affects the amplitude of ENSO fluctuations,” says climate scientist Michael McPhaden, working at the US National Oceanic and Atmospheric Administration in Seattle.
According to him, the identified trend aligns with both direct observation results and climate model data. All three lines of evidence point to the same trend, so scientists can be more confident that their conclusions are correct.
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