Faster Net Zero Cuts Could Curb a Centuries-Long El Niño-Like Shift in the Pacific

One of the most consequential questions in climate science is not simply how much the planet will warm, but how quickly the climate system can recover once humanity stops emitting greenhouse gases. A new modelling study published in Communications Earth and Environment suggests that for the tropical Pacific, the answer is sobering: even after carbon dioxide emissions cease entirely, changes to the region’s ocean temperatures, winds and rainfall patterns could persist for hundreds of years. The research, led by scientists at the University of Melbourne and the ARC Centre of Excellence for Weather of the 21st Century, also carries a clear message of agency, showing that the sooner net zero is reached, the smaller and less risky these long-lasting changes become.

The tropical Pacific is arguably the most influential single region in the global climate system. Its year-to-year swings, governed by the El Niño–Southern Oscillation, or ENSO, alternate between El Niño, La Niña and neutral phases, and these fluctuations ripple outward across the planet. El Niño events in particular can reshape rainfall, drought, flooding, heatwaves and bushfire conditions in Australia and far beyond, affecting agriculture, water security and ecosystems on multiple continents. Understanding how this pivotal oscillation will behave under continued warming, and crucially after emissions stop, is therefore essential for anticipating the climate risks that future generations will face.

To probe that question, the research team first analysed simulations from nine Earth system models, establishing how the tropical Pacific’s average state changes as the planet warms. They then turned to the Australian ACCESS Earth System Model to run a more demanding experiment: examining how the region evolves for a full 1,000 years after emissions are halted at different points within the 21st century. This two-stage approach allowed the researchers to separate the immediate response of the atmosphere and ocean surface from the much slower adjustments occurring deep in the ocean, where the majority of the excess heat trapped by greenhouse gases is stored.

The simulations revealed a consistent pattern across the models. As warming proceeds, the tropical Pacific’s average conditions shift towards a more El Niño-like state. This does not mean the Pacific becomes permanently locked into an El Niño event; rather, the characteristic temperature contrast between the warm western Pacific and the cooler eastern Pacific weakens. During a typical El Niño, warm surface water extends farther east across the basin, the upwelling of cold, nutrient-rich water along the eastern equatorial Pacific weakens, and major rainfall patterns migrate. The modelling suggests that this configuration could increasingly become part of the Pacific’s background climate rather than an occasional episodic departure from it.

What makes the new findings particularly striking is their persistence. The researchers found that stopping emissions would not immediately return the tropical Pacific to its pre-industrial condition. Instead, the altered ocean temperatures, winds and rainfall patterns could endure for centuries after the last tonne of carbon dioxide is released. The reason lies in the sheer thermal inertia of the ocean. Lead author Aditya Sengupta of the University of Melbourne explained that 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, giving the ocean a long memory of previous greenhouse-gas emissions.

This ocean memory has profound implications for how irreversible certain aspects of climate change really are. Even once humanity reaches net zero and the surface stops warming, the Southern Ocean and the deep ocean continue to take up and redistribute heat, sustaining altered circulation patterns in the tropical Pacific long after the atmospheric driver has been removed. According to the study, key aspects of tropical Pacific climate change could therefore be effectively irreversible on timescales of centuries. The deep ocean acts simultaneously as a buffer that slows surface warming and as a reservoir that locks in regional changes once they have been established.

The study’s central practical conclusion is that timing matters enormously. The longer emissions continue, the more heat accumulates within the climate system, and the greater the risk of committing the tropical Pacific to a substantially altered state. Reaching net zero earlier limits the magnitude of the changes occurring within the ocean and reduces the burden passed on to future generations. In other words, while some degree of long-lasting change may already be unavoidable, the difference between reaching net zero in the coming decades versus late in the century could determine whether the El Niño-like shift remains modest or becomes a defining feature of the Pacific for centuries to come.

Co-author Associate Professor Andrew King of the University of Melbourne emphasised that the results reinforce the value of rapid emissions reductions. The research team reported finding clear benefits from reducing emissions quickly, noting that earlier net zero limits the changes occurring within the ocean and reduces the risk of leaving future generations with a substantially altered tropical Pacific. Sengupta added that the findings make it imperative for rapid emissions cuts and long-term climate adaptation planning to go hand in hand, even after net zero is achieved, because the changes already set in motion will not vanish the moment emissions stop.

The 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, and was based on computational simulation and modelling rather than new observations. For a region whose variability shapes droughts, floods and heatwaves across the globe, the study reframes the stakes of climate policy. Stopping emissions remains essential, but the deep ocean’s centuries-long memory means that every additional year of delay deepens the commitment to a changed Pacific, strengthening the scientific case for cutting emissions as quickly as possible while preparing adaptation strategies for the changes that are already underway.

Subject of Research: Long-term response of the tropical Pacific and El Niño–Southern Oscillation to net zero carbon dioxide emissions

Article Title: Reaching net zero sooner limits long-lasting El Niño-like shift in the Pacific

Article References: Reaching net zero sooner limits long-lasting El Niño-like shift in the Pacific. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: El Niño, ENSO, tropical Pacific, net zero, climate modelling, ocean thermal inertia, deep ocean warming, Communications Earth and Environment, University of Melbourne, emissions reductions, climate adaptation, Earth system models