Today's Unsettling Comparison to 'The Great Dying'

The crust through which it bubbled contained vast coal and limestone deposits from an earlier time. As it burned through this fossilized organic material, it released huge amounts of carbon.

Today, by burning fossil fuels, humans are again releasing carbon sequestered long ago, and at a similarly rapid rate.

"There may be some pretty direct parallels between the end-Permian extinction and today," says Jonathan Payne, professor of geological and environmental sciences at Stanford University in Palo Alto, Calif.

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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.

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.