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Today’s unsettling comparison to ‘the great dying’

250 million years ago, rising greenhouse-gas levels set off catastrophic changes.

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The eruption and release of greenhouse gas was just the beginning. The warmer atmosphere heated the ocean surface, effectively capping the seas with a warmer layer. The result: The overturning of the ocean’s water, which keeps deep waters oxygenated, likely stopped. Deeper waters became oxygen-depleted.

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Meanwhile, erosion accelerated on land, says Lee Kump, professor of geosciences at Penn State University, University Park, dumping more fertilizers, like phosphorus, into the seas. High nutrient influx led to plankton blooms. As the organic matter decomposed, it sucked up what oxygen remained – the same process now observed in the world’s dead zones.  Widespread ocean anoxia (oxygen depletion) suffocated much oxygen-dependent marine life.

Then came the final blow. In waterways that are anoxic beyond a certain depth, like today’s Black Sea, oxygen-dependent organisms live near the surface and oxygen-avoiding microbes live deeper. Scientists call the boundary between them the “chemocline.” Organisms below the chemocline “breathe” sulfates, not oxygen. Just as oxygen-dependent organisms exhale CO2, these bacteria give off hydrogen sulfide, a gas toxic in high concentrations to many life forms, including plants and animals. The gas neatly explains one of the mysteries of the Permian die-off: how an extinction event that began at sea could have decimated life on land.

Scientists find molecular “signatures” of anaerobic organisms at  what was the water’s surface in end-Permian times. Lack of oxygen let sulfate-breathers rise from the ocean deep and spew hydrogen sulfide directly into Earth’s atmosphere.

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.

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.”