Methane Bomb Squad Part 5: Shakhova, Schmidt
October 20, 2014
UPDATE: a reader sends a link to a more recent (june 2014) Shakhova interview. The sound quality not as good, which makes it difficult to follow, nevertheless,I am posting this in the interest of furthering the discussion. This is a “part 1” of 3, the others being available at the you tube link.
In the past week, I’ve been posting a number of excerpted interviews relating to the topic of undersea methane releases in the arctic.
This has not actually been something I planned out just this way. The idea was, I wanted to interview as many folks behind the scenes and find out as much as I could, then synthesize everything into a nice tidy video, and release that. But events have kind of overtaken that.
In September, the Royal Society held an event in the UK which drew a number of experts on arctic ice and the rapid changes observed there. Among those was Dr. Gavin Schmidt, of NASA, one of the most highly respected experts on the planet in areas of global climate. Dr. Schmidt gave a well attended lecture on the issue of arctic methane, specifically to push back against what has become somewhat of a Youtube cottage industry, methane disaster porn. It’s not hard to find material proclaiming the imminent extinction of humanity due to the massive release of methane from arctic ocean shelves – and I agree with Dr. Schmidt that this kind of material is irresponsible and divorced from reality.
The scientific source often cited (and then, I think, exaggerated) for such dire pronouncements is the work of Dr. Natalia Shakhova, the University of Alaska in Fairbanks, and her associate Dr. Igor Semilitov. Short story, Shakhova et al were not at the UK shindig, leading to charges of conspiracy to exclude them from the discussion. Since I started this series, I’ve even been getting emails darkly hinting that perhaps I was a tool of that conspiracy, as well. I don’t think a preference to step carefully is indicative of a conspiracy, but never mind.
Among Dr. Schmidt’s strongest arguments are datasets that he says indicate little or no methane breakout in the relatively near paleo-record, from 2 previous eras.
First, the Holocene warm period, – at about the time the planet was coming out of the last ice age, the arctic was warm, possibly warmer than today, for a period substantially longer than the current warmup. The other period, known as the Eemian, was an interglacial, a temperate spell, like ours, just before the plunge in to the most recent ice age, about 120,000 or so years ago. We know it was pretty warm for quite a while – thousands of years – because sea level was high – maybe 15, or 20 feet higher, and a substantial portion of the Greenland ice sheet seems to have melted.
Yet, Schmidt says, we saw no methane breakout.
This paleo argument is compelling to me, as I have always considered the fossil record a powerful indicator of how the current climate will behave.
Could we have more of a buffer for big methane belches than we know? Here are two slides from Schmidt’s lecture – which is not available to my knowledge online, although may be soon, at least in audio.
There’s a lot more material on this, much of which I have not digested myself, but I wanted to get as much of this under-seen and under-discussed material online where people can read it, and spark a tiny bit more informed discussion on the issue, something I am sure Dr. Schmidt, Dr. Shakhova, and all concerned, would like to see happen. There are a lot more perspectives on this than what most people have heard, so now is as good a time as any to hear them.
Clearly this is an important discussion, and more information is needed. Happy to hear from anyone who has additional useful resources on this issue, and ideas about how to broaden the discussion.
Methane from the Siberian continental shelf
The Siberian continental shelf is huge, comprising about 20% of the global area of continental shelf. Sea level dropped during the last glacial maximum, but there was no ice sheet in Siberia, so the surface was exposed to the really cold atmosphere, and the ground froze to a depth of ~1.5 km. When sea level rose, the permafrost layer came under attack by the relatively warm ocean water. The submerged permafrost has been melting for millennia, but warming of the waters on the continental shelf could accelerate the melting. In equilibrium there should be no permafrost underneath the ocean, because the ocean is unfrozen, and the sediment gets warmer with depth below that (the geothermal temperature gradient).
Ingredients of Shakhova et al (2013)
- There are lots of bubbles containing mostly methane coming up from the shallow sea floor in the East Siberian Arctic shelf. Bubbles like this have been seen elsewhere, off Spitzbergen for example (Shakhova et al (2013)). Most of the seep sites in the Siberian margin are relatively low flow but a few of them are much larger.
- The bubbles mostly dissolve in the water column, but when the methane flux gets really high the bubbles rise faster and reach the atmosphere better. When methane dissolves in the water column, some of it escapes to the atmosphere by evaporation before it gets oxidized to CO2. Storms seem to pull methane out of the water column, enhancing what oceanographers call “gas exchange” by making waves with whitecaps. Melting sea ice will also increase methane escape to the atmosphere by gas exchange. However, the concentration of methane in the water column is low enough that even with storms the gas exchange flux seems like it must be negligible compared with the bubble flux. In their calculation of the methane flux to the atmosphere, Shakhova et al focused on bubbles.
- Sediments that got flooded by rising sea level thousands of years ago are warmer than sediments still exposed to the colder atmosphere, down to a depth of ~50 meters. This information is not directly applied to the question of incremental melting by warming waters in the short-term future.
- The study derives an estimate of a total methane emission rate from the East Siberian Arctic shelf area based on the statistics of a very large number of observed bubble seeps.
Is the methane flux from the Arctic accelerating?
Shakhova et al (2013) argue that bottom water temperatures are increasing more than had been recognized, in particular in near-coastal (shallow) waters. Sea ice cover has certainly been decreasing. These factors will no doubt lead to an increase in methane flux to the atmosphere, but the question is how strong this increase will be and how fast. I’m not aware of any direct observation of methane emission increase itself. The intensity of this response is pretty much the issue of the dispute about the Arctic methane bomb (below).
What about the extremely high methane concentrations measured in Arctic airmasses?
Shakhova et al (2013) show shipboard measurements of methane concentrations in the air above the ESAS that are almost twice as high as the global average (which is already twice as high as preindustrial). Aircraft measurements published last year also showed plumes of high methane concentration over the Arctic ocean (Kort et al 2012), especially in the surface boundary layer. It’s not easy to interpret boundary-layer methane concentrations quantitatively, however, because the concentration in that layer depends on the thickness of the boundary layer and how isolated it is from the air above it. Certainly high methane concentrations indicate emission fluxes, but it’s not straightforward to know how significant that flux is in the global budget.
What about methane hydrates?
There are three possible sources of the methane in the bubbles rising out of the Siberian margin continental shelf:
- Decomposition (fermentation) of thawing organic carbon deposited with loess (windblown glacial flour) when the sediment was exposed to the atmosphere by low sea level during the last glacial time. Organic carbon deposits (called Yedoma) are the best-documented carbon reservoir in play in the Arctic.
- Methane gas that has been trapped by ice, now escaping. Shakhova et al (2013) figure that flaws in the permafrost called taliks, resulting from geologic faults or long-running rivers, might allow gas to escape through what would otherwise be impermeable ice. If there were a gas pocket of 50 Gt, it could conceivably escape quickly as a seal breached, but given that global gas reserves come to ~250 Gt, a 50 Gt gas bubble near the surface would be very large and obvious. There could be 50 Gt of small, disseminated bubbles distributed throughout the sediment column of the ESAS, but in that case I’m not sure where the short time scale for getting the gas to move comes from. I would think the gas would dribble out over the millennia as the permafrost melts.
- Decomposition (melting) of methane hydrates, a peculiar form of water ice cages that form in the presence of, and trap, methane.
Methane hydrate seems menacing as a source of gas that can spring aggressively from the solid phase like pop rocks (carbonated candies). But hydrate doesn’t just explode as soon as it crosses a temperature boundary. It takes heat to convert hydrate into fluid + gas, what is called latent heat, just like regular water ice. There could be a lot of hydrate in Arctic sediments (it’s not real well known how much there is), but there is also lot of carbon as organic matter frozen in the permafrost. Their time scales for mobilization are not really all that different, so I personally don’t see hydrates as scarier than frozen organic matter. I think it just seems scarier.
The other thing about hydrate is that at any given temperature, a minimum pressure is required for hydrate to be stable. If there is pure gas phase present, the dissolved methane concentration in the pore water, from Henry’s law, scales with pressure. At 0 degrees C, you need a pressure equivalent to ~250 meters of water depth to get enough dissolved methane for hydrate to form.
The scariest parts of the Siberian margin are the shallow parts, because this is where methane bubbles from the sea floor might reach the surface, and this is where the warming trend is observed most strongly. But methane hydrate can only form hundreds of meters below the sea floor in that setting, so thermodynamically, hydrate is not expected to be found at or near the sea floor. (Methane hydrate can be found close to the sediment surface in deeper water depth settings, as for example in the Gulf of Mexico or the Nankai trough). The implication is that it will take centuries or longer before heat diffusion through that sediment column can reach and destabilize methane hydrates.