Extreme El Nino events, which have cost New Zealand hundreds of millions of dollars in the past, are likely to become twice as common under climate change, a study has found.
And the Australian scientists behind the research, published today in major journal Nature Climate Change, say the frequency of these events was projected to increase even if nations could meet the Paris Agreement ambition of halting global mean temperature at 1.5C above pre-industrial levels.
In New Zealand, the ocean-driven system typically brings cooler, wetter conditions, bringing higher rainfall to regions that are normally wet, and often drought to areas that are usually dry.
Farmers in the western, wetter parts of the country often faced significant damage to pastures from too much rainfall, and it was also harder for stock to thrive in the constant wet.
Those in the east, faced with dry conditions, needed to consider food availability for stock - one disastrous El Nino in the summer of 1997/1998 cost the country hundreds of millions of dollars.
CSIRO researcher and lead author Dr Guojian Wang said the growing risk of extreme El Nino events did not stabilise in a stabilised climate, due to a continuation of faster warming in the eastern equatorial Pacific.
"Currently the risk of extreme El Nino events is around five events per 100 years."
This doubled to approximately 10 events per 100 years by 2050, when the study's modelled emissions scenario hit the 1.5C peak.
"After this, as faster warming in the eastern equatorial Pacific persists, the risk of extreme El Nino continues upwards to about 14 events per 100 years by 2150.
"This result is unexpected and shows that future generations will experience greater climate risks associated with extreme El Nino events than seen at 1.5C warming."
The research was based on five climate models that provided future scenarios past the year 2100, and run using the Intergovernmental Panel on Climate Change's lowest emissions scenario, RCP2.6, which requires negative emissions late in the century.
Study co-author, Dr Wenju Cai, director of CSIRO's Centre for Southern Hemisphere Oceans Research, said the research continued important work on the impacts of climate change on the El Nino-Southern Oscillation (ENSO), which was a significant driver of global climate.
"The most severe previous extreme El Nino events occurred in 1982/83, 1997/98 and 2015/16, years associated with worldwide climate extremes," Cai said.
"Extreme El Nino events occur when the usual El Nino Pacific rainfall centre is pushed eastward toward South America, sometimes up to 16,000km, causing massive changes in the climate."
The further east the centre moved, Cai said, the more extreme the El Nino.
"This pulls rainfall away from Australia bringing conditions that have commonly resulted in intense droughts across the nation.
"During such events, other countries like India, Ecuador, and China have experienced extreme events with serious socio-economic consequences."
While previous research suggested that extreme La Nina events would double under a 4.5C warming scenario, the new study indicated that under a scenario of climate stabilisation, there was little or no change to these La Nina events.
Commenting on the findings, the Australian Bureau of Meteorology's head of climate research Dr Scott Power said most small island states in the Pacific had only a limited capacity to cope with major floods and drought.
"Unfortunately, this paper indicates that these vulnerable nations could be exposed to El Nino impacts more frequently in the future, even if global warming is restricted to 1.5C," he said.
"To make matters worse, our recent study published in Nature Communications indicates that the risk of major disruptions to Pacific rainfall have already increased.
"And, unfortunately, these El Nino-related impacts will add to the other challenges of climate change, such as rising sea levels, ocean acidification and increasing temperature extremes."
Associate Professor Pete Strutton, of the University of Tasmania's Institute for Marine and Antarctic Studies, said while the Paris targets were admirable, it was becoming "increasingly unlikely" the world would meet them.
"Even if we do, this study shows that the climate effects will be very serious," Strutton said.
"This study is consistent with previous work by these authors and others that shows that the intensity and frequency of El Nino is favoured by a climate that is warmer.
"The paper also suggests that the effects on El Nino will continue long after mean global temperatures are stabilised, if they are.
"This is important because it illustrates the 'inertia' in the climate system.
"This issue is probably even more pertinent for processes in the ocean such as acidification."
Past El Nino events
• 2015/16: While a typical strong El Nino pattern of southwesterly winds occurred in December 2015, a change to more northerly-quarter winds than usual for January and February brought more rain and alleviated fears of El Nino-associated drought in many parts of the country.
• 1997/98: In 1997, it became much drier than normal in the east from as early as July, with drought areas particularly pronounced from the Bay of Plenty down to Canterbury over the following summer. Officials estimated the likely cost of the drought on farm gate returns would be $256 million for the year ending June 30, 1998, and $169m for the following year, giving a total damage cost of $425m.
• 1982/83: The once-in-a-century "super El Nino" that hit New Zealand in the summer of 1982/83 was characterised as stormy, with low pressures and frequent southwest winds over the country. It also caused severe drought in eastern regions.