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Global warming triggered mass extinction, scientists claim

“It got hotter and hotter until it reached a critical point,” said Peter Ward of the University of Washington, lead author of a new study on the extinction. “It was a double-whammy of warmer temperatures and low oxygen, and most life couldn’t deal with it.” Ninety percent of all marine life and nearly three-quarters of land-based life died.

Ward’s findings contradict previous ones. Earlier studies supported the notion that a comet or asteroid impact triggered the carnage.

But in a paper published Jan. 20 on an advance online edition of the research journal Science, Ward and colleagues say they have found evidence that the culprit was warming caused by greenhouse gases, which are gases that trap heat in the atmosphere. They might have come from erupting volcanoes, the researchers said.

The extinction occurred at the boundary between the periods known as the Permian and Triassic, at a time when all land was concentrated in a supercontinent called Pangea. The event is not to be confused with the better-known Cretaceous-Tertiary extinction, which killed off the dinosaurs 65 million years ago.

“Animals and plants both on land and in the sea were dying at the same time, and apparently from the same causes – too much heat and too little oxygen,” said Ward.

The work provides a glimpse of the possible consequences of long-term climate warming, Ward said. Most scientists believe global warming is also taking place today, and some have presented evidence that droughts are spreading because of it (see the World Science Jan. 12 story: Global warming causing spreading droughts, scientists say)

The Karoo Basin of South Africa has provided the most intensively studied record of Permian-Triassic vertebrate fossils, according to Ward and colleagues. They used chemical, biological and magnetic clues to compare layers of Earth in the Karoo to similar layers in China. Previous research has tied the Chinese layers to the marine extinction at the end of the Permian period.

Evidence from the marine extinction is “eerily similar” at both sites, Ward said. 

Over seven years, his team collected 126 reptile or amphibian skulls from a nearly 1,000-foot thick section of exposed Karoo deposits from the time of the extinction. They found two patterns. One showed gradual extinction over about 10 million years leading up to the boundary between the Permian and Triassic periods; the other, a sharp increase in extinction rate at the boundary that then lasted another 5 million years.

The scientists said they found nothing in the Karoo that would indicate a massive object hit around the time of the extinction, though they looked for clues of it. These signs would include particular types of clays called impact clays, or material ejected from a crater left by such an impact. 

Evidence from the Karoo, they said, is consistent with a mass extinction resulting from catastrophic ecosystem changes over a long time scale, not sudden changes caused by an impact.

In this case, there is ample evidence that the world got much warmer over a long period because of continuous volcanic eruptions in an area known as the Siberian Traps, Ward asserted. 

As volcanism warmed the planet, large stores of methane gas frozen on the ocean floor might have been released to trigger runaway greenhouse warming, he said. But evidence suggests that species began dying out gradually as the planet warmed until conditions reached a critical threshold beyond which most species could not survive.

“It appears that atmospheric oxygen levels were dropping at this point also,” he said. “If that’s true, then high and intermediate elevations would have become uninhabitable. More than half the world would have been unlivable, life could only exist at the lowest elevations.”

He noted that the normal atmospheric oxygen level is around 21 percent, but evidence indicates that at the time of the Great Dying it dropped to about 16 percent – the equivalent of trying to breathe at the top of a 14,000-foot mountain. 

Other research teams supporting the theories of a comet or asteroid impact, however, have specifically disputed the notion that the “Great Dying” was a gradual process. 

They have also provided chemical evidence to support their claims. For instance, Luann Becker, a geologist at the University of California, found that soccer ball-shaped molecules called “fullerenes” (or “buckyballs”) inside rocks formed around the time of the extinction held an unusual number of unusual isotopes of the element helium and argon. These isotopes are more common in space than on Earth. Something, like a comet or an asteroid, must have brought the fullerenes to our planet, Becker argued.

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