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Knowing the rate of emission and mechanism of permafrost degradation is a prerequisite to meaningful predictions of near-future methane releases in the Arctic. In this interview with two of the leading authors of this paper, Dr Natalia Shakhova and Dr Igor Semiletov, we learn that the decay of the subsea permafrost, even that which was submerged relatively recently (less than 1000 years ago) is currently occurring and, due to manmade global warming, there is no known countervailing force to stop the trend of further decay and increased emissions.
What is the East Siberian Arctic Shelf?
The East Siberian Arctic Shelf (ESAS) is the largest and the shallowest shelf in the worlds ocean with a mean depth of around 50m. The total area of the ESAS is 2,000,000 sq Km’s with a seabed of frozen organic matter called subsea permafrost. This coastal permafrost (ground that remains less than or equal to 0ºC for 2 or more years) developed when the northern hemisphere cooled around 2.5 million years ago.
As the glaciers eventually melted, the sea-level rose submerging the permafrost. Inundation of the shelf with seawater has changed the permafrost properties due to an increase in temperature of as much as 17ºC.
Warming of the ESAS began about 12-13 thousand years ago when the entire shelf area was exposed above sea level. When the inundation occurred, numerous thaw lakes underlain by taliks, existed on the surface of the permafrost. A talik is a layer within the permafrost that is above 0ºC.
It is the behaviour of this permafrost that has occupied Dr Shakhova and Dr Semiletov in their studies of the ESAS because beneath it is the largest pool of methane gas predicted to exist in the world.
Gas migration paths building in degrading permafrost acts like a Champagne cork
Dr. Shakhova: We use an analogy where we compare the East Siberian Arctic Shelf to a bottle of champagne. So the gas produces within this bottle and it keeps accumulating as long as the cork serves as an impermeable lid.
This lid is subsea permafrost. Before it was just permafrost [on land] but after it was submerged it became subsea permafrost and served to preserve an increasing amount of gas produced from its release to the ocean and atmosphere above. While this lid is impermeable, there is nothing to worry about.
But when this lid loses its integrity, this is when we start worrying. This is where the methane is releasing and the amounts of methane currently releasing makes us think it will increase as a result of the disintegration of this permafrost body.
Nick Breeze: How can the changes observed more recently in a little over three decade period be conclusive?
Dr. Shakhova: For the permafrost, three decades is not a huge period of time, because the processes, the consequences of which we are studying right now and have to deal with, started long long ago. This was triggered by natural warming associated with replacement of the cold climate epoch with the warm interglacial period and followed by permafrost inundation by sea water. Scientists agree that submerged permafrost would eventually start degrading, but how soon and at what pace this degradation would occur became the major point of disagreement between them.
It was suggested by some scientists that subsea permafrost would keep its integrity for millennia, which means that in the areas submerged less than 1000 years ago (as we investigated in our study) it should not have occurred yet. Our study proved that not only has it ;already occurred, but it has been progressing to higher rates, which have almost doubled since this degradation started.
It is most likely that we are now dealing with the consequences of when natural warming is enhanced with anthropogenic warming and together they are accelerating the pace of natural processes. This appears to be continuing the processes of permafrost degradation at levels that we have never observed before.
Shakhova explains that during the period between ice ages, called an interglacial period, the permafrost starts to thin due to the warming. It has been pointed out that in previous interglacials, the temperatures were even higher than they are now but the methane hydrates were not released from the ESAS.
Dr. Shakhova: Despite the fact that in the Eamian (the interglacial period that occurred 130,000 - 115,000 years ago) the temperatures reached higher numbers but the duration of this optimum period was shorter (about 2 thousand years) and was followed by cooling; in the Holocene, there is still no cooling after more than 5 thousand years of warming.
For subsea permafrost, it was long thought that because the duration of warming is more important than surface temperatures themselves, in order to start thawing, it must first reach an equilibrium with the surrounding environment. For that reason, it only matters that the temperature of the surrounding environment reaches the level at which permafrost thaws; after that, it makes no difference if the temperature reaches +5ºC, or +7ºC; once thawed, it is no longer permafrost. We also demonstrated in our latest paper, that there are more intricate mechanisms of permafrost disintegration, not known before, that allow gas migration pathways to form well before the whole permafrost body is thawed through.
What is important is that it is above the thaw point and how long this warming lasted. This is what effects the permafrost more effectively than the temperature itself.
So the thinning can only continue if the duration of the warmer period is long enough to cause the taliks that lead to gas migration pathways that allow for the passage of methane from the sediments below.
Dr. Shakhova: As we showed in our articles, in the ESAS, in some places, subsea permafrost is reaching the thaw point. In other areas it could have reached this point already. And what can happen then? The most important consequence could be in terms of growing methane emissions… a linear trend becomes exponential.
This edge between it being linear and becoming exponential is very fine and lays between frozen and thawed states of subsea permafrost. This is what we call the turning point. To me, I cannot take the responsibility in saying there is a right point between the linear and exponential yet, but following the logic of our investigation and all the evidence that we accumulated so far, it makes me think that we are very near this point. And in this particular point, each year matters.
This is the big difference between being on the linear trend where hundreds and thousands of years matter, and being on the exponential where each year matters.
Shakhova and Semiletov currently estimate that of the 2,000,000 sq km’s that comprise the ESAS, 200,000 sq km’s (10%) are what they would call hotspots, areas where methane emissions are observed as being far greater than in the lower background area.
Nick Breeze: Does a sudden burst of methane become feasible as the subsea permafrost is destabilised?
Dr. Shakhova: The difference between emissions in background areas and hotspots is orders of magnitude.
It’s about… try the difference between about 3 milligrams per square metre per day [for background areas] or 3,000 grammes per square metre per day. How many orders [of magnitude]? It’s 3-5 orders of magnitude between this.
This is exactly what is the difference between the linear and exponential. If the areas we call hotspots increase about two times there would be huge difference in the scale of emissions. Three times, there would be even bigger difference. If there could be an outburst like a gigatonne release, I don’t know if I can exclude this scenario, and what would be the argument to exclude this scenario?
Because when we see the difference between two different areas releasing methane at rates divided by 5 orders of magnitude (like 3 milligrams and 3 kilograms), that indicates to me that up on progression of permafrost degradation, the area of gas migration pathways will grow and the area of hotspots will grow accordingly.
Gas in the areas of hotspots is releasing from the seabed deposits, in which free gas has accumulated for hundreds of thousands, or even for a million years. This is why the amount of this gas and its power in releasing (due to its high pressure) is tremendous.
That would allow large releases of methane and whatever you call it - outburst, bomb, or whatever, I see no point to say no to such a possibility. I’m afraid to say yes because we still have to learn so much about the mechanism.
Nick Breeze: In relation to the ESAS, how do you know these hydrates are there and that they are a potential threat?
Dr. Shakhova: The importance of hydrates involvement in methane emissions is overestimated. The hydrate is just one form of possible reservoirs, in which pre-formed methane could be preserved in the seabed if there are proper pressure/temperature conditions; it is just the layer of hydrates composes just few hundred of meters – this is a very small fraction compared to thousands of meters of underlying gas-charged sediments in the ESAS.
Dr. Semiletov added that the 5 billion tonnes of methane that is currently in the Earth’s atmosphere represents about one percent of the frozen methane hydrate store in the East Siberian Arctic Shelf. He finishes emphasising “…but we believe the hydrate pool is only a tiny fraction of the total.”
Dr. Shakhova: The second point is that the hydrates are not all of the gaseous pool that is preserved in this huge reservoir. This huge area is 2 million square kilometres. The depth of this sedimentary drape is a few kilometres, up to 20 kilometres at places. Generally speaking, it makes no difference if gas releases from decaying hydrates or from other free-gas deposits, because in the latter, gas also has accumulated for a long time without changing the volume of the reservoir; for that reason, gas became over pressurised too.
Unlike hydrates, this gas is preserved free; it is a pre-formed gas, ready to go. Over pressured, accumulated, looking for the pathway to go upwards.
The point Shakhova and Semiletov are making is that the question of whether there are methane hydrates present beneath the permafrost is really not important. The estimated amount of hydrates, 1500 billion tonnes, is actually only a tiny proportion of the actual pressurised methane store beneath the gas hydrate stability zone.
Dr. Shakhova: The third point is that the hydrates, despite disbelief from some scientists, have already been found in the ESAS. We know from personal communication that the South Korean expedition was accomplished in 2016 and they sampled the hydrates. I believe, this data will be published soon. However, hydrates could only be sampled if they remain stable. After hydrates are destabilised, we can only sample gas releasing from these decaying deposits.
In our observations, we have accumulated the evidence that this gas front is propagating in the sediments. To me as a scientist, these points are enough to be convinced that methane release in the ESAS is related to disintegration of subsea permafrost and associated destabilisation of seabed deposits whether it is hydrates or free gas accumulations.Nick Breeze: Why do some scientists say there can be no hydrates in the ESAS?
Natalia Shakhova: They believe the required pressure could only be built by the overlaying water column, creating so called hydrostatic pressure. With a mean depth of water column in the ESAS of about 50 meters, these scientists think the hydrostatic pressure would not be enough and, thus, hydrates would not form. This is a misunderstanding, because this argument only works if the seafloor is considered the top boundary of the hydrates stability zone (HSZ).
In areas of permafrost, there are a few specific features altering the pressure/temperature conditions required for the formation of hydrates:
1) Because hydrates are associated with permafrost, their top boundary could be well below the sea floor, closer to the bottom of permafrost. It is natural that grounds composing seabed could build up pressure even more efficiently than the water column. This is because grounds are denser, thus, to create 1 atmosphere of pressure it only requires 5-8 meters compared to 10 meters of water column.
This means that the top boundary of the HSZ would occur tens to hundreds meters below the seafloor.
2) High inter-pore pressure could also form during sediment freezing of shelf sediments while shelf exposure above the sea level in cold epochs, which means that the upper boundary of the gas hydrate stability zone could occur at shallower depths (less than 100 metres).
3) Because a major fraction of hydrates form when the shelf is dry, very low temperatures occurring in the grounds are pared with lower pressure requirements than it is for oceanic hydrates. It is unclear to me why scientists ignore these facts while arguing the presence of hydrates in the ESAS.
The claim that there is not enough water depth for hydrates has absolutely no scientific reasoning.
However, all the opposite opinions are based exclusively on modelling results, which, until now, are based on lack of knowledge of subsea permafrost physics, as well as a lack of observational data to calibrate the models.
Nick Breeze: Could the 10% area of hotspots in the ESAS be very significant in releasing methane and impacting the global climate?
Dr. Shakhova: The area of hotspots is determined by the fraction of subsea permafrost that is disintegrated. The process of permafrost degradation started thousands of years ago and it is now a key driver triggering methane emissions from these long-preserved deposits.
Emissions that are occurring right now are the result of a combined effect of natural and anthropogenic warming and they will be accelerated until warming is turned to cooling. Even after it happens, there is no mechanism to stop permafrost disintegration in the ESAS besides shelf exposure above the sea level that would serve to freeze the gas migration paths so that they integrate with the permafrost. Before that, the amount of methane that is releasing will increase while the supply lasts.
As gas within the sedimentary basins of the ESAS have been accumulating for a million years with no way to be released earlier, the supply for currently occurring emissions is tremendous. Because the shelf area is very shallow (mean depth is less than 50 metres), a fraction of these emissions will reach the atmosphere. The problem is that this fraction would be enough to alter the climate on our planet drastically.
Dr Semiletov and Dr Shakhova are coauthors of a new paper published in Nature Communications Journal titled: ‘Current rates and mechanisms of subsea permafrost degradation in the East Siberian Shelf’ and are about to begin new work to accurately assess the quantity of carbon preserved in the sedimentary drape of the East Siberian Arctic Shelf.
Interview conducted in April 2017 by Nick Breeze (@NickGBReeze)
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