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The Great Dying at the end of the Permian Period 250 million years ago may have been amplified by El Niño events far stronger and longer lasting than any today.

These mega El Niños caused wild swings in the climate that killed off forests and many land animals, says at the University of Bristol in the UK.

They also triggered feedback processes that helped make this mass extinction as bad as it was, he says. “There are knock-on effects of this sort of El Niño event becoming stronger and lasting longer.”

Around 90 per cent of all species alive at the time may have gone extinct during the end-Permian extinction, making it the worst ever mass extinction. It is widely thought that it was triggered by massive volcanic eruptions in what is now Siberia.

These eruptions released huge quantities of carbon dioxide – possibly by heating rocks full of fossil carbon – that led to extreme global warming. The ocean became stagnant and low in oxygen, killing off marine creatures.

But this doesn’t explain everything. In particular, land species started going extinct tens of thousands of years earlier than those in the sea.

Many ideas have been put forward to explain this, from volcanic winters to the loss of the ozone layer. The idea that extreme El Niños might be involved emerged from studies of past ocean temperatures, based on oxygen isotopes in fossils, led by at the China University of Geosciences in Wuhan.

Now, Farnsworth and his colleagues have run computer models to explore what might have happened at end of the Permian that could explain Sun’s findings.

Today, El Niño occurs when warm water in the western Pacific spreads eastwards across the surface of the ocean. This creates an area of abnormally warm water that heats the atmosphere and affects weather across the planet.��

Before the Permian extinction began, the researchers found, El Niños were probably of a similar intensity and duration as today. That is, the anomalously warm water was about 0.5°C (0.9°F) hotter than average and the events lasted for a few months.

These events, however, were happening in a massive ocean called Panthalassa, which was 30 per cent wider at the equator than the Pacific Ocean is today. This means the area of anomalously warm water during El Niños was much larger than today, and thus had a bigger planetary impact.

As CO2 levels rose at the end of the Permian, these El Niño events got stronger and lasted longer, the team’s models suggest. They caused extreme swings in the weather on land that killed off forests, which stopped soaking up CO2 and started releasing it, leading to more warming and even more extreme El Niños.

In the sea, the temperature variations would have been less severe, and marine animals can more easily migrate to avoid them. This explains why marine extinctions happened later, when global warming got more intense. “The killer extreme global warming that was the cause of marine extinction was worse because of these El Niños taking away the carbon sink,” says Farnsworth.

By the peak of the extinction, the temperature anomaly during El Niños was up to 4°C (7.2°F), with each event lasting more than a decade, he says.

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It isn’t clear if something similar will happen in the future. Computer models vary in their forecasts of how El Niños will change as the planet warms, says Farnsworth. But they are already having a bigger impact because they are happening in a warmer world.

“The El Niño we just had was helping set record temperatures everywhere and leading to a huge amount of forest fires,” he says. “And the thing that disturbs me most is tentative signs during this El Niño of dieback in the Amazon.”

The study shows that under specific climate conditions, El Niño events can cause extinctions, says at the University of Colorado, Boulder. But these mega El Niños couldn’t occur today because the Pacific is smaller than Panthalassa, they say.

“These results are very exciting to understand the past, not so much the near future. To answer what El Niño will do, we need to look at intervals in the past with similar continental configurations as today,” says DiNezio.

“I think it’s a compelling study,” says at the University of Münster in Germany, who found the first direct evidence for the loss of the ozone layer during the Permian extinction.

“I don’t think that this and other extinction drivers, including ozone degradation, are mutually exclusive,” he says. “The deadly thing about the end-Permian mass extinction seems to be that a lot of things were happening at once, and interacting with each other as they cascaded through the Earth system.”