Human greenhouse gas emissions aren’t driving major changes in massive air circulation across the Pacific Ocean, but reduction in aerosols over time could lead to a weakening of processes that influence the length and frequency of El Niño climate events.
These are the findings of an Australian-American collaboration investigating the impacts of carbon emissions on the Pacific Walker Circulation, published in the journal Nature.
The Walker Circulation is the driving force behind the El Niño Southern Oscillation (ENSO), which shifts climate patterns between El Niño and La Niña ‘phases’ in the Pacific Ocean.
These phases influence seasonal weather patterns in nations within and lining the Pacific, among them Australia, New Zealand, the Americas and parts of Asia.
But the Walker Circulation has changed over the last 800 years, according to modelling performed by researchers from Australia and the United States.
Led by Dr Georgy Falster, a climate scientist from ANU and the Centre of Excellence in Climate Extremes, and drawing contributions from colleagues at three US-based universities, the group originally wanted to analyse the impact of human greenhouse gas emissions on the Walker circulation.
While their expectation was for global warming to weaken the circulation’s strength throughout the industrial age, it appears atmospheric aerosols have had a counteractive effect.
Atmospheric aerosols are tiny particles that can cool the planet by reflecting sunlight back into space. They include molecules like sulphur dioxide, which is part of the aerosol layer spewed out during large volcanic eruptions; desert dusts and sulphates from burning fossil fuels.
“The expected influence of global warming on the Walker circulation is that it gets weaker,” Falster tells Cosmos. “That’s what climate models thought would happen, that’s what we expected.”
“We didn’t see that.”
Instead, Falster and her colleagues speculate the simultaneous effects of atmospheric aerosols and greenhouse gas emissions cancel each other out.
“So global warming is pushing the water saturation to be weaker, aerosols are pushing the water circulation to be stronger, which means that on average, they’ve just sort of cancelled out to nothing.”
But some atmospheric aerosols are reducing. It’s been speculated, though not yet confirmed, that efforts from the global shipping industry to reduce their sulphate emissions in recent years have exacerbated the warming effects of the current El Niño on world temperatures.
El Niño, while a natural phenomenon, typically results in higher-than-average global temperatures. In July 2023, the world experienced its hottest month on record. The loss of one of the atmosphere’s important reflective barriers threatens to make future El Niño years worse.
Walker weakening may result in longer El Niños
The Walker circulation refers to ENSO’s atmospheric component. In essence, it’s a massive air cycle.
In neutral phases – neither those of El Niño and La Niña – air travels west along the Pacific and rises into the atmosphere around New Guinea, moves back east at high atmospheric levels and then descends again in oceans near the Americas.
Falster’s research suggests that as the preventative effects of aerosols lessen, the effect of global warming will begin to exert its influence on the Walker Circulation, leading to a weakening of trade winds. Such an effect is consistent with El Niños. Ultimately, they predict a slower transition between ENSO phases – potentially leading to multiple El Niño or La Niña years before moving towards neutral conditions.
“We’ve seen this subtle change in the length of time it takes for the Walker Circulation to switch from one state to the next, it’s now switching more slowly between these El Niño-like and La Niña phases,” Falster says.
She points to the current atmospheric conditions in the Pacific to explain this. While the US-based NOAA and WMO have declared El Niño as being underway, Australia’s Bureau of Meteorology looks for a coupling of Walker Circulation changes to the marine component of the ENSO.
Because of that, it’s yet to declare El Niño as underway in Australia.
“This year is a good example of that, right,” Falster says.
“We’ve got this El Niño that is sort of developing in the Pacific Ocean, but the Walker Circulation is slower to respond than usual. That’s why the BOM hasn’t declared an El Niño yet.
“[With a slowing of the Walker] maybe we’ll have more El Niños that are two or three years in a row, same with La Niña. And again slower on the transition, so maybe longer spent in neutral phase.”
Massive volcanoes exacerbate weakening
Falster’s team also found that massive volcanic eruptions can have a similar, albeit temporary “El Niño-like” effect on the process.
Specifically, they found the eruptions of Indonesia’s Mount Agung (1963), Mexico’s El Chichón (1982) and the Philippines’ Mount Pinatubo (1991) provided a far clearer indication of shifting the Walker circulation than greenhouse gas emissions.
And although huge volcanic eruptions also release aerosols, these are pumped into the uppermost levels of the atmosphere and are distributed across the globe, limiting their cooling effects around the Pacific.
“So you can essentially say a volcano erupts and then the whole world has this aerosol effect,” Falster says. “Compared with human-emitted aerosols, which are really spatially concentrated in small areas, and that’s why they have the opposite effect.”