RESOLVING THE LEDGER
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Researchers sample corals growing near the Galapagos Islands
NOAA
A super El Niño that is gathering strength in the Pacific Ocean may have been made worse by climate change, according to new analysis of coral records that suggest El Niños have become more intense as global temperatures have climbed.
The impacts of El Niño – droughts, floods and storms – are already exacerbated by rising global temperatures, because a warmer atmosphere intensifies both rainfall and droughts.
But scientists have long struggled to untangle the effect of climate change on the El Niño Southern Oscillation (ENSO) cycle, which drives El Niño events. Part of the difficulty is that detailed observational records only began in the 1980s. A much longer timescale than that is needed to tease out a trend from natural variability.
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Julia Cole at the University of Michigan and her colleagues hope corals can provide the answer. They collected living and dead corals from around the Galapagos Islands in the eastern Pacific ocean, where El Niños first develop.
They used uranium and thorium dating to identify the age of the corals, then analysed the chemistry of their skeletons to track changes to sea temperatures during their lifetime. In that way, they built a time series of sea surface temperatures in the eastern Pacific spanning the past 1000 years.
El Niño and La Niña events are measured by changes to sea surface temperatures in the Pacific, so by building this time series, Cole and her colleagues could track the fluctuating ENSO cycle through the centuries.
“The chemistry of the coral skeleton preserves a record of the environment in which it grew, and certain aspects of the chemistry are especially sensitive to temperature changes,” says Cole.
The team found that, over the past 40 years, variability in the ENSO cycle has increased by 36.5 per cent compared with the pre-industrial period. This increase in variability has been driven largely by more intense El Niño events, the analysis found.
The corals collected by the team covered about half the 1000-year time period. Nevertheless, Cole is confident in the overall trend identified. “I think we show pretty clearly that the increase in variability in ENSO is tracking global temperature closely,” she says.
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The findings add to an emerging body of evidence that suggests the ENSO cycle is changing in response to rising temperatures. “The evidence is better than 50-50 that climate change is affecting El Niño,” says Mark Cane at Columbia University in New York, whose own work used tree ring data to identify an increase in El Niño activity in recent years.
Although the research does not include the recent El Niños in 2015, 2023 and 2026, Cole says they are consistent with the trend identified by the corals. When the current El Niño peaks at the end of this year, sea surface temperatures in the east-central Pacific Ocean are projected to reach 4°C above normal, a 60 per cent increase over the old record.
Michael McPhaden at the US National Oceanic and Atmospheric Administration (NOAA) agrees. “If the current El Niño turns out to be as strong as we expect, that past decade will have witnessed three major El Niños – a sequence not seen before in the instrumental record dating back to the late 19th century,” he says. “It will add to the mounting evidence that climate change is amplifying the ENSO cycle… which is a concern considering the far-reaching global impacts that ENSO has on humans and natural systems.”
Journal Reference:
Science
DOI: 10.1126/science.ady2660
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