Strong El Niños primed Earth for mass extinction

The greatest extinction in Earth’s history may have begun not just with a bang, but also a change in the wind.

Some 250 million years ago, more than 80% of marine species and two-thirds of those on land died off in the end-Permian mass extinction—the closest life ever came to annihilation. Most scientists think massive volcanic eruptions in what is now Siberia triggered the event by spewing carbon dioxide (CO₂) and warming the planet. But according to a modeling study published this week in Science, the Great Dying was primed by a mega–El Niño pattern in the global ocean of the time, leading to weather extremes that killed off forests and kicked off the extinctions.

Volcanic eruptions and asteroid impacts are familiar culprits for mass extinctions. The notion that weather patterns could amplify their impact, says Isabel Montañez, a geochemist at the University of California, Davis, “is a breath of fresh air.”

Records of ancient sea surface temperature, based on swings in oxygen isotopes in the fossilized teeth of eellike creatures called conodonts, suggest the vast connected oceans of the time, Tethys and Panthalassa, were warmer in the west and cooler in the east. That’s similar to the Pacific Ocean today, where steady trade winds pile up warm water in the west. But records show that as the extinction approached, this pattern disappeared—suggesting warm water periodically spread east, much as it does today during El Niño events, says Yadong Sun, a paleoclimatologist at the China University of Geosciences and lead author of the study.

Those temperature records are blurry, so Sun turned to climate modelers to sharpen the picture. At the time of the Permian extinction, Earth was dominated by a supercontinent, Pangaea, which ran from pole to pole, leaving Tethys and Panthalassa to encircle much of the planet. The team plugged this geography into a climate model developed by the United Kingdom’s Met Office and dialed in factors such as the rise in greenhouse gases driven by the Siberian eruptions. This let them re-create snapshots of the planet’s climate across the Great Dying, which lasted 100,000 years or more. “And luckily, everything fits,” Sun says.

Geological evidence suggests that before the eruptions, atmospheric CO₂ levels were likely about 400 parts per million (ppm), similar to the present day. As they began to tick up, the model showed profound El Niño events began to occur, says Alexander Farnsworth, a study co-author and paleoclimate modeler at the University of Bristol. Warm western waters at the equator spread east, preventing deep, cold waters from reaching the surface. Chaotic weather would have been unleashed worldwide, as happens during today’s El Niños. As CO₂ levels rose to 800 ppm and beyond, the El Niños grew more fearsome than any in history, lasting up to 7 years. “You’re seeing this incredible heat buildup, which just persists,” Farnsworth says.

Pangaea at the time was lush on the coasts but arid across its vast interior—something like a “super-Australia,” says Wolfram Kürschner, a paleoclimatologist at the University of Oslo. This could have left it more vulnerable to the swings of drought and excessive rainfall caused by El Niño and its opposite, La Niña. The seesawing weather extremes could help explain Permian geological records that show periods of heavy flooding and an absence of coal deposits, suggesting forests had vanished. “This is a major step forward in our understanding,” he says.

Most important, Montañez says, it is the first kill mechanism that can adequately explain why fossil records show extinctions on land beginning thousands of years before those in the ocean. That was a puzzle in the volcano theory, because global warming from the erupted CO₂ should have killed life on land around the same time as it stifled ocean circulation and mixing, killing marine species through a lack of oxygen. El Niño swings could account for the lag, because many marine species survive them better than land species—with the notable exception of coral reefs. And indeed, the only marine species to suffer an early death were corals.

Mega–El Niños do not rule out other methods of extinction—indeed, they could have helped nudge them along. By killing forests, El Niños could have liberated more carbon into the atmosphere, helping keep levels high between bouts of eruptions, Farnsworth says. Meanwhile, noxious chemicals from the eruptions or even an uptick in ultraviolet radiation from ozone-destroying chemicals injected into the stratosphere could all have played a role. “The extinction rate is so high, you must have multiple factors,” Sun says.

The study raises the disturbing prospect that today’s rising greenhouse gas levels could trigger Permian-like mega–El Niños. So far, climate models that project the future of El Niño under present-day warming offer mixed forecasts. “No one knows what to expect,” says Judson Partin, a paleoclimatologist at the University of Texas at Austin.

But Sun takes little solace in the uncertainty, given that the end-Permian disruptions seemed to begin when CO₂ levels were so similar to today’s. “Everything that happens today, happened before.” Only this time, humanity is the volcano.

This post was originally published on this site