[ExI] peak fossil by 2020

Eugen Leitl eugen at leitl.org
Fri Mar 29 15:47:46 UTC 2013


On Thu, Mar 28, 2013 at 07:07:20AM +0100, Tomaz Kristan wrote:

> Deep down there is no warming. I very much doubt it is on the surface, but
> sure none deep down, where those deposits are waiting.

http://en.wikipedia.org/wiki/Clathrate_gun_hypothesis

...

Current outlook

Most deposits of methane clathrate are in sediments too deep to respond
rapidly, and modelling by Archer (2007) suggests the methane forcing should
remain a minor component of the overall greenhouse effect.[11] Clathrate
deposits destabilize from the deepest part of their stability zone, which is
typically hundreds of metres below the seabed. A sustained increase in sea
temperature will warm its way through the sediment eventually, and cause the
deepest, most marginal clathrate to start to break down; but it will
typically take of the order of a thousand years or more for the temperature
signal to get through.[11] One kind of exception is recent changes to the
Gulf Stream causing widespread gas hydrate destabilization:[12] "It is
unlikely that the western North Atlantic margin is the only area experiencing
changing ocean currents; our estimate of 2.5 gigatonnes of destabilizing
methane hydrate may therefore represent only a fraction of the methane
hydrate currently destabilizing globally."

Main article: Arctic methane release

Another kind of exception is in clathrates associated with the Arctic ocean,
where clathrates can exist in shallower water stabilized by lower
temperatures rather than higher pressures; these may potentially be
marginally stable much closer to the surface of the sea-bed, stabilized by a
frozen 'lid' of permafrost preventing methane escape. Research carried out in
2008 in the Siberian Arctic has shown millions of tons of methane being
released, apparently through perforations in the seabed permafrost,[13] with
concentrations in some regions reaching up to 100 times normal.[14][15] The
excess methane has been detected in localized hotspots in the outfall of the
Lena River and the border between the Laptev Sea and the East Siberian Sea.
Some melting may be the result of geological heating, but more thawing is
believed to be due to the greatly increased volumes of meltwater being
discharged from the Siberian rivers flowing north.[16] Current methane
release has previously been estimated at 0.5 Mt per year.[17] Shakhova et al.
(2008) estimate that not less than 1,400 Gt of carbon is presently locked up
as methane and methane hydrates under the Arctic submarine permafrost, and
5–10% of that area is subject to puncturing by open taliks. They conclude
that "release of up to 50 Gt of predicted amount of hydrate storage [is]
highly possible for abrupt release at any time". That would increase the
methane content of the planet's atmosphere by a factor of twelve,[18][19]
equivalent in greenhouse effect to a doubling in the current level of CO2.
In 2008 the United States Department of Energy National Laboratory system[20]
and the United States Geological Survey's Climate Change Science Program both
identified potential clathrate destabilization in the Arctic as one of four
most serious scenarios for abrupt climate change, which have been singled out
for priority research. The USCCSP released a report in late December 2008
estimating the gravity of this risk.[21] According to data released by the
EPA[22] atmospheric methane (CH4) concentrations (ppb) remained between
400–800ppb (between years 600,000 BC to 1900) and since 1900 have risen to
levels between 1600–1800ppb.



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