Today's Unsettling Comparison to 'The Great Dying'

In 1980, scientists Luis Alvarez and his son, Walter, proposed a new explanation for the dinosaurs' disappearance 65 million years ago: a meteor strike. Initially, the idea was met with resistance. But the evidence was convincing: a sediment layer high in iridium, an element common in asteroids, was found the world over, along with a 110-mile-wide impact crater in the Yucatán of the same age. What started as a fringe idea has gone mainstream.

Now scientists are rethinking another of earth's great die-offs. The end-Permian extinction 251 million years ago was the worst of earth's five mass extinctions. Ninety percent of all marine life and 70 percent of terrestrial life disappeared. It took five million years, perhaps more, for the biosphere to recover.

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But while the die-off was uniquely devastating, evidence of a single cataclysmic event, like an asteroid strike, hasn't been found in the geological record. Scientists now suspect that "the mother of all mass extinctions" was of Earth's own making. And the more they learn about it, the more parallels they see to today's world: A bout of greenhouse-gas-induced global warming, much like today's, set off a chain of events that culminated in oxygen-depleted oceans exhaling poison gas.

And as in today's human-dominated earthscape, life was already stressed.

"Something came along and kicked it over the edge," says Linda Elkins-Tanton, an assistant professor of geology at the Massachusetts Institute of Technology in Cambridge, Mass. She heads a recently launched multidisci­plin­ary effort to study the extinction. "Should there be a great kick [now], we are in a position for a great die-off," she says.

Two hundred and fifty million years ago, Earth was emerging from a period of glaciation. The transition from icehouse to greenhouse was already stressing life, scientists think. Then magma began bursting through the crust of what is now Siberia. The eruption was tremendous, says Professor Elkins-Tanton. Over the course of maybe 1 million years, enough lava flowed to cover the continental United States half a mile deep.