Yahoo News 28 May 08;
Melting of methane ice unleashed runaway global warming some 635 million years ago, according to a study released Wednesday that has implications for today's climate-change crisis.
Release of the potent greenhouse-gas, at first in small amounts and then in massive volumes, brought a sudden end to the planet's longest Ice Age, its authors believe.
During the "Snowball Earth" era, Earth froze over completely, with glaciers that crept down into the tropics and possibly even reached the equator.
The chill was self-sustaining, because the ice formed a brilliant white shell that reflected the Sun's rays, preventing the surface from warming.
After a frozen slumber lasting 155 million years, Earth warmed dramatically.
How this happened has been fiercely disputed, although all agree that the event changed the planet's climate system and ocean chemistry forever.
Publishing in the weekly British journal Nature, scientists in the United States and Australia point the finger at methane clathrates -- methane-rich ice that forms under ice sheets at specific temperatures and pressures.
The researchers believe that the ice sheets on Snowball Earth became unstable, which released pressure on the clathrates.
They began to evaporate, releasing the methane, which helped to warm the planet slightly. This thawed more clathrates and fuelled the warming and so on, creating a vicious circle or "positive feedback" in scientific parlance.
Methane is a prodigious greenhouse gas, being 30 times more efficient than CO2 in trapping solar heat.
Martin Kennedy, a geologist at the University of California Riverside who led the study, said the evidence comes from hundreds of marine sediment samples taken in South Australia.
Analysis of them for oxygen isotopes gave a signature of melting waters in ice sheets and destabilisation of clathrates by the meltwater.
Kennedy says the findings have a bearing on a much-feared positive feedback today -- the release of methane from frozen soil in Canada, Siberia and Alaska, and from clathrates that are below sea level, at the continental margins of the ocean.
Billions of tonnes of methane are locked up in these reservoirs, and the big worry is that it could take a relative small rise in temperature to start unleashing the gas, which would then trigger an unstoppable warming cycle.
"One way to look at the present human influence on global warming is that we are conducting a global-scale experiment with Earth's climate system," said Kennedy.
"We are witnessing an unprecedented rate of warming, with little or no knowledge of what instabilities lurk in the climate system and how they can influence life on Earth."
If the end of Snowball Earth is a guide, positive feedbacks, "once initiated, change the climate to a wholly different state," he observed.
If the mechanism for clathrates' feedback is now clearer, the scientists have still to explain how much forcing was needed for the vicious circle to set in motion -- and whether we are approaching any similar threshold today with the CO2 from fossil fuels.
Meanwhile, other research, also published in Nature, explains a mysterious fall in temperature which was noted in 1945 and has irked climate investigation for decades.
The fall was not due to a sudden cooling as some have suggested, but to different ways in which sea temperatures were measured by ocean-going ships, it says.
Runaway Global Warming 635 Million Years Ago
LiveScience.com Yahoo News 28 May 08;
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.
