PARIS (AFP) -- 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
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
Methane is a prodigious greenhouse gas, being 30 times more efficient than CO2 in trapping solar heat.
Martin Kennedy, a geologist at the
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
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.
An abrupt release of methane, a powerful greenhouse gas, about 635 million years ago from ice sheets that then extended to Earths low latitudes caused a dramatic shift in climate, triggering a series of events that resulted in global warming and effectively ended the last snowball ice age, a UC Riverside-led study reports.
Our findings document an abrupt and catastrophic means of global warming that abruptly led from a very cold, seemingly stable climate state to a very warm also stable climate state with no pause in between, said Martin Kennedy, a professor of geology in the Department of Earth Sciences, who led the research team.
This tells us about the mechanism, which exists, but is dormant today, as well as the rate of change, he added. What we now need to know is the sensitivity of the trigger: 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.
Study results appear in the May 29 issue of Nature.
According to the study, methane clathrate destabilization acted as a runaway feedback to increased warming, and was the tipping point that ended the last snowball Earth. (The snowball Earth hypothesis posits that the Earth was covered from pole to pole in a thick sheet of ice for millions of years at a time.)
Once methane was released at low latitudes from destabilization in front of ice sheets, warming caused other clathrates to destabilize because clathrates are held in a temperature-pressure balance of a few degrees, Kennedy said. But not all the Earths methane has been released as yet. These same methane clathrates are present today in the Arctic permafrost as well as below sea level at the continental margins of the ocean, and remain dormant until triggered by warming.
This is a major concern because its possible that only a little warming can unleash this trapped methane. Unzippering the methane reservoir could potentially warm the Earth tens of degrees, and the mechanism could be geologically very rapid. Such a violent, zipper-like opening of the clathrates could have triggered a catastrophic climate and biogeochemical reorganization of the ocean and atmosphere around 635 million years ago.
Today, the Earths permafrost extends from the poles to approximately 60 degrees latitude. But during the last snowball Earth, which lasted from 790 to 635 million years ago, conditions were cold enough to allow clathrates to extend all the way to the equator.
According to Kennedy, the abruptness of the glacial termination, changes in ancient ocean-chemistry, and unusual chemical deposits in the oceans that occurred during the snowball Earth ice age have been a curiosity and a challenge to climate scientists for many decades.
The geologic deposits of this period are quite different from what we find in subsequent deglaciation, he said. Moreover, they immediately precede the first appearance of animals on earth, suggesting some kind of environmental link. Our methane hypothesis is capable also of accounting for this odd geological, geochemical and paleooceanographic record.
Also called marsh gas, methane is a colorless, odorless gas. As a greenhouse gas, it is about 30 times more potent than carbon dioxide, and has largely been held responsible for a warming event that occurred about 55 million years ago, when average global temperatures rose by 4-8 degrees Celsius.
When released into the ocean-atmosphere system, methane reacts with oxygen to form carbon dioxide and can cause marine dysoxia, which kills oxygen-using animals, and has been proposed as an explanation for major oceanic extinctions.
One way to look at the present human influence on global warming is that we are conducting a global-scale experiment with Earths climate system, Kennedy said. 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. But much the same experiment has already been conducted 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 the climate to a wholly different state.
As part of their research, Kennedy and his colleagues collected hundreds of marine sediment samples in
Next in their research, Kennedy and his colleagues will work on estimating how much of the temperature change that occurred 635 million years ago was due solely to methane.