Global heating has likely intensified a climate pattern in the Pacific since the 1960s that has driven extreme droughts, floods and heatwaves around the globe, according to a new study.
The scientists said they had shown for the first time that greenhouse gas emissions were likely already making El Niños and La Niñas more severe.
The shifts in ocean temperatures and atmospheric conditions in the Pacific – known as the El Niño Southern Oscillation (Enso) – affect weather patterns around the globe, threatening food supplies, spreading disease and impacting societies and ecosystems.
Scientists have struggled to work out if adding greenhouse gases to the atmosphere – trapping enormous amounts of heat in the ocean – has already changed Enso.
But because the system has natural swings spanning decades and actual observations have been too sparse, the scientists looked instead at more than 40 models of the climate, analysed in several ways.
Dr Wenju Cai, lead author of the study from Australia’s CSIRO science agency, said the models showed a “human fingerprint” from 1960 onwards.
This meant climate change had likely made both El Niños and La Niñas “more frequent and more extreme,” he said.
But some other scientists not involved in the study had reservations about the findings, raising concerns about the reliance on modelling.
The study has been in the works for five years, and Cai said it showed “we are experiencing a vastly different climate to that of the distant past” and would help scientists understand how Enso will change in the future “given sea surface temperatures are continuing to increase”.
During La Niña periods, sea surface temperatures in the equatorial Pacific are cooler than a long-term average – dampening global temperatures. During El Niños, the opposite is true.
In a world with enhanced greenhouse warming, the models showed the sea surface temperature extremes had intensified by about 10% when they compared the 60 years before 1960 to the 60 years after.
Dr Mike McPhaden, a senior research scientist at the US government’s National Oceanic and Atmospheric Administration (NOAA) and a co-author on the paper, said scientists had grappled for 30 years with the question of whether human activity was altering Enso.
He said the researchers had thrown “everything at this problem” to find an answer, and had wrung out “every bit of information” from models to make their case.
He said: “The big events pack the most punch, so even though 10% doesn’t sound like much, it juices up the strongest and most societally relevant year-to-year climate fluctuation on the planet.
“In practical terms, this translates into more extreme and frequent droughts, floods, heatwaves, wildfires and severe storms, just like we observed during the recent triple dip La Niña that ended in March.”
Because Enso can swing naturally across decades even 150 years of sea surface temperature observations was not enough to tease out the human-forced changes from those natural swings, McPhaden said.
He said the study relied on the latest climate models that had known limitations. “However, they are the best tools available to us for addressing this problem,” he said.
Dr Sarah Kirkpatrick, a climate scientist at UNSW Canberra who was not involved in the research, said she was “a bit apprehensive” because of the reliance on modelling.
“But if we’re going to rely just on observations then we would need hundreds of years of data as a bare minimum and we don’t have that long to wait,” she said.
“This has almost been in the ‘too hard’ basket for scientists. But it’s the first study that’s really tried to address the question.
“[The authors] have come out with an interesting result that’s going to need to be explored further.”
Dr Shayne McGregor, an expert on Enso at Monash University in Melbourne, who was not involved in the research, said he was “not convinced” the study had shown that human-caused climate change was having an effect on El Niños or La Niñas.
“It’s possible, but for me there are a lot of question marks still. Just because the models agree, that doesn’t mean they’re correct [about what’s happening in the real world].”
He said Enso was “hugely variable” and while the research “provides evidence to support the idea that greenhouse gas warming may have influenced Enso” this evidence was “suggestive” rather than conclusive.
The study appears in the journal Nature Reviews Earth and Environment.
