Australia: The Land Where Time Began

A biography of the Australian continent 

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.

Sources & Further reading

  1. Wadhams, P., 2016, A Farewell to Ice, Penguin Books Ltd.


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