How climate change could be making El Niño more extreme
It’s the phenomenon behind some of Australia’s hottest, driest years, and climate change could be making it more extreme. Here’s why that doesn’t guarantee catastrophe this year — or in the future.
7 September 2023
El Niño in a changing climate
The prospect of a possible El Niño summer, with the hot and dry conditions it's known for in Australia, can be frightening.
The winter has already been dry in parts of the country. Record summer temperatures in Europe, although not related, have added to a sense of foreboding.
But the oscillation between El Niño and La Niña is a complex beast. An El Niño year can help create the conditions for bushfire weather and drought — but it's not a guarantee of either, this year or any other.
What is becoming clearer to researchers with every passing year, though, is that global warming will make the hot and dry years even hotter and drier.
And the newest research suggests a double whammy: climate change may be changing the very nature of the El Niño-Southern Oscillation itself.
It all begins on the other side of the world.
The Pacific Ocean is a place of immense forces.
The winds that swirl across its vast expanse have helped shape human history, from the earliest Pacific migrations and explorations, to shipping routes that still exist today.
Coupled together with ocean temperatures, they also form one of Earth's most important climate phenomenon: the El Niño-Southern Oscillation, or ENSO.
Sea Surface Temperature Anomaly (Min -2.5°C - Max 2.5°C)
The pendulum that swings between the La Niña and El Niño phases of this cycle can drive drought, wildfires, heatwaves, cyclones and floods in countries on either side of the Pacific — including Australia.
"ENSO is the strongest year-to-year fluctuation in the climate system on the planet," says Mike McPhaden, a senior scientist at the US National Oceanic and Atmospheric Administration. "We know when a big one occurs, it rings the earth like a bell."
Normally, trade winds rush from east to west along the equator, towards the warmer waters close to Australia and Indonesia, where clouds form and much-needed rain falls.
But if sea surface temperatures in the eastern Pacific begin to rise, those clouds instead start to form above the newly warming ocean, shifting rainfall away from Australia.
The winds lose momentum and can stall or even reverse.
With nothing to disperse those warmer eastern waters, El Nino feeds on itself.
For the last three years, though, La Nina has prevailed.
The eastern Pacific has cooled and stronger winds have pushed the warmer waters towards Australia, unleashing record rainfall over parts of the country.
Now, the pendulum is swinging yet again.
Early this year, sea surface temperatures in the eastern Pacific began rising.
By winter, they had reached an El Niño-like state.
At the same time, rainfall in parts of Australia began to diminish.
May was dry almost everywhere, while pockets of Victoria recorded their driest July on record.
Rainfall (Min -100% - Max 100%)
August was another dry month, especially in southern and eastern Australia.
All that remains for an official El Niño is for the Pacific wind to change.
El Niño was the missing ingredient when bushfires torched swathes of eastern Australia in 2019 and 2020.
What Alen Slijepcevic remembers then is the silence.
Not the freight-train roar of the flames, not the forest splitting and cracking and exploding, but what came immediately afterwards.
Driving through East Gippsland bushland in early January 2020, the landscape scythed by fire, the veteran Country Fire Authority Victoria deputy chief was compelled to stop every so often and get out of his car.
"There was nothing left. It was burned so horribly that there was nothing left on the ground. There were no leaves on any of the trees, you couldn't hear the birds — nothing."
The year preceding the Black Summer fires still stands as the hottest and driest on record in Australia. "In June, we knew it would be a bad fire season," Slijepcevic says.
Two climatic phenomena combined to turn the country into a tinderbox in 2019: a positive Indian Ocean Dipole and a negative Southern Annular Mode, which both decrease rainfall in Australia, especially in the south.
Four years later, Gippsland is beginning to dry out again, and this time an El Niño summer is all but a certainty.
The Bureau of Meteorology has hesitated to declare a full-blown El Niño until the winds and convection respond to the warming eastern Pacific sea surface, but their own projections suggest it's highly likely to form.
Regardless of an official El Niño event, the drier weather means there will be a lot more fires in Victoria and elsewhere in Australia this summer than there have been over three wet La Niña years, Slijepcevic says.
The Australasian Fire And Emergency Services Authorities Council's spring bushfire outlook predicted an increased risk for large parts of the Northern Territory, Queensland and New South Wales, as well as regions in Victoria and South Australia.
But Slijepcevic regards any headlines warning of a horror bushfire season as overblown. "This is not a year like Black Summer or Black Saturday. It's not the same settings. We want people to be alert to the risk, but not alarmed in a year like this — because where do you go, in a really horrible year that we know we will have?"
The reason for his restraint comes down to the conditions.
While this winter has been dry, the land — for the most part — is not, Slijepcevic's colleague, CFA research manager Sarah Harris says.
"A lot of those significant seasons with high-impact fire events were preceded by multi-year droughts, and that's what we haven't had this year."
Thanks to La Niña, many dams are full. Soil moisture in most areas is good.
The wet weather has fed grassland and forest growth, but it will take "significant drying" to turn all of that into fuel, Harris says.
Still, Harris' own research, alongside others, has found a strong link between ENSO and fire weather in Australia. A study she co-authored in 2019 found that El Niño conditions in spring, in particular, corresponded to more high fire danger days in both spring and summer. There were some variations, but the connection existed in all states and territories.
But the relationship between El Niño and the twin risks of drought and bushfire is not linear. How bad it could get, and which parts of the country will be affected, are the outcomes of a complex system involving not just ENSO but other major climate patterns, along with how dry the land already is.
El Niño years have produced wildly different rainfall patterns in Australia. The common factor is that it's always drier than normal somewhere, usually in the east.
CSIRO climate and natural hazards researcher Nandini Ramesh says this year's El Niño is predicted to be moderate to strong, but that doesn't map directly onto Australia.
"What you might see in forecasts of El Niño are often about the strength of the event, and when they say strength, they're really talking about those temperatures in the Pacific — they're not talking about how impactful that is [on land]."
Even where El Niño first forms — in the east close to South America, or further out in the centre of the Pacific — can make a difference, Ramesh says. "Right now it's looking like an eastern Pacific El Niño, which historically has had less of a strong impact on Australian rainfall."
Other climate patterns can work both with and against El Niño and La Niña. "There's the Indian Ocean Dipole for example, and that is forecast to be in a positive state over the coming months, which tends to exacerbate dry conditions over south-eastern Australia. That's likely to play a role."
Ramesh cautions against reading too much into the record temperatures set in Europe and elsewhere over their summer. "I realise that people are also seeing the massive fires in the northern hemisphere, and that's not a forecast of what's going to happen to us, necessarily."
To Melbourne University bushfire researcher Hamish Clarke, those northern hemisphere heat waves are concerning for another reason: they are further evidence of "an unmistakable rising trend" in global temperatures. Along with making summers hotter, they create an ever-thirstier atmosphere that sucks moisture from the soil.
"The scary thing about the trend is that the Black Summer conditions were part of this longer trend — and if that holds, then it's a question of when, not if, we see those conditions again."
The knot that researchers have been trying to disentangle is whether those rising temperatures also affect ENSO itself — and how. Does a hotter planet mean every El Niño or La Niña period will be worse than the last?
On the far side of the Pacific Ocean, it's a question that's preoccupied Seattle-based Michael McPhaden for three decades, through to his current role as director of the Global Tropical Moored Buoy Array programme. "Climate change's effects on ENSO is one of the most compelling problems in earth systems science today."
The cycle's "tremendous natural variability" means progress has been hard-won, he says.
The Global Tropical Moored Buoy Array programme both monitors the current state of the ocean and provides valuable data for research on ENSO and other climate systems.
"This is an issue of signal-to-noise detection in a very noisy world.
"From the observational instrumental record, which only goes back about 150 years, it's hard to look at that record and say unambiguously that climate change has a fingerprint on it."
That's exactly what McPhaden and a group of researchers led by one of Australia's foremost climate researchers, Wenju Cai, have been trying to do.
Earlier this year, they published a paper concluding that climate change was not only making the swings of the El Niño-La Niña cycle slightly larger, it was also making the biggest swings more frequent.
They teased those findings out of dozens of different climate change models, averaging out the different imperfections in each model to see what consistent pattern remained.
What they found was only a small increase in the strength of the whole cycle — about 10 per cent — but enough to project that extreme El Niño events will happen twice as often this century as they did during the 1900s, and strong La Niña events will happen more often too.
"If you throw on a little more amplitude — you're making the big events a little bit bigger — you've got this extra boost," McPhaden says.
"We're seeing the beginnings of this now… Extreme [La Niña] rainfall in Australia for the past couple of years, the historic and prolonged drought on the Horn of Africa."
Their conclusions differ from the Intergovernmental Panel on Climate Change, which most recently expressed low confidence — based on the research that was available at the time — that climate change was already affecting ENSO.
McPhaden and his colleagues believe their work is an improvement on the previous research. "They were looking at 30-year segments from these models and saying, they're all over the place, you can't tell whether there's a signal or not," he says.
"It's only when you begin to compare longer records that you can have more confidence that the variance or the magnitude has changed."
But even without the group's findings, researchers who spoke to ABC agree that climate change will make the effects of El Niño and its La Niña counterpart more intense, simply because average temperatures everywhere are increasing.
"That we do have more certainty about," Nandina Ramesh says. "When you have a higher baseline temperature and an El Niño event sets off a heatwave, your extremes are going to be higher and that heatwave will be more dangerous."
La Niña years now are hotter than La Niña years from previous decades, and El Niño years are hotter still.
For every degree warmer, the atmosphere can hold 7 per cent more moisture. That means soils can dry out more quickly during hot periods. When the rain finally unleashes, it does so with more ferocity.
"Even if we put aside the question of whether ENSO has increased in magnitude, just the simple fact that ENSO is occurring on a warming background is compounding the impacts of the two together," McPhaden says.
"From a societal point of view, we talked about climate change producing more extremes. One of the ways that will be manifest is through more extreme ENSO events. That will mean more extreme droughts, more extreme floods, more extreme wildfires, more extreme heatwaves, more extreme storms."
But in the same way that a strong El Niño does not necessarily forecast disaster for Australia now, people should not expect catastrophic drought, fire or flood every time El Niño or La Niña pays a visit in the future.
"You have to try to hold those two things in your head at the same time," Hamish Clarke says. "Yes, El Nino does ramp up the risk, but no, it doesn't guarantee that it's going to be a really bad fire season everywhere."
And although El Niño can be a predictor of year-to-year fire risk in parts of Australia, research has found that climate change itself is a more important ingredient in the long-term rise in fire danger and activity. One 2019 paper estimated that a 2°C increase in average temperatures would lead to 38 per cent more fire activity in Victoria.
"We need to be planning for this kind of steady marginal warming, but it's not that it's just going to be a super-high risk year every year," Clarke says. "It's going to be some more intense ones, and some milder ones."
But he understands the fear people are feeling. "It's just this foreboding sense that ok, we've had this triple La Nina and now things are switching around," he says. "There's always a certain inevitability to fire: if it's not this year then it's next year, or the year after."
For this year at least, forecasters, farmers and firefighters alike are still watching to see how the dance between ocean and atmosphere will unfold in the coming months, and how far the El Niño pendulum will swing.
"There's a really high degree of probability the ocean's going to remain warm," Mike McPhaden says. "The question is how warm will it be?"
"When a big one happens, the whole earth resonates."
Credits
- Reporting:
- Development: Katia Shatoba
- Design: Alex Palmer
Notes about data used in this story:
Monthly averaged sea surface temperature anomaly spatial data was sourced from the US National Oceanic and Atmospheric Administration and monthly averaged wind spatial data from the Copernicus ERA5 database. Rainfall totals and 30-year climatologies (1991-2020 averages) were sourced from the Australian Bureau of Meteorology and used to calculate monthly anomalies. Niño 3.4 index data was sourced from the BOM, while data for projected El Niño frequency was provided by Wenju Cai and his research collaborators.
ABC