Feb. 26, 2008 -- Vast releases from the oceans of a potent greenhouse gas may have helped trigger the ends of past ice ages, says a researcher studying methane levels in ancient ice cores from Antarctica.
Researchers point out the phenomenon could happen again today, possibly accelerating global warming.
The signals of sudden methane releases during ice ages were extracted from 400,000 years-worth of ice drilled out of Antarctica's Lake Vostok. The pattern suggests that there have been spikes in methane releases over and over--right at the times when things are coldest, driest, and the most common modern source--wetlands--are the scarcest.
"There's no way that the wetlands can work," said researcher Kieran O'Hara of the University of Kentucky in Lexington. "Even modern wetlands can't explain the source."
O'Hara modeled methane releases seen in the ancient ice to try and find out their source. His results appear in an article in the February issue of Geophysical Research Letters.
What O'Hara found is that the methane releases happened relatively suddenly and then the methane persisted--which is weird because methane breaks down rather quickly in the atmosphere. This means that wherever the methane came from, it kept coming for a while, replenishing the methane that was being destroyed in the atmosphere.
That suggests there was a really big methane source. The biggest source that was available in those dry, icy times would be the frozen gas hydrates that exist throughout the world in the cold depths of the oceans.
The gas hydrates would have been vulnerable, said O'Hara, because the ice age sea level was close to 120 meters lower, which would expose shallower gas hydrates to lower pressures that that would make them unstable.
While sea levels are not lower today, another factor--warming ocean temperatures--may be making gas hydrates similarly vulnerable.
"A reduction in pressure by lowering sea-level, or an increase in ocean temperature, via global warming, can lead to hydrate destabilization, converting generally immobile and innocuous methane hydrate (a solid), into more buoyant and mobile methane gas," explained gas hydrate researcher Matthew Hornbach of the University of Texas at Austin.
"Now, here's the rub: While one of these processes acts as negative feedback mechanism for climate change (namely sea-level lowering during glacial periods)," said Hornbach, "the other mechanism (ocean temperature warming) triggers a positive feedback mechanism that potentially injects large quantities of methane into an already warming climate."
There's also the danger that the warming of hydrates could weaken undersea slopes and cause massive submarine landslides that belch huge amounts of methane into the atmosphere, Hornbach told Discovery News. That may even be what was happening in those ice age methane releases.
"In fact, two studies back in 2003 and 2004 indicate that near-critical gas pressures may exist at many gas hydrate provinces, indicating that many ... are at the brink of instability, and only very minor changes in temperature or pressure could lead to structural failure, and methane gas release," he said.
Nor will the slight additional rise in sea level stave off these troubles by putting today's gas hydrates under more pressure, said Hornbach.
"A few tens of meters of sea level rise, will not significantly offset the effects of a few degrees of ocean temperature warming," Hornbach said.
Related Links:
Larry O'Hanlon's blog: Earth Impacts
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