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Australia: The Land Where Time Began |
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Arctic Methane
Offshore permafrost and warm water
The potential catastrophe represented by methane in the Arctic arises
from a combination of 2 phenomena: the retreat of sea ice and the
continued existence in Arctic seas of offshore permafrost. There are 3
layers that comprise the water structure of the deep Arctic Ocean.
The upper layer, polar surface water, is about 150 m deep and is
at or near the freezing point. The next layer down is the Atlantic
water, which continues down to 900 m and it contains the heat that has
been brought from the North Atlantic, which sinks at the edge of the
ice, and moves into the middle depths of the Arctic Ocean at middle
depths. The cold water layer that is the next in depth is the bottom
water that extends to the ocean bed. Therefore if the water above a
continental shelf is only 50-100 m deep there is a single layer of polar
surface water present, as the Atlantic water, that is deeper and warmer,
remains outside the shelf break. This polar surface layer was covered by
sea ice even in summer before to 2005, which provided a kind of air
condition system that blocked incoming solar radiation and the water
could not be warmed by the radiation as the first role of the radiation
was to melt some of the ice. The ice cover similarly kept the local air
temperatures at about 0oC. As the summer ice was melted away
completely beginning in 2005, solar radiation has been free to penetrate
the shelf water and so warm it. The polar surface water is no longer
being held to 0oC as it used to be by the summer ice, the
temperature of the polar surface water now rises throughout the ice free
summer. A surface temperature of 7oC was detected by a NASA
satellite in 2011 in the Chukchi Sea, which is as warm as the North Sea
in winter. Wadhams said that during a recent voyage in August 2014 of a
US Coast Guard icebreaker extraordinary temperatures were recorded in
the Chukchi Sea with air temperatures being 19oC and sea
surface temperatures of 17oC.
Significant waves can now be formed by winds above these wide areas that
are now free of ice, and as a result of this warmed water is being mixed
down to the bottom, so water above freezing is now impinging of the sea
bed for the first time in several thousand years.
This warmer water encounters frozen sediment on the sea bed. These
sediments represent a seaward extension of the terrestrial permafrost,
relics from the last glacial period. Methane hydrates or clathrates are
trapped within these sediments. Though this material has the appearance
of ice it burns. It is a compound of methane gas and water that has an
open crystal structure, and is stable only under the conditions of low
temperature and/or high pressure. It is usually found in deep water in
the sediments of various oceans where the pressure of the water column
above it maintains it in a stable state. It has been estimated that the
amount of methane stored in hydrate deposits is more than 13 times the
amount of carbon in the atmosphere, totalling 10,400 Gt (gigatons). As
the water above the Arctic shelves is shallow the hydrates should be
unstable but have remained stable because of the frozen sediment that
caps the sea bed which supplies enough overpressure to hold them in the
stable state. As a result of the recent loss of ice covering in these
areas the warm summer water thaws these sediments so that they can no
longer provide a solid cap above the hydrates. When sea levels were
lower during the last Ice Age the frozen sediments formed on land and
were then flooded as the sea level rose between 15,000-7,000 years ago
as the shallow East Siberian Sea formed in what has been termed the
‘Holocene transgression’ as the ice sheets melted. These hydrates are
now disintegrating as the sediment thaws after remaining stable beneath
the sea bed for tens of thousands of years to produce pure methane gas
which is exiting from the sediments and rising to the surface in large
bubble plumes. If the plume arises in deep water the methane is oxidised
by the water as it rises towards the surface, as has been seen off the
coast of Svalbard in 400 m of water (Westbrook et
al., 2009), all the methane
dissolves and the plume fades away before it reaches the surface. The
situation is different if the water is 50-100 m deep, in which case the
methane reaches the surface before it can dissolve in the sea water and
is released to the atmosphere at the surface. Wadhams suggest it must be
remembered that the situation at the present is entirely new with a new
melt phenomenon occurring, and that substantial open water has existed
on the Arctic shelves only since 2005.
The methane emerges as huge clouds of bubbles rising through the water
column, the process being called
ebullition. The bubbles can be recognised as a series of individual
plumes, similar to plumes that arise from the sea bed when there is a
gas-oil blowout, that emerge from a number of point sources on the sea
bed. The water is exceptionally shallow above the East Siberian Arctic
Shelf, being less than 40 m deep over more than 75 % of its entire area
of 2.1 million km2, so most methane gas bubbling from the
sediment doesn’t have time to oxidise before it reaches the surface and
is emitted to the atmosphere. Above the sea surface atmospheric
concentrations have been found to be as much as 4 times higher than
normal atmospheric levels. According to Wadhams it has been widely
assumed that methane could not be emitted from the Arctic shelf during
the period when it is covered by winter ice. However, it is suggested by
new observational data that methane ebullition, as well as other
emissions, are now occurring throughout the year. Winter emissions have
been strongly suggested by methane fluxes from European Arctic polynyas
where concentrations have been found to be 20-200 times higher in
methane than the ocean average. It has also been observed that methane
was accumulating under winter ice. It is strongly suggested by this that
methane is able to escape from the sediments throughout the year once
the sediment has been thawed by summer melt.
The first discovery and observations of the powerful bubble plumes on
the East Siberian Shelf in summer, by Natalia Shakhova and Igor
Semiletov, was recorded by some dramatic underwater pictures. They have
estimated that there are 400 Gt of methane equivalent that is held in
these sediments, and within a very few years of this warming gathering
pace 50 Gt could be released from the uppermost 10s of metres of the
sediment. The sediments closest to shore, in 10 m of water, have been
looked at by modellers such as Igor Dmitrenko (Dmitrenko
et al., 2011), who
have estimated that the thawing time scales and the release of methane
are slow, on the order of 1,000 years. Though further out to sea, other
things are going on.
Attention has been drawn to the role of
taliks, irregularities in the
subsea layers of permafrost, which are caused by faults or localised
irregularities, which have been found to provide a route by which
methane can find it way to the surface of the sediment from hydrates
that are buried deep within the sediments. It was found by Shakhova that
many of the methane plumes that had been observed in the East Siberian
Sea contained methane that was being released from the top of a talik.
According to Wadhams there is a parallel between the role played by
moulins on the Greenland ice sheet and the role of taliks in the East
Siberian Sea, they permit the occurrence of thermal processes deeper
inside the material than was suspected by modellers. A talik provides a
route for the escape of methane molecules from their hydrate cage up
past the barrier that was supposed to be offered by the permafrost on
the seabed, and eventually into the atmosphere. Therefore the rate of
emission does not depend of consecutive layers of sediment which would
give up its methane gradually as the permafrost thaws.
Methane is exceptionally powerful as a greenhouse gas, being 23 -100
times more effective than CO2 in its warming potential,
depending on how it is calculated. Wadhams suggests the main cause of
the beginning of the rise again after 2008, after they had stabilised
about 2000, in global atmospheric levels of methane may well be
Antarctic offshore emissions. Fracking with its leakage did not begin
until more recently. There are a number of unknowns such as the amount
of methane that is available, when will it emerge and what effect will
it have on the climate. It is expected that it will further increase the
speed of retreat of the ice, reduce the amount of the solar energy being
reflected into space, and increase the speed with which the melting of
the Greenland ice sheet accelerates. It is also expected that the
ramifications of vanishing ice will be felt far from the poles.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |