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Saturday, August 3, 2013

A methane spike

Recently (early 2013) there have been back and forth arguments about the possibility of a rapid methane emission from the Arctic Continental Shelves and especially from the very wide shelf off the north coast of Russia.  Sea level was 120 meters lower during the last glacial and apparently there is still permafrost under the sediment of the ocean bottom from this period despite the overlying layer of water which is above zero degrees centigrade. This layer is said to be up to 1.5km deep.   Although we don't yet have a very good handle on the subject, it is hypothesized that this permafrost is locking in enough methane as methane hydrate, either within or below the permafrost, to greatly increase global warming if it was released suddenly.

Since the permafrost is apparently still there and has been over the  10,000 years since the last glacial ended, the conduction of heat downward to this layer must be very gradual and hence,,  so the argument goes,, a sudden belch of methane is unlikely.  For the purpose of this blog, I will assume that such a reservoir does exist and speculate on a mechanism(s) by which it could be released suddenly.

Before starting, though, we should look at the true strength of Methane as a green house gas.  While it is oft quoted as 20 or 25 times as effective as Carbon dioxide, this is "on a 100 year basis".  As odd as it seems, instantaneously, methane is more than 100 times as effective as Carbon dioxide and hence a 4ppm increase in methane in the atmosphere would have a greater effect in the short term (a few decades) than our present 400ppm Carbon dioxide.    Click on the above link to see why this is so.  Reverse engineering the figures, I came up with a figure of 140.  In other words, the approximately 2ppm methane in the atmosphere at present has the warming effect of 280ppm carbon dioxide.  Just recently (Dec 2013) the NSIDC site quoted a figure of x86.

It should also be noted that ever increasing amounts of methane are being observed bubbling out of Arctic Ocean.  It is possible that this may is due to more intense observation.  Whether or not methane emissions are actually increasing will become apparent over the next few years.  Curiously enough, despite a likely increase in methane emissions over the past decade or two, methane levels in the atmosphere have hardly increased and this needs some explanation.

This is an exerp. from the NSIDC web site on the subject.

The Siberian continental shelf is a vast region of shallow-water covered continental crust, comprising about 20% of the global area of the continental shelf. During the last glacial maximum, much of the shelf was exposed to the cold atmosphere and froze to a depth of about 1.5 kilometers (about 1 mile). Layers of sediment below the permafrost slowly emit methane gas, and this gas has been trapped for millennia beneath the permafrost. As sea levels rose at the end of the ice age, the shelf was once again covered by relatively warm ocean water, thawing the permafrost and releasing the trapped methane. Methane is a potent greenhouse gas but is relatively short-lived in the atmosphere (about 12 years), leading to reduced global warming potential over time. In the short-term however, methane has a global warming potential 86 times that of carbon dioxide.

So what mechanisms could lead to rapidly increasing breakdown of Clathrates in or under the permafrost.

The added 120m layer of water over the Arctic continental shelves will have added extra stability to any underlying clathrates due to the increased pressure.  Therefore a greater temperature rise will be necessary to start the disintegration than before the sea covered these deposits .  Once enough heat has reached the clathrates to start this break down, the pressure will begin to rise.  If the overlying cap of permafrost is strong enough and continuous enough, this increase in pressure will have a negative feed back on the further break down of the underlying clathrate*. 

* Think of putting a piece of clathrate into a very strong sealed container at room temperature.  As the clathrate begins to break down, pressure in the vessel increases.  The warmer it is, the higher the pressure has to rise before clathrate break down ceases.  For instance, clathrates are stable at 17 degrees centigrade at a pressure equal to a depth of 1600m.

The problem arises if  pressure from the   methane which has been released from the clathrate is sufficient to crack the overlying permafrost and create a tunnel or crack up to the ocean bottom.  Now instead of the weight of the sediment (SG about 2), the pressure of the overlying water and the mechanical strength of the frozen sediment keeping the pressure on the clathrates, you have only the pressure of the water column from the ocean surface to the clathrate layer.  Some of the clathrate has already broken down and the methane is just waiting for a breach in the overlying  permafrost for it to rise to the surface.

On the other hand clathrates have latent heat just as does ice which creates a negative feed back on the rate of clathrate break down.  Clathrates can only break down as fast as the inflow of heat allows.  Already broken down clathrate will release its methane suddenly but remaining clathrate will break down only as fast as heat can reach it.  As more an more gas is evolved, the tunnelling increases and sea water with it's heat content gains access to these layers.  You have a sort of geyser as in Yellowstone park.  The process accelerates.

You also have an air (methane) lift effect.  Gas rising through any channel between  the clathrate deposit and the bottom of the ocean further reduces the pressure on the clathrate increasing its break down. The shallower the bottom of the sea where such a break occurs, the greater the reduction of pressure on the clathrate deposit.  Now yet another effect is kicks in.

At some locations along the continental slope, it is likely that all that is holding the sediment together is the permafrost and clathrate ice.  Once this layer starts to loose it's integrity due to the break down in the clathrates, small tremors can induce large slumps, releasing the pressure on large deposits of clathrate.  Picture the land slide on Mt St Helen that released the pressure on underlying gas-saturated magma.  I'm not suggesting anything so dramatic but the basic principle is the same.

Another factor at play is that the clathrate itself likely caps deeper deposits of free methane.  Heat from the centre of the earth seeps upward to meet the "cold" seeping down from the sea floor*.  Above about 200C clathrates don't form. On average, temperatures rise 25 degrees per km you go down into the earth.  Below the permafrost layer, one would expect to find free methane.    If methane is seeping up from deep deposits of liquid or gas hydrocarbons, from coal measures or from  shale, as it hits the deep cold pore water of sediments it is absorbed by water and forms clathrates. This caps underlying methane and any crack is quickly sealed as methane seeps up such cracks and forms clathrate.  It has been observed that some of the methane seeping out of permafrost areas on land is young methane (likely from the break down of organic material) and some is old (likely from deeper hydrocarbon deposits).   Such rising methane will sit below its cap of clathrate ice just waiting to be released.

When it was initially calculated how fast our ice sheets could melt, only thermodynamics was taken into account.  At that time it wasn't realized the effect of, for instance, moulons increasing the slide of ice into the sea and it also wasn't realized that a warmer ocean was melting floating ice sheets from below.  As these ice sheets disintegrated, ice flow to the ocean increased and the contribution to sea level of ice was larger than thermodynamic considerations would have predicted.

We may be making the same mistake here with clathrates as we only consider how fast heat can be conducted down through the layers of sediment toward the deposits of clathrates.  We could be in for some wee surprises as some of the above "convection" type phenomenon cut in.

ps.  There is another wrinkle in this story.  If the permafrost isn't conventional permafrost; in other words frozen ground, but is itself methane clathrate; ie permafrost with a methane component dissolved in it, it will not melt at just above 00C.  It's melting temperature will depend on how much methane is in the ice and on what depth, and hence what pressure, it is at.  It would be very instructive to have a few hundred cores taken on the Arctic continental shelf to see what is actually down there and at what depth.  Methane clathrate can exist at 200C with sufficient pressure.  Such cores would allow a much better estimate of how prone we are to a sudden release of methane.

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