New research produces 400-year El Niño record and reveals astonishing change
Australian scientists have developed an innovative method using cores drilled from coral to produce a world first 400-year long seasonal record of El Niño events, a record that some in the field had described as impossible to extract.
The record published today in Nature Geoscience detects different types of El Niño and shows that the nature of El Niño events has changed in recent decades.
This understanding of El Niño events is vital because they produce extreme weather across the globe with particularly profound effects on precipitation and temperature extremes in Australia, South East Asia and the Americas.
The 400-year record revealed a clear change in El Niño types, with an increase of Central Pacific El Niño activity in the late 20th Century and suggested future changes of the strength of Eastern Pacific El Niños.
“Prior to this research, we did not know how frequently different types of El Niño occurred in past centuries,” said study co-author Dr Ben Henley, from the Monash School of Earth, Atmosphere and Environment, and the Centre of Excellence for Climate Extremes.
“Now we do,” he said.
“We are seeing more El Niños forming in the central Pacific Ocean in recent decades, which is unusual relative to the past 400 years,” said lead author Dr Mandy Freund, who completed the work as a PhD student at the Centre of Excellence for Climate Extremes at Melbourne University.
“There are even some early hints that the much stronger Eastern Pacific El Niños, like those that occurred in 1997/98 and 2015/16 may be growing in intensity,” said Dr Freund.
This result was drawn out of information about past climate from coral cores spanning the Pacific Ocean. It was made possible because coral cores - like tree rings – have centuries-long growth patterns and contain isotopes that can tell us a lot about the climate of the past. However, until now, they had not been used at a seasonal time resolution to detect the different types of El Niño events.
This meant El Niño researchers were constrained by what they could say about El Niño behaviour because the instrumental record was too short and it was hard to judge whether or not recent decadal changes were exceptional.
The key to unlocking the El Niño record was the understanding that coral records contained enough information to identify seasonal changes in the tropical Pacific Ocean. However, using coral records to reconstruct El Niño history at a seasonal timescale had never been done before and in fact many people working in the field considered it a major challenge.
It was only after Dr Freund took her innovative approach to a team of climate scientists and coral experts: Dr Ben Henley, Professor David Karoly, Associate Professor Helen McGregor, Associate Professor Nerilie Abram, and Dr Dietmar Dommenget that they were able to proceed with the idea.
While the approach was considered challenging, leading Australian experts on past corals said that, while the approach might be unconventional, it was worth a shot.
After carefully refining the technique to reconstruct the signature of El Niño in space and time using new machine learning techniques, the scientists were able to compare recent coral results with the instrumental record. Dr Freund found a strong agreement between the coral cores and recorded events. This confirmation allowed the team to extend the record back in time.
The researchers found there has been an unprecedented increase in the number of El Niños forming in the central Pacific over the past 30 years, compared to all 30 year periods in the past 400 years.
At the same time the stronger Eastern Pacific El Niños were the most intense El Niño events, according to both the 100-year long instrumental record and the 400-year long coral record.
As a result, Australian researchers have produced a world-first seasonal El Niño record extending 400 years and a new methodology that will likely be the basis for future climate research.
“The El Niño phenomenon is one of the most important features of global climate, and changes to its behaviour have very serious implications for weather patterns and extreme events around the world,” said Dr Henley.
“This gives us a new window into El Niño behaviour and an opportunity to further explore how global warming may change El Niño in the future, including what this means for future weather and climate extremes,” he said.
- Mandy Freund (+61 403 802 357)
- Benjamin Henley (+61 404 278 355)