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.

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