A sudden and extreme case of runaway global warming 635 million years ago was caused by an abrupt release of methane, a powerful greenhouse gas, scientists said today.

The methane seeped from ice sheets that covered much of the planet toward the end of a frigid era called Snowball Earth. The gas escaped gradually at first and then very quickly from clathrates, or methane ice that forms and stabilizes beneath water ice sheets. As the water ice melted, pressure was relieved on the clathrates and they began to de-gas.

The transition represents one of the earliest known cases of what scientists now call a climatic tipping-point.

The big question scientists are now pondering: Could it happen again?

"Our findings document an abrupt and catastrophic global warming that led from a very cold, seemingly stable climate state to a very warm, also stable, climate state — with no pause in between," said geologist Martin Kennedy of the University of California at Riverside, who led the research team.

"What we now need to know is the sensitivity of the trigger," Kennedy said. "How much forcing does it take to move from one stable state to the other — and are we approaching something like that today with current carbon dioxide warming?"

Also called marsh gas, methane is a colorless, odorless gas. As a greenhouse gas, it is about 30 times more potent than carbon dioxide.

Methane clathrates still exist in Arctic permafrost and beneath the oceans at continental margins. Kennedy said it's possible that very little warming could unleash this trapped methane, potentially warming the planet by tens of degrees.

Kennedy and colleagues collected hundreds of marine sediment samples in South Australia for stable isotope analysis, an important tool used in climate reconstruction. They found the broadest range of oxygen isotopic variation ever reported from marine sediments, which they attribute to melting waters in ice sheets as well as destabilization of clathrates by glacial meltwater.

"Today we're conducting a global-scale experiment with Earth's climate system," Kennedy said, "and witnessing an unprecedented rate of warming, all with little or no knowledge of what instabilities lurk in the climate system and how they can influence life on Earth."

He said Nature did a similar experiment 635 million years ago, "and the outcome is preserved in the geologic record. We see that strong forcing on the climate, not unlike the current carbon dioxide forcing, results in the activation of latent controls in the climate system that, once initiated, change climate to a completely different state."

The research, detailed in the May 29 issue of the journal Nature, was supported by the National Science Foundation and NASA's Exobiology Program.

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