Arctic methane outgassing on the E Siberian Shelf part 1 - the background
by John Mason, Skeptical Science, January 15, 2012
Reports of extensive areas of methane - a powerful greenhouse gas - bubbling up through the shallow waters of the East Siberian Arctic Shelf (ESAS) have been doing the rounds in the media recently, with some articles taking the apocalyptic approach and others the opposite. So what IS going on in the far North? In this two-part post we will first examine the data available to date and then in part two we go on to discuss the findings of 2011 with the research team who have been doing the work on the ground.
Background
To understand the goings-on up at the ESAS in context, we need to go back to the time of the last glacial maximum, some 20,000 years ago. Although the climate was cold, much of Siberia remained unglaciated for the simple reason that the climate was also extremely dry: the main area of glaciation was in the Verkhoyansk Range in the east, which rises to nearly 2500m. The low-lying plains of central Siberia saw the development of permafrost - defined as soil that remains at below freezing point for two or more years. The prolonged cold of the last glacial period saw permafrost develop to great depths - over 1000m in places. Extensive areas of this old permafrost, albeit thinner than at the last glacial maximum, exist at the present day. On land, permafrost occurs several metres below surface, and is overlain by the so-called active layer, soil which seasonally thaws out and in which the Siberian flora grows.
Above: Bathymetric map (source - NOAA) of the Arctic with key features noted and the subject area highlighted in red.
During the last glacial maximum, global sea levels fell by over a hundred metres, with the result that the shallow seas of the ESAS became dry land, which allowed permafrost to develop there. Climate warming in the Holocene melted the big ice sheets in N America and NW Europe, leading to sea level rise and flooding of the ESAS, which once again became an extensive shelf sea, averaging some 45 metres in depth. The incoming seawater raised the temperature of the seawater-seabed interface dramatically so that it is considerably (>10C) warmer today than the annual average temperature over the adjacent land permafrost areas. This warming led to a certain amount of seabed permafrost degradation but until recently the remaining subsea permafrost layer was thought to be relatively stable, acting as a cap or lid to the methane that was expected to be present in and beneath it.
Permafrost degradation and methane release on land are things that most people will be familiar with: footage of people igniting methane on frozen Siberian lakes has been broadcast many times. This is primarily biogenic methane - formed via microbial decay of organic matter such as plant-debris. As permafrost degrades due to the warming climate, the organic matter, trapped in the frozen ground for thousands of years, is freed and bacterial decay rapidly sets in, releasing methane to the atmosphere.
At greater depths in the sedimentary column, methane may exist in a second form, trapped in clathrate molecules. A clathrate is a naturally-occurring chemical substance which consists of one type of molecule forming a cage-like crystalline lattice - the host - which traps a second type of molecule - the guest. In the case under discussion here, the host is water and the guest is methane, hence the commonly-used term 'methane hydrate'. Methane hydrate looks just like ice: it is a white, crystalline solid but is only stable at low temperatures and/or high pressures: otherwise it decomposes, liberating its methane content.
Permafrost degradation and methane release on land are things that most people will be familiar with: footage of people igniting methane on frozen Siberian lakes has been broadcast many times. This is primarily biogenic methane - formed via microbial decay of organic matter such as plant-debris. As permafrost degrades due to the warming climate, the organic matter, trapped in the frozen ground for thousands of years, is freed and bacterial decay rapidly sets in, releasing methane to the atmosphere.
At greater depths in the sedimentary column, methane may exist in a second form, trapped in clathrate molecules. A clathrate is a naturally-occurring chemical substance which consists of one type of molecule forming a cage-like crystalline lattice - the host - which traps a second type of molecule - the guest. In the case under discussion here, the host is water and the guest is methane, hence the commonly-used term 'methane hydrate'. Methane hydrate looks just like ice: it is a white, crystalline solid but is only stable at low temperatures and/or high pressures: otherwise it decomposes, liberating its methane content.
Above: methane hydrate forming irregular white masses embedded in the marine sediment of Hydrate Ridge, in the Pacific Ocean off Oregon.
This sensitivity to temperature and pressure means that outside of very deep water environments, methane hydrate typically occurs at considerable depths in the sedimentary column (ref. 1): values of 200-500m beneath surface are commonly cited as being within the 'gas hydrate stability zone' (GHSZ). Any deeper than that and temperatures tend to be too high due to the geothermal gradient; any shallower and temperatures are again too high - except, perhaps, where the hydrates are locked-in and kept at low temperatures by extensive, bonded permafrost. Within the GHSZ, methane hydrate occurs as pore-filling cements in coarse-grained sediment such as sand; conversely, in finer-grained sediment such as mud it forms pure masses of irregular shape. Typical concentrations in sandy sediment are a few percent of pore-volume. Estimates of the total amount present globally vary: although some very high values have been suggested, more commonly-cited figures are 10,000 Gt carbon or less. This is still a substantial figure when compared to e.g. estimates of carbon in global coal reserves.
Methane hydrate has been exploited on a limited scale as a fossil fuel. At Messoyakha, in western Siberia, the Soviets extracted methane trapped beneath a dome of permafrost 450m in thickness; at least a third of the resource, exploited over 13 years, was thought to exist as hydrate which was artificially destabilised by pumping hot water and solvents into the wells in order to collect the gas.
Methane hydrate has been exploited on a limited scale as a fossil fuel. At Messoyakha, in western Siberia, the Soviets extracted methane trapped beneath a dome of permafrost 450m in thickness; at least a third of the resource, exploited over 13 years, was thought to exist as hydrate which was artificially destabilised by pumping hot water and solvents into the wells in order to collect the gas.
Recent observations on the East Siberian Arctic Shelf
That the sea in this area of the Arctic has warmed up significantly should come as no surprise to anybody who has been following the unfolding reductions in sea-ice and other developments in that region. A 2011 paper (ref. 2), citing hydrographic data collected since 1920, reported a dramatic warming of the bottom water layer over the ESAS coastal zone (<10 m depth), since the mid-1980s, of 2.1°C. The warming was attributed to atmospheric changes involving enhanced summer cyclonicity, reduction in ice extent, the consequent lengthening of the summer open-water season and - consequential to that - solar heating of the water column.
Until relatively recently, the subsea permafrost of the ESAS saw little or no attention compared to the onshore permafrost: it was simply assumed that it was unlikely to be a source area for methane because it was all frozen solid. That assumption was turned on its head in 2003 when the first of a series of field expeditions by scientists from the University of Alaska at Fairbanks took place and resulted in an ominous discovery: surface and especially bottom waters were super-saturated with methane, implying that outgassing from the sea-bed was occurring. Further fieldwork went on to discover plumes of methane gas bubbling up to the surface. In deeper waters, methane does not make it all the way up to the atmosphere - it all dissolves in seawater - but over the shallower waters of the ESAS this is not the case. Air sampling surveys over the ESAS revealed great variability in methane levels: against the global background level of 1.85ppm, they were elevated by typically 5-10% up to 1800m in height, with local spikes over gas-productive areas as high as 8ppm. The researchers calculated the annual total methane flux from the ESAS to the atmosphere to be 7.98Tg C-CH4, which in plain English is 10.64 million tonnes of methane per year, a figure similar to what, up until now, was thought to be the methane emissions of the entire world's oceans (ref. 3). This figure needs to be seen in the context of other sources, however: domestic animals emit about 80 million tonnes a year, for example.
More worryingly though, the same team made estimates of the methane present as free gas and methane hydrate beneath or within the ~1.5 million sq km of the submarine permafrost of the ESAS. The total came to >1000 Gt. The area of this permafrost affected by active fault zones and by open taliks - zones of permafrost that have melted - was stated to be 1-2% and 5-10% of the total area respectively. As such zones are exactly those through which buried methane can escape from under the permafrost, they went on to suggest that up to 50Gt of methane hydrate was at risk of destabilisation leading to "abrupt release at any time" (ref. 4).
That is a colossal figure, when put against annual anthropogenic methane emissions which in 2010 were approximately 275 million tonnes (or 0.275 Gt). Methane is a far more potent greenhouse gas than carbon dioxide - by a factor of 25 (global warming potential as stated in the IPCC AR4) - so that a 50 Gt methane release would be like releasing 40 years' worth of anthropogenic carbon dioxide emissions (at 2009 emission levels) all at once. However, there are some issues with cranking atmospheric methane levels up in this drastic way.
The first problem is that in none of the glacial-interglacial transitions of the past 400,000 years has a sudden large methane-spike been recorded. Ice-core data instead reveal transitions from 0.4ppm (glacials) to 0.8ppm (interglacials) and back. Such records would tend to suggest that no such releases occurred during this period of geological time despite drastic fluctuations in climate. Does that suggest that large-scale abrupt methane outbursts are rather unlikely? Leading on from that, the second problem is finding a physical mechanism by which such an abrupt release of that magnitude could actually happen: so far, on a subshelf environment where major undersea landslides are unlikely, nobody has proposed a detailed mechanism by which that could happen.
Furthermore, a recent permafrost modeling study (ref. 2) has indicated that permafrost melting lags behind changes in surface temperature: after 25 years of the summer seafloor warming reported above, in the model the upper boundary of the subsea permafrost deepened by only a metre. This is one of the current controversies associated with ESAS research: has the model accurately depicted the actual situation? The team who did the modelling study have attibuted the observed methane outgassing to "degradation of subsea permafrost that is due to the long-lasting warming initiated by permafrost submergence about 8000 years ago rather than from those triggered by recent Arctic climate changes". Although they accept that severe subsea permafrost degradation will occur in about a thousand years' time, they see the current degassing as nothing new. Which of these views will turn out to be correct?
Controversy, however, does not invite complacency. Any increased Arctic methane flux, tapping into vast stores of steadily destabilising methane hydrate, has the potential to keep going over a considerable time-period as a response to warmer (and rising) sea temperatures. We certainly do not need any feedbacks that bring additional natural sources of powerful greenhouse gases to the table, yet that is exactly what we risk up in the Siberian Arctic. The big questions that we now need the answers to are for how long has this outgassing been going on, does it appear to be intensifying and how might a colossal and rapid outburst occur. These are among the points we will be raising with the people on the ground and the answers from our interview with Dr Natalia Shakhova, part two of this post, will soon be appearing, here on Skeptical Science. In the meantime, David Archer, who has worked extensively with gas hydrates, looks at some release scenarios over at Realclimate, here and here.
More worryingly though, the same team made estimates of the methane present as free gas and methane hydrate beneath or within the ~1.5 million sq km of the submarine permafrost of the ESAS. The total came to >1000 Gt. The area of this permafrost affected by active fault zones and by open taliks - zones of permafrost that have melted - was stated to be 1-2% and 5-10% of the total area respectively. As such zones are exactly those through which buried methane can escape from under the permafrost, they went on to suggest that up to 50Gt of methane hydrate was at risk of destabilisation leading to "abrupt release at any time" (ref. 4).
That is a colossal figure, when put against annual anthropogenic methane emissions which in 2010 were approximately 275 million tonnes (or 0.275 Gt). Methane is a far more potent greenhouse gas than carbon dioxide - by a factor of 25 (global warming potential as stated in the IPCC AR4) - so that a 50 Gt methane release would be like releasing 40 years' worth of anthropogenic carbon dioxide emissions (at 2009 emission levels) all at once. However, there are some issues with cranking atmospheric methane levels up in this drastic way.
The first problem is that in none of the glacial-interglacial transitions of the past 400,000 years has a sudden large methane-spike been recorded. Ice-core data instead reveal transitions from 0.4ppm (glacials) to 0.8ppm (interglacials) and back. Such records would tend to suggest that no such releases occurred during this period of geological time despite drastic fluctuations in climate. Does that suggest that large-scale abrupt methane outbursts are rather unlikely? Leading on from that, the second problem is finding a physical mechanism by which such an abrupt release of that magnitude could actually happen: so far, on a subshelf environment where major undersea landslides are unlikely, nobody has proposed a detailed mechanism by which that could happen.
Furthermore, a recent permafrost modeling study (ref. 2) has indicated that permafrost melting lags behind changes in surface temperature: after 25 years of the summer seafloor warming reported above, in the model the upper boundary of the subsea permafrost deepened by only a metre. This is one of the current controversies associated with ESAS research: has the model accurately depicted the actual situation? The team who did the modelling study have attibuted the observed methane outgassing to "degradation of subsea permafrost that is due to the long-lasting warming initiated by permafrost submergence about 8000 years ago rather than from those triggered by recent Arctic climate changes". Although they accept that severe subsea permafrost degradation will occur in about a thousand years' time, they see the current degassing as nothing new. Which of these views will turn out to be correct?
Controversy, however, does not invite complacency. Any increased Arctic methane flux, tapping into vast stores of steadily destabilising methane hydrate, has the potential to keep going over a considerable time-period as a response to warmer (and rising) sea temperatures. We certainly do not need any feedbacks that bring additional natural sources of powerful greenhouse gases to the table, yet that is exactly what we risk up in the Siberian Arctic. The big questions that we now need the answers to are for how long has this outgassing been going on, does it appear to be intensifying and how might a colossal and rapid outburst occur. These are among the points we will be raising with the people on the ground and the answers from our interview with Dr Natalia Shakhova, part two of this post, will soon be appearing, here on Skeptical Science. In the meantime, David Archer, who has worked extensively with gas hydrates, looks at some release scenarios over at Realclimate, here and here.
References
1. Archer, D (2006): Destabilization of methane hydrates: a risk analysis. A Report Prepared for the German Advisory Council on Global Change (40pp). PDF
2. Dmitrenko, I.A., Kirillov, S.A., Tremblay, L.B., Kassens, H., Anisimov, O.A., Lavrov, S.A., Razumov, S.O. & Grigoriev, M.N. (2011): Recent changes in shelf hydrography in the Siberian Arctic: Potential for subsea permafrost instability. Journal of Geophysical Research, 116, C10027.Abstract
3. Shakhova NE, Semiletov I, Salyuk A, Yusupov V, Kosmach D, Gustafsson O (2010): Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic shelf. Science, 327:1246-1250. Abstract
4. Shakhova NE, Semiletov I, Salyuk A, Yusupov V, Kosmach D (2008). Anomalies of methane in the atmosphere over the East Siberian shelf. Geophysical Research Abstracts 10, EGU2008-A-01526. Abstract
2. Dmitrenko, I.A., Kirillov, S.A., Tremblay, L.B., Kassens, H., Anisimov, O.A., Lavrov, S.A., Razumov, S.O. & Grigoriev, M.N. (2011): Recent changes in shelf hydrography in the Siberian Arctic: Potential for subsea permafrost instability. Journal of Geophysical Research, 116, C10027.Abstract
3. Shakhova NE, Semiletov I, Salyuk A, Yusupov V, Kosmach D, Gustafsson O (2010): Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic shelf. Science, 327:1246-1250. Abstract
4. Shakhova NE, Semiletov I, Salyuk A, Yusupov V, Kosmach D (2008). Anomalies of methane in the atmosphere over the East Siberian shelf. Geophysical Research Abstracts 10, EGU2008-A-01526. Abstract
Posts
No comments:
Post a Comment