Large methane release could cause abrupt climate change
as happened 635 million years ago
30.05.2008
UCR-led research team says methane-triggered global
warming ended last 'snowball' ice age; dramatically reorganized Earth system
An abrupt release of methane, a powerful greenhouse gas, about 635
million years ago from ice sheets that then extended to Earth’s 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.
The researchers posit that the methane was released gradually at first
and then in abundance from clathrates – methane ice that forms and
stabilizes beneath ice sheets under specific temperatures and pressures.
When the ice sheets became unstable, they collapsed, releasing pressure on
the clathrates which began to degas.
“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 Earth’s 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 it’s 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 Earth’s 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 Earth’s 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 South Australia for stable isotope analysis, an
important tool used in climate reconstruction. At UCR, the researchers
analyzed the samples and found the broadest range of oxygen isotopic
variation ever reported from marine sediments that they attribute to melting
waters in ice sheets as well as destabilization of clathrates by glacial
meltwater.
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.
Iqbal Pittalwala | Quelle: EurekAlert!
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