for National Geographic News
A massive release of methane, a potent greenhouse gas, may have triggered rapid melting of the last "snowball Earth" about 635 million years ago, a new study suggests.
According to the snowball theory, ancient Earth experienced periods of global glaciation when ice sheets extended all the way to the Equator.
Methane ice forms and stabilizes beneath glaciers under certain temperatures and pressures, noted lead study author Martin Kennedy, a geologist at the University of California, Riverside.
But ice sheets are inherently unstable. Once they reach a certain size, they begin to fall apart.
The collapse of ancient ice sheets at the Equator would have unleashed trapped methane deposits and pushed global temperatures higher.
This warming would have caused ice sheets at slightly higher latitudes to melt, unleashing even more methane and causing Earth to warm even more.
"You can see the feedback there, that pretty soon you'll unzipper the entire reservoir" of methane, Kennedy said, which could have caused the abrupt transition from a very cold state to a much warmer climate.
A similar mechanism, the authors say, could uncontrollably accelerate global warming today.
Kennedy and colleagues collected and analyzed hundreds of marine sediment samples from a region of South Australia state that was near the Equator about 635 million years ago.
They found a broad range of chemical signatures in the sediments consistent with melting ice sheets and destabilization of methane deposits.
In addition, they found evidence that ice-sheet collapse and methane release preceded a rise in global sea levels.
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