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Coral cores reveal 400-year long El Niño record

by TR Pakistan

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.

El Niño is a complex weather patterns resulting from variations in ocean temperatures in the Equatorial Pacific. It occurs when sea surface temperatures in the tropical Pacific Ocean rise to above-normal levels for an extended period of time.

The record published recently in the journal Nature Geoscience detects different types of El Niño and shows the nature of these events has changed in recent decades.

This understanding of these 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 to the strength of Eastern Pacific El Niños.

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“We are seeing more El Niños forming in the central Pacific Ocean in recent decades, which is unusual across the past 400 years,” said lead author Dr. Mandy Freund. “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.”

This result was discovered out of information about past climate from coral cores spanning the Pacific Ocean, as part of Freund’s PhD research at the University of Melbourne and the Center of Excellence for Climate Extremes. 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 to detect the different types of El Niño events.

This meant El Niño researchers were constrained by what they could say about the behavior of these events because the instrumental record was too short and it was hard to judge whether recent decadal changes were exceptional.

Researchers dive down to corals to extract coral cores that will give us information about Earth’s past climate. Credit: Jason Turl

“Prior to this research, we did not know how frequently different types of El Niño occurred in past centuries. Now we do,” said co-author Dr. Ben Henley of the Center of Excellence for Climate Extremes.

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. According to a press release, using coral records to reconstruct El Niño history at a seasonal timescale had never been done before and many people working in the field considered it impossible.

A team of climate scientists and coral experts were able to compare recent coral results with the instrumental record, after carefully refining the technique to reconstruct the signature of El Niño in space and time using new machine learning techniques. Using this unconventional approach, they found a strong agreement between the coral cores and recorded events. This confirmation allowed the team to extend the record back in time.

The team 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 ever recorded, according to both the 100-year long instrumental record and the 400-year long coral record.

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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.

It took three years of hard work to achieve the result and now the team is excited to see how this work can be built upon.

“The El Niño phenomenon is one of the most important features of global climate, and changes to its behavior have very serious implications for weather patterns and extreme events around the world,” said Dr. Henley.

“This gives us an opportunity to more accurately explore how global warming may change El Niños and what this means for future weather and climate extremes,” he added.

“Having a better understanding of how different types of El Niños have affected us in the past and present will mean we are more able to model, predict and plan for future El Niños and their wide-ranging impacts,” said Dr. Freund.