The strength and frequency of La Niña and El Niño were once determined entirely by natural forces, but now the climate patterns are showing the fingerprints of humans.
A new study led by researchers at CSIRO set out to determine the impact of greenhouse gas emissions on the major climate driver, known as the El Niño-Southern Oscillation (ENSO).
Up until now there had been limited understanding about the role climate change has already played on ENSO, with research primarily looking at future projections.
Lead researcher Wenju Cai said their research yielded significant results, with evidence that El Niño and La Niña events had become more frequent and intense due to increasing emissions of greenhouse gases.
"Previous research projected how El Niño and La Niña will change in the future but was unable to tell whether human-caused climate change has already affected [them]," he said.
"The current paper provides modelling evidence that climate change has already made El Niño and La Niña more frequent and more extreme."
The swinging pendulum of ENSO plays a major part in year-to-year climate, with recent La Niña and El Niño events having played a hand in devastating flooding and drought events in Australia.
On a global scale, no other single phenomenon yields a bigger influence on whether a year will be warmer, cooler, wetter, or drier than average.
It is a climate pattern that has been operating for millions of years, according to palaeoclimatic evidence.
Strong El Niño events doubled, strong La Niña increased nine-fold
The CSIRO study, published in the journal Nature Reviews Earth and Environment, examined extensive outputs from models without greenhouse warming, each for hundreds to over thousands-of-year time scales, to examine how unusual the last 60 years have been.
To understand the change, they then compared ENSO in the 60 years pre- and post-1960.
They found that strong El Niños increased from two events in the pre-1960 to four events in the post-1960, and strong La Niñas from one event to nine events.
Dr Cai said the observed strength was extremely unusual if climate change had not had an impact.
Even without changes to ENSO itself, Dr Cai said the impacts of El Niño and La Niña were expected to be more intense because of climate change.
"Global warming makes their impact more extreme because a warmer atmosphere holds more water, so when it rains it rains harder, and evaporation is higher making droughts more severe, their onsets earlier and harder to get out," he said.
But Dr Cai said with the changes to the frequency and strength of ENSO the impacts were likely to be even stronger.
"The result suggests that the extreme floods and droughts we have seen in Australia are at least in part attributable to climate change through the increasing El Niño-La Niña," he said.
Dr Cai said their findings would be of broad interest and consequence across the globe.
"That the increased strength has likely caused a greater loss, and is likely to increase further, is of broad interest to policy makers, resource managers, and the general public," he said.
"Our result should inspire mitigation and adaptation strategy because the increased ENSO and its further strengthening will exacerbate the economic risks of global warming."
