Reaching net zero sooner would limit long-lasting changes to El Niño and the tropical Pacific, new climate modelling suggests, although heat stored deep in the ocean means changes to the tropical Pacific could persist for centuries.
The study, published today in Communications Earth & Environment and led by researchers from the University of Melbourne and the ARC Centre of Excellence for Weather of the 21st Century, examined what happens to the tropical Pacific both while the planet is warming and after carbon dioxide emissions cease.
The research found that stopping emissions would not immediately return the tropical Pacific to its pre-industrial state. Instead, changes to ocean temperatures, winds and rainfall patterns could endure for hundreds of years.
Lead author University of Melbourne Aditya Sengupta said that stopping emissions remained essential but would not instantly reverse all the changes already set in motion.
"The atmosphere and the ocean surface can adjust relatively quickly, while the deep ocean absorbs and releases heat over hundreds or even thousands of years. This gives the ocean a long memory of previous greenhouse-gas emissions," Mr Sengupta said.
"These findings indicate that key aspects of tropical Pacific climate change could be effectively irreversible over centuries. It strengthens the case for reaching net zero quickly - the longer emissions continue, the more heat accumulates within the climate system and the greater the risk of long-lasting changes.
"It is imperative that rapid emissions cuts and long-term climate adaptation planning go hand in hand even after we reach net zero."
The tropical Pacific has a major influence on weather and climate around the world. Much of this influence comes through the El Niño–Southern Oscillation, or ENSO, which alternates between El Niño, La Niña and neutral phases.
These events can influence rainfall, droughts, floods, heatwaves and bushfire conditions in Australia and across many other regions. Understanding how ENSO will respond to climate change is essential for preparing for future weather and climate risks.
The researchers first analysed simulations from nine Earth system models. They then used the Australian ACCESS Earth System Model to examine changes for 1,000 years after emissions were stopped at different points within the 21st century.
Across the models, warming shifted the tropical Pacific's average conditions towards a more El Niño-like pattern. This does not mean that the Pacific becomes permanently locked in an El Niño event. Rather, the usual temperature difference between the warmer western Pacific and cooler eastern Pacific becomes weaker.
During an El Niño event, warm surface water extends farther east across the tropical Pacific, cold-water upwelling weakens and major rainfall patterns shift. The simulations suggest that this pattern could increasingly become part of the Pacific's average background climate.
Further simulations suggested that these changes could last for centuries.
The study's co-author, University of Melbourne Associate Professor Andrew King, said the results reinforced the value of rapid emissions reductions.
"We found clear benefits from reducing emissions quickly," Professor King said.
"Reaching net zero earlier limits the changes occurring within the ocean and reduces the risk of leaving future generations with a substantially altered tropical Pacific."
This work was a collaboration between researchers at the University of Melbourne, the Australian National University, the University of New South Wales, Monash University, the University of Reading and Australia's national science agency, CSIRO.
