SINGAPORE - Climate change has made El Nino and La Nina events stronger and more frequent in recent decades, scientists say in a study that underscores mankind’s growing impact on the planet.

The peer-reviewed study, led by researchers at Australia’s national science agency, CSIRO, bolsters previous research which has said humanity should expect more intense and frequent El Nino and La Nina events in the future, and more frequent swings from a strong El Nino to a strong La Nina.

El Nino and La Nina are naturally occurring events in the Pacific Ocean involving changes in ocean surface temperature, wind patterns and rainfall. Both events have a powerful influence on the world’s weather.

El Nino typically brings drier, hotter conditions to South-east Asia and Australia, while La Nina usually brings wetter and cooler conditions. They alternate every few years and, together, the two phases are known as the El Nino-Southern Oscillation, or Enso.

The latest findings by the CSIRO-led team come after the United Nations World Meteorological Organisation said earlier in May that the chances of El Nino developing were 60 per cent by the end of July and 80 per cent by the end of September. A three-year La Nina phase petered out earlier in 2023 after being linked to record rains and flooding in Australia, South-east Asia and Pakistan.

“The research provides compelling modelling evidence that climate change has already made El Nino/La Nina more frequent and more extreme, and makes the human fingerprint on this climate pattern a lot more clear,” said lead author Wenju Cai from the CSIRO. The study was published on Thursday in the journal Nature Reviews Earth & Environment.

“This is the most substantial study to date that has used climate models to answer the question of whether greenhouse forcing was increasing the intensity of El Nino/La Nina,” he told The Straits Times.

In recent decades, these events have triggered extreme heatwaves, droughts and wildfires and record floods, costing lives, destroying livelihoods and leading to billions of dollars in damage.

Until now, scientists had limited understanding of how man-made climate change affected El Nino and La Nina.

“Previous research projected how El Nino/La Nina will change in the future,” said Dr Cai, a global expert on the links between climate change and Enso.

“But it was unable to tell us whether human-caused climate change has already affected El Nino/La Nina.”

So, the research team set about trying to better understand how greenhouse gas emissions were affecting El Nino and La Nina and when the impact started to occur.

The research, five years in the making, involved running a complex set of 43 climate models to produce computer simulations of the earth’s climate system, Dr Cai and co-author Agus Santoso, from the CSIRO and University of New South Wales, explained in a commentary on The Conversation news website published on Thursday.

The research team first compared simulations from between 1901 and 1960 with those from 1961 to 2020. They found that strong El Nino and La Nina events have occurred more frequently than average since 1960 and that this finding was consistent with what has actually occurred over the same period.

The team then examined climate simulations over hundreds of years before humans started releasing huge amounts of greenhouse gas emissions into the air, mainly from burning fossil fuels and deforestation, and compared these with the simulations after 1960.

This analysis showed even more clearly the very strong variability in Enso after 1960 – with variability referring to a departure from the average, Dr Cai and Dr Santoso explained.

The strong variability has contributed to more extreme and frequent droughts, floods, heatwaves, bush fires and storms around the world, they said.

What do the findings mean for the future?

Global warming, caused by greenhouse gases trapping growing amounts of heat in the atmosphere, has caused ocean temperatures to steadily rise. The oceans have absorbed more than 90 per cent of the excess heat trapped in the atmosphere and the global average ocean surface temperature reached a new record in April of 21.1 deg C.

Warmer oceans feed more heat and moisture into the air, causing more rainfall and more powerful storms.

“We now know that increased greenhouse gas activity leads up to higher Enso variability. That has far-reaching impact on societies, economies and livelihoods given Enso affects almost every continent in the world,” Dr Cai said.

“Essentially, our legacy of accumulated greenhouse gas emissions has put us on a trajectory of more intense El Nino and La Nina events well into the future,” he said.

Reducing greenhouse gas emissions in the coming years will help stabilise Enso pattern in the centuries ahead.

“Now is the time for policymakers, resource managers and the public to start to prepare for the social and economic risks ahead,” Dr Cai said.