Wyrd - OE: that which has become; fate-shaping; destiny-unfolding

Thursday, 29 August 2013

Of monsters, methane and the burdensome sea

It's the ultimate tipping point - the so-called Methane Burp. Gigatons (Gt) of methane are released into the atmosphere by some sudden phase change in the earth's climate system.
Plumes of methane in Arctic Ocean (Westbrook et al, 2009)

With its vastly greater global warming potential - up to 100 times more powerful than CO2, over short timescales - a sudden pulse of methane would accelerate human-caused global warming. It could even cause a cascade of other knock-on effects (Amazon deforestation, collapse of the ice-sheets) so creating a runaway greenhouse effect.

It's that sort of a scenario - with 50Gt of methane gushing from a source in the Arctic over 10-20 years - that was recently modeled by economists in Nature. They put a $60 trillion price tag for the global economy from the consequences of ratcheting upwards man-made climate change. A scary monster, made even more real by describing it in the language our leaders understand most - hard cash.

But the Methane Burp is also complete fantasy - at least according to some recent detractors. Many climate scientists believe the methane locked up in frozen soils, or the curious, icy 'methane-hydrates' under the sea-floor, are a real threat - but only as a slow-burn cooker for the globe's climate. As the planet warms, the natural emissions from these sources will increase, they believe. But not at a rate that will be catastrophic. They struggle to see how anything like as large as 50,000 million tonnes of methane can erupt so quickly from Arctic permafrost, or sub-sea methane hydrates. Such sources have been thought to have been stable in the recent geological past.



Now a paper,studying one of the most vulnerable of these 'carbon time-bombs' - the methane hydrates frozen in the sub-sea sediments of the East Siberian Arctic Shelf - have found a worrying trigger for setting off the bomb. And it isn't the warming water itself that's the culprit - just the fact that there's more of it.

Figure prepared by Robert A. Rohde from published data.
Global warming has already been shown to be raising sea-levels at rates approaching 3cm (1.2 inches) per decade, much from the melting of the world's glaciers and ice-sheets. The most recent IPCC estimate sees that rate reaching 10 cm (3.6 inches) per decade, over the next century. Some reckon it could even get as high as 20cm (7.2 inches). Sea-level rises like these are a worrying problem in themselves, of course.

But such high rates aren't just potentially disastrous for those coastal cities likely to be impacted by storm surges. Higher seas from melted ice mean millions of tons of extra water sitting over the continental shelves. And that weight gain is putting areas like East Siberian Arctic Shelf under a lot more pressure. That's something the new study, published in Geology believes is a real cause for concern.

The loose sediments piled under the shallow Arctic seas fronting East Siberia are not just packed with methane hydrates. They are also riven by faults, and littered with signs of spectacular slips along the Siberian continental slope. It is looking at these old slope failures, which happened between 15,000 and 8,000 years ago, that has revealed just how vulnerable the area is to collapse. And unveiled a viable mechanism for the potential release of vast quantities of methane, fast.

This is one of the first studies to look at the reasons for these subsea landslides. The scientists involved looked at the rise in sea-level over the last 25,000 years - as the last Ice Age ended, and the ice melted - and calculated how much extra stress the rising waters would place on East Siberia's underwater fault systems. They found the additional force would be enough to trigger slips along those faults, and so produce the massive slides now seen at the foot of the continental  slope.

Given that those slides have been dated to coincide with the maximum period of sea-level rise, it seems very likely that rising seas are not good news for the stability of the East Siberian Arctic Shelf. And that's not good news for us. As well as the tsunamis that the movement of such large volumes of sediment would produce, they also 'unroof' the methane-hydrates trapped by permafrost in the area.

And the sea-level rise our global warming is causing could soon reach the same levels that were last seen 10,000 years ago. That takes the catastrophic release of multi-gigatons of methane out of the realm of pure fantasy. It seems the Methane Burp is a monster quietly biding its time in the far north - just waiting for the next tremor  to throw the gates open.

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