Tuesday, 24 January 2012

Arctic Methane: What can AIRS tell us. Part 2.

This post won't really make much sense unless you've read the original post. It would also help to check out the videos of AIRS methane retrievals that I've previously posted.

Dr Yuragonov was good enough to point to some significant problems with a conclusion I drew in my previous post. In the meantime something else has occurred to me. So this post in essence is intended to add some major caveats to my claimed observations of apparent recurrent small scale anomalies in the AIRS images. The thrust of this will be towards the Laptev Sea anomaly because of it's possible linkage to Semiletov and Shakhova's work in that area. However these caveats also apply to the areas of the Lena River and the Yukon.



Dr Yurganov's first point was about the method of retrieval used to make the AIRS images. A key element is the assumed surface emissivity i.e. as I understand it, if the surface is assumed to be sea-ice but is actually a lead or polnya, or indeed wet ice as opposed to dry, this can create spurious indications of the presence of methane. For example I presume that this is what is happening over the ice tongue off Greenland in the Fram Strait. Given that this is the case it may be unsound to claim that AIRS is showing a localised signature of methane emissions in the Laptev Sea.

Secondly Dr Yurganov stressed the importance of the fact that maximum concentration does to mean maximum emission. Emission drives the rate of concentration increase. Concentration itself is a result of addition from sources, loss from chemical reactions in the atmosphere and atmospheric fluxes in and out of a given area. I was aware of this but failed to make it clear in the previous post.

In relying upon patterns that appear regularly, as opposed to occasional patterns, I was trying to get at patterns that are tied geographically to a given location. My reasoning being that such localised patterns could only occur repeatedly over a period of many years because of something that was going on at the surface - the common factor over years with different weather. Assuming that chemical loss of methane in the Arctic atmosphere can be neglected as a factor common across the Arctic, and that interannual methane fluxes in and out of a given area are randomised by different years of weather, the common factor underlying any small scale pattern that shows up over repeated years has to be methane fluxes from the surface (although I have some further comments regarding surface emissivity). To assist people in seeing the Laptev Sea pattern here is March:

2003 Present.
2004 Present
2005 Present.
2006 Present.
2007 Present.
2008 Obscured.
2009 Present.
2010 Obscured.
2011 Present.

I should stress that the pattern I'm looking at is very specific - higher concentration in a band north of the New Siberian Islands, extending off, and to the east of, the Lena River delta, see the final graphic in my previous post.

Could this pattern be being caused by incorrect surface emissivity? I've checked Bremen AMSRE images and can't see any regular large leads or polnyas, however the transpolar drift draws sea-ice from the Siberian coast, and the presence of islands and coastal topography may cause regular leads in the area. Terra/Aqua would be a better bet, having higher resolution, but these aren't available in winter. So perhaps the pattern is being caused by errant surface emissivity. It is worth noting however that coastal flaw leads are a common feature of all the Arctic coast, yet the small scale patterns in other parts of the Arctic do not reveal persistent patterns that repeat from year to year. This argument also applies to the possibility of leads allowing enhanced methane flux into the atmosphere from the ocean.

In the time since posting another issue has occurred to me: The Arctic winter atmosphere is dominated by surface inversions, these are due to both cooling of the surface layers by IR emission, and influx of warmer sub-arctic air masses (Zhang et al). From radiosondes (and reanalysis datasets) the inversions are thick, being of the order of 600 to 800 metres (800 to 1000 metres). So their presence could stop convection from drawing surface methane up to the 400mb level. However Alam & Curry modelled the atmospheric impact of leads and found that for leads over 600m in width, latent and sensible heat fluxes have the potential to penetrate heights of around 700m into the Arctic atmosphere. So it is possible that the presence of the persistent methane anomaly over the Laptev Sea is a combination of recurrent leads over an area of increased ocean methane concentration - the latent heat flux from the leads causing local convective plumes which entrain methane up into the troposphere above the inversion, thence feasibly mixing could cause them to be detected at the 400mb level.

In closing, my previous conclusion of a detection of a methane anomaly at around 400mb over the area Semiletov & Shakhova have identified as a source of methane flux from clathrates should be treated as speculative. That said, I still find it suspicious at least that AIRS shows a persistent small-scale anomaly in a region where there are significant concentrations of methane in the ocean, and consequent fluxes to the atmosphere. However as with all posts here, whilst I try to base my posts on what the science says, the writing and opinion is my own, and I'm not a professional.

PS I should add that nothing I've seen in the AIRS methane retrievals makes me think we're headed towards some sort of methane driven apocalypse.


Alam & Curry, 1995, Lead induced atmospheric circulations.

Zhang et al, 2010, Climatological Characteristics of Arctic and Antarctic Surface-Based Inversions.

4 comments:

Anonymous said...

Could you please answer a simple question?
Methane emissions at the Antarctic are low, and the CH4 concentration is very low (the lowest globally ?).
Methane emissions for the Arctic are also supposedly low. Why then does the CH4 concentration seem to be so high (the highest globally?) over the Arctic circle?

Kevin O'Neill said...

@Anonymous: I think the premise of your question is wrong. The historical ice core record does not show a significant difference between arctic (Greenland) and antarctic methane levels. The graph is from Historic CH4 Records from Antarctic and Greenland Ice Cores, Antarctic Firn Data, and Archived Air Samples from Cape Grim, Tasmania, Etheridge et al, 2002.

During the austral winter Antarctica has a far higher incidence of polar stratospheric clouds compared to the arctic and should, if I understand the chemistry correctly, lead to a reduction in atmospheric methane. I'm guessing here, but I suspect that might account for any observed difference.

Chris R said...

Anonymous (& Kevin),

Before I start rambling (always a danger with me), to quickly answer the question:

AIRS shows that the atmospheric concentration around the Arctic climbs in winter, this is because emissions continue during the winter (although at a reduced rate) but oxidation (hence destruction) of methane slows markedly during the winter, due to cold. AIRS shows prominent increased Arctic methane in winter as compared to mid lattitudes. So I think most of the increased atmospheric concentration in the Arctic is due to reduced destruction of methane during the winter.

It would be interesting to examine this - using monthly means averaged over lattitude bands. However the sources of methane data I've come across all seem to be daily and are of a complex format, so getting the data into Excel isn't trivial, and I've not got the motivation together to tackle the problem - it's a big hassle.

Methane levels are higher over the Arctic and NH mid lattitudes than other lattitude bands. See here (second graphic down). Kevin's source shows a difference of perhaps 100ppb, this seems to be suported by the preceding link. It would be interesting to see if there's a change with time in lattitudinal concentration - maybe I should boot myself up the arse and deal with the data.

Also the ratio of Carbon13/Carbon12 isotopes in methane shows a strong lattitudinal bias, with the Arctic having a much lower ratio than other lattitudes, see here (my own calculation - note that there are only a few stations in the SH so I'd only take the NH graphs as reliable).

The lower C12/C13 ratio shows that much of the source in the Arctic is biological. That graph suggests that there is a role for the Arctic in the recent increase of methane, this is supported by research I've read. The graph also shows a similarly high annual range toward the end of the last period of rise; methane had been stable between about 1998 and 2006. Dlugokencky (2003), PDF, finds that the early part of this period of stability was due to a reduction of emissions from north of 50degN. So that finding tallies with the C13/C12 ratio I've graphed.

So I wouldn't agree that the Arctic is a low emitter, for it's area it bats above it's weight. (That's another calculation I could do if I got to grips with the concentration data)

Chris R said...

Kevin,

I think the biggest issue, as the C13/C12 ratio suggests is that the NH has more wetlands, including tundra, which emit CH4. However in Yurganov's London Presentation (see my most recent post on methane by clicking on the methane tag at the bottom of this post) he notes an interesting Antarctic methane anomaly, who's origin is not known.