A Monash University-led study is prompting scientists to rethink how the El Niño-Southern Oscillation (ENSO) system evolved and how it might behave in the future as our climate continues to change.
The most comprehensive study to date of the climate system in the Early Eocene period, one of the warmest known periods in history, found ENSO's two phases, El Niño and La Niña, were both stronger and occurred over longer time intervals than they do today.
During the Early Eocene, global temperatures were up to 15°C warmer than today.
The research, published in the prestigious Nature Communications, found this warmth, combined with a much wider tropical Pacific Ocean and different global geography, reshaped the winds and ocean currents that are responsible for regulating ocean temperatures.
ENSO is a recurring climate pattern in the tropical Pacific driven by interactions between ocean temperatures and atmospheric winds, with two distinct phases, El Niño and La Niña.
El Niño is generally responsible for warmer and drier weather in Australia, while La Niña typically brings higher rainfall.
In addition to drought or flooding risks that come with a strong El Niño or La Niña, changes in line with those observed during the warmer Early Eocene period could lead to an altered climate variability, with far-reaching impacts on global ocean and atmospheric systems.
Lead researcher Dr Abhik Santra, Research Fellow at Monash School of Earth, Atmosphere and Environment, said the findings not only enhance our understanding of past climate but also shed light on how future warming could influence ocean-atmosphere-coupled climate variability in the tropical Pacific.
"In the Early Eocene, the tropical Pacific Ocean was about 1.5 times wider than it is today," Dr Santra said.
"That changed the way the ocean and atmosphere interacted, resulting in a stronger ENSO with longer cycles than we observe in the present day.
"We have used these findings to get a better understanding of the basic processes behind El Niño and La Niña. Our results also offer important clues about how ENSO could behave in a future climate that remains persistently warm."
To understand how ENSO might respond to future global warming, researchers had to separate the effects of tectonic changes from those of greenhouse gas-driven warming.
They did this by running targeted climate model experiments, revealing that tectonics and greenhouse warming can affect ENSO in opposing ways.
"We still don't have a clear consensus on how ENSO will change in the future," Dr Santra said.
"But by examining periods of sustained warmth in Earth's history, our study brings us closer to understanding its possible evolution."
While many climate models suggest ENSO variability could increase in the coming decades, there's still significant uncertainty.
"Once the warmer climate reaches equilibrium and stops further rising in temperature, ENSO is likely to become slightly weaker, relative to its height during the increase of global warming, although still likely stronger than today," Dr Santra said.
"It challenges the idea that ENSO will simply strengthen in a warmer world.
"Our study shows the relationship between global warming and ENSO behaviour is more complex than previously thought."
Dr Santra and the Monash paleoclimate research team are now building on this work to further explore prehistoric tropical climate variability.
By uncovering more about Earth's ancient climate systems, they aim to gather vital clues about the planet's future under continued global warming.
Read the full paper in Nature Communications at: www.nature.com/articles/s41467-025-59263-7
