The current El Niño is already the strongest on record and is forecast to get even stronger, peaking around November or December. Though it should be over by June next year, the effects of this monster event will continue to be felt long after that.
“I expect that this super El Niño event will drive multiple climate regime shifts like we’ve seen after past super El Niño events,” says hurricane specialist Jeff Masters. “Regime shifts tend to cause considerable trouble – significant disruptions to ecosystems and to the humans that depend on them.”
For instance, this El Niño is expected to lead to a devastating drought in the Amazon that could kill off some of the remaining areas of rainforest. This would release huge quantities of carbon dioxide and also make the whole region permanently drier, because much of the rainfall in rainforests comes from the water evaporating from trees.
El Niños occur when warm water that has built up in the western Pacific spills east across the equator, making the surface of the eastern Pacific warmer than usual. An El Niño is said to be happening when the sea-surface temperature in one region along the equator is 0.5°C above average; at the time of writing, it is 3.4°C and climbing.
One reason why El Niños end is that the warm water that spreads across the Pacific slowly disperses. Some of it flows away from the equator and the water also cools as it transfers vast amounts of heat to the atmosphere, which leads to a sudden jump in global surface temperatures.
This year’s average temperature is on track to tie with 2024’s at around 1.6°C above the pre-industrial level, and 2027 is expected to reach 1.7°C or more. The next year could be a bit cooler if a La Niña makes the Pacific soak up more heat than usual, but we are on the brink of permanently crossing the 1.5°C threshold we were meant to avoid.
What happens below the surface of the Pacific also plays a key role in bringing about the end of El Niños. For instance, the arrival of warm water in the eastern Pacific triggers gigantic underwater waves known as Rossby waves that travel westwards. When these hit the continental shelf in the western Pacific months later, they bounce back eastwards in the form of cold, faster-moving Kelvin waves. These thin the layer of warm water in the eastern Pacific when they arrive several weeks later, helping deeper, colder water reach the surface and bring the El Niño to an end.
“Many different processes are involved in the onset, maturation and demise of El Niño events,” says Michael McPhaden at the US National Oceanic and Atmospheric Administration.
When will the El Niño end?
The combined forecasts of computer models suggest that this El Niño will be over by June next year, but individual models vary. Some suggest it could persist well into the northern hemisphere’s summer.
We can’t predict with any certainty how this event will end, says McPhaden. The 1997 El Niño ended with a bang, with sea-surface temperatures in the eastern equatorial Pacific plummeting by 8°C in a month, followed by a strong, long-lasting La Niña. But the next strong El Niño in 2015 faded gradually and was followed by a weak, short La Niña.
“So, at this point, it is difficult to know precisely how the current El Niño will wind down,” says McPhaden. “Nature loves to confound us with new wrinkles in the epic saga of El Niño, so be prepared for surprises.”
What is certain is that the effects of this event will persist after it ends. Some will be temporary. For instance, the Indian Ocean can become warmer after El Niños – on top of the warming due to global heating – leading to floods in some nearby regions. Some of the worst floods in the Yangtze river occurred in the summer of 1998, after the El Niño that ended in May.
The biggest concern is that the current El Niño could lead to changes that last decades or more. Last year, Jong-Seong Kug at Seoul National University in South Korea and his colleagues published a study showing that the combination of extreme El Niños and global warming can produce climate regime shifts.
After the 2015 super El Niño, there was a sudden increase in sea-surface temperatures in the Gulf of Mexico that has persisted ever since. The El Niño wasn’t the only factor involved, says Kug, but it contributed to this jump.
The even-stronger El Niño now under way is more likely to trigger such shifts. “We are experiencing a very strong El Niño event,” says Kug. “Based on our study, such an extreme event could substantially increase the likelihood that various components of the Earth system are pushed toward, or even across, regime-shift thresholds.”
In other words, in some parts of the world, the climate will never go back to how it was. “Our civilisation is adapted to the climate of the 20th century, and it is costly and disruptive to be forced to deal with sudden climate regime shifts,” says Masters.