Today's Unsettling Comparison to 'The Great Dying'

Hydrogen sulfide would have also eaten holes in the earth's protective ozone layer. Plants and animals either suffocated directly – atmospheric oxygen levels plummeted to 15 percent (it's about 21 percent today) – or succumbed to the combination of long-term stresses.

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And the lessons for today? At the Permian boundary, "you're in a state of gradual warming, then as you approach that boundary, the warming in­­creases dramatically," says Jeff Kiehl, a senior scientist at the Na­­­tion­­­al Center for Atmospheric Re­­search in Boulder, Colo. "It wasn't a linear warming." Says Professor Kump: "This shows us what could happen if we push the system too hard…. We don't know where the intermediate thresholds are."

We're still some way from the atmospheric CO2 levels hypothesized at the end-Permian extinction – which were perhaps 10 times preindustrial levels, or 2,800 ppm. Yet, according the Intergovernmental Panel on Climate Change, if trends continue we're still approaching 1,000 ppm of CO2 by 2100. That's not Permian-extinction levels, but it would be the highest CO2 concentration in 80 million years, and a level at which both ocean anoxia and lesser extinctions have occurred.

"Do we want to put ourselves on a very risky path of possibly repeating earth's history, or do we want to be more cautious?" says Dr. Kiehl. "I would hope as a conscious species that we would choose the latter."