Stopping emissions would not return the Pacific Ocean to its pre-industrial state for centuries, leaving extreme El Niño events far more frequent long after the world reaches net zero, a new study has found.

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Significantly, these extreme events will become more frequent the later emissions stop. In the study’s main climate model, extreme events made up 1.73 per cent of El Niño events before industrialisation. When emissions stopped in 2030, the share rose to 16.4 per cent, nearly 10 times as high. When they stopped in 2060, it reached 26.3 per cent, about 15 times as high.

For Southeast Asia, El Niño brings dry weather that lets fires spread and haze build, cut rice and palm oil harvests and push up palm oil prices. The findings suggest that this risk, which has cost the region billions of dollars in fires and haze alone, will outlast the energy transition, and grow larger the longer that transition takes.

The study, published on 13 October in Communications Earth & Environment, was led by researchers at the University of Melbourne and the ARC Centre of Excellence for the Weather of the 21st Century. Its 1,000-year projections come from a single Australian climate model, though in a wider group of models the share of extreme events also roughly doubled under warming and stayed high after emissions ceased.

The authors trace the persistence to the Southern Ocean and the deep ocean, which keep warming for centuries after emissions stop; that warming weakens the trade winds and keeps the eastern Pacific warm, so the temperature gap between the western and eastern Pacific does not recover. With the Pacific’s average state held closer to El Niño conditions, extreme events become easier to trigger.

“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,” lead author Aditya Sengupta of the University of Melbourne said in a statement.

The earlier emissions stopped, the smaller the change the authors found. “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,” said co-author Associate Professor Andrew King.

El Niño brings warmer and drier weather to Singapore and the surrounding region, which raises the risk of transboundary haze, according to the Meteorological Service Singapore; the agency has said that such conditions are conducive to peatland and vegetation fires. Indonesia’s two most recent very strong El Niño years, 1997 and 2015, were both severe fire years, a peer-reviewed study of the period found.

Indonesia’s fires during the El Niño of 2015 burned 2.6 million hectares and were estimated to have cost the country at least US$16.1 billion, according to the Center for International Forestry Research (CIFOR); haze from the 1997–1998 fires caused about US$4.5 billion in tourism and short-term health costs across the region.

The damage reaches beyond the air – drought and heat in past El Niño years have led to declines in annual rice and corn production in the region and, in some years, lower palm oil output, according to the US Department of Agriculture. Palm oil prices tend to rise in El Niño seasons as supply tightens, Mizuho Bank analysts noted in 2024; Indonesia and Malaysia together account for more than 80 per cent of global oil palm fruit production.

By 7 September, fires in Indonesia had released 76 million tonnes of carbon this year, putting 2026 on the same trajectory as 2015, according to the Global Fire Emissions Database.

Sengupta said the findings had implications for adaptation as well. “This means it is imperative that rapid emissions cuts and long-term climate adaptation planning go hand in hand even after we reach net zero.”